REPORT ◥ NANOMATERIALS Epitaxial lift-off of electrodeposited single-crystal gold foils for flexible electronics Naveen K. Mahenderkar, 1 Qingzhi Chen, 2 Ying-Chau Liu, 2 Alexander R. Duchild, 2 Seth Hofheins, 2 Eric Chason, 3 Jay A. Switzer 2 * We introduce a simple and inexpensive procedure for epitaxial lift-off of wafer-size flexible and transparent foils of single-crystal gold using silicon as a template. Lateral electrochemical undergrowth of a sacrificial SiO x layer was achieved by photoelectrochemically oxidizing silicon under light irradiation. A 28-nanometer-thick gold foil with a sheet resistance of 7 ohms per square showed only a 4% increase in resistance after 4000 bending cycles. A flexible organic light-emitting diode based on tris(bipyridyl)ruthenium(II) that was spin-coated on a foil exploited the transmittance and flexibility of the gold foil. Cuprous oxide as an inorganic semiconductor that was epitaxially electrodeposited onto the gold foils exhibited a diode quality factor n of 1.6 (where n = 1.0 for an ideal diode), compared with a value of 3.1 for a polycrystalline deposit. Zinc oxide nanowires electrodeposited epitaxially on a gold foil also showed flexibility, with the nanowires intact up to 500 bending cycles. S ingle-crystal silicon (Si) is the bedrock of semiconductor devices; its high crystal- line perfection minimizes electron-hole recombination, and its dense SiO x native oxide minimizes surface states. There is interest in moving beyond the planar structure of conventional Si-based chips to produce flex- ible electronic devices such as wearable solar cells, sensors, and flexible displays (1–5). Flexible devices and beautiful architectures have been produced using ultrathin foils of Si (6–8). Con- ductive polymers (9), carbon nanotubes (10), graphene (11), and metal nanostructures (12, 13) have been used as transparent and flexible sub- strates for flexible electronics. Ultrathin (5 to 30 nm) metal films (14, 15) have relatively high optical transmittance, flexibility, improved device efficiency, and low sheet resistance. However, they usually are grown by vacuum evaporation or sputtering, which gives a polycrystalline or textured deposit. Polycrystalline electronic mate- rials suffer from electron-hole recombination at grain boundaries (16). To expand the palette of electronic materials beyond planar Si, an in- expensive source of highly ordered material is needed that can serve as an inert substrate for the epitaxial growth of grain boundary–free semicon- ductors, optical materials, and superconductors. We show that wafer-size transparent and flexible single-crystal foils of gold (Au) can be produced by a simple and inexpensive lift-off procedure using single-crystal Si as the template for electrochemical epitaxial growth. The trans- parency of these single-crystal Au foils is exploited to fabricate a flexible organic light-emitting diode (OLED) based on tris(bipyridyl)ruthenium(II). The single-crystal nature of the Au is used to produce an inorganic diode based on epitaxial cuprous oxide (Cu 2 O) that has more ideal diode char- acteristics than a diode based on polycrystal- line Cu 2 O. Epitaxial lift-off of films on single-crystal sub- strates by dissolving a sacrificial adhesion layer can produce free-standing single-crystal foils (17). The epitaxial lift-off procedure for ultra- thin single-crystal foils of Au electrodeposited onto Si(111) substrate is shown in Fig. 1. Electro- deposition of epitaxial Au on a Si(111) substrate was carried out using the method developed by Allongue and co-workers (18, 19). Previously, we showed that epitaxial electrodeposition of Au on Si(111), Si(100), and Si(110) substrates can serve as a proxy for bulk single-crystal Au (20). Single-crystal Si(111) with a 0.2° miscut toward ½11 2and a resistivity of 1.15 ohm·cm was used as the substrate to grow the Au foils (Fig. 1A). The deposition was performed at room temper- ature in a solution containing 0.1 mM HAuCl 4 , 1 mM KCl, 1 mM H 2 SO 4 , and 0.1 M K 2 SO 4 with a Si electrode that was prepolarized at –1.9 V versus Ag/AgCl before inserting it in the solution (Fig. 1B). Prepolarizing the electrode inhibited the formation of an amorphous native oxide layer on the surface of Si and enabled epitaxial growth of Au. After the epitaxial growth of Au, lateral under- growth of a sacrificial SiO x layer was achieved by photoelectrochemically oxidizing Si under ir- radiation of light at +0.75 V versus Ag/AgCl in 0.5 M H 2 SO 4 solution (Fig. 1C). Au/n-Si forms a Schottky (rectifying) junction, so irradiation of light is required to generate holes at the Au/n-Si interface to oxidize the Si. The epitaxy of the Au foil was maintained during the lateral undergrowth of SiO x , analogous to the epitaxial lateral over- growth process in silicon-on-insulator technology (21). A polymer adhesive (tape or hot glue) was applied to the Au surface as a support and fa- cilitator for foil separation (Fig. 1D). The SiO x layer was etched using dilute (5%) hydrofluoric acid to detach the Au foil from the Si substrate (Fig. 1E); this enabled an effortless separation of the foil (Fig. 1F). Both the tape and hot glue have high optical transmittance in the visible range (400 to 800 nm) and are resistant to chemical etching procedures. After the foil separation, the Si substrate was etched using a 0.6 M KI and 0.1 M I 2 solution to dissolve any residual Au and then reused. Because only a 2- to 3-nm-thick layer of SiO x is removed during each fabrication cycle without extensive roughening, the Si should be reusable thousands of times. High-resolution transmission electron micros- copy (HRTEM) was used to study the interfacial changes during photoelectrochemical oxidation of Si. The as-deposited 30-min film (i.e., 30 min of deposition time) did not show an interfacial SiO x layer between the Si(111) and Au(111), as seen by the abrupt transition at the interface in Fig. 2A. Electron diffraction patterns for the as- deposited layers of Au on Si showed a spot pattern with in-plane and out-of-plane order (fig. S1). After the photoelectrochemical oxidation of Si, an amorphous SiO x interface with a thickness of 2.45 nm was evident in Fig. 2B. Native oxide formation on the Si surface in ambient air is limited to 0.5 to 1 nm because of the dense pinhole- free oxide layer that protects the Si from further oxidation. However, during photoelectrochemical oxidation of Si, tunneling of electrons through the insulating oxide layer caused the SiO x layer to grow thicker. The tunneling of electrons com- pletely ceased when the oxide layer reached a thickness of 2.0 to 2.5 nm and the photocurrent dropped nearly to zero (fig. S2). The electron dif- fraction pattern of Au on Si with an interfacial oxide layer showed a similar spot pattern, in- dicating that the epitaxy was maintained after the lateral undergrowth of SiO x (fig. S3). Both micrographs (Fig. 2, A and B) are viewed along the ½11 2zone axis, and the measured d-spacings for Au and Si are consistent with bulk values. Scanning electron microscopy (SEM) was per- formed to study the surface morphology and the initial nucleation and growth of Au foils. All of the foils for SEM were separated with an adhesive tape as the support layer. Figure 2C shows a segment of Au foil formed after 5 min with a fractal morphology that had been reported for evaporated ultrathin films of Au (22, 23). We attribute these fractal features to the diffusion- limited aggregation of Au on the surface of Si during the initial nucleation period. This Au foil has a coverage of 56% and appears to be at the percolation threshold thickness (5 to 6 nm), in RESEARCH Mahenderkar et al., Science 355, 1203–1206 (2017) 17 March 2017 1 of 4 1 Department of Materials Science and Engineering and Graduate Center for Materials Research, Missouri University of Science and Technology, Rolla, MO 65409, USA. 2 Department of Chemistry and Graduate Center for Materials Research, Missouri University of Science and Technology, Rolla, MO 65409, USA. 3 School of Engineering, Brown University, Providence, RI 02912, USA. *Corresponding author. Email: [email protected]on October 10, 2020 http://science.sciencemag.org/ Downloaded from
Transcript
REPORT◥
NANOMATERIALS
Epitaxial lift-off of electrodepositedsingle-crystal gold foilsfor flexible electronicsNaveen K. Mahenderkar,1 Qingzhi Chen,2 Ying-Chau Liu,2 Alexander R. Duchild,2
Seth Hofheins,2 Eric Chason,3 Jay A. Switzer2*
We introduce a simple and inexpensive procedure for epitaxial lift-off of wafer-size flexible andtransparent foils of single-crystal gold using silicon as a template. Lateral electrochemicalundergrowth of a sacrificial SiOx layer was achieved by photoelectrochemically oxidizingsilicon under light irradiation. A 28-nanometer-thick gold foil with a sheet resistance of 7 ohmsper square showed only a 4% increase in resistance after 4000 bending cycles. A flexibleorganic light-emitting diode based on tris(bipyridyl)ruthenium(II) that was spin-coated on afoil exploited the transmittance and flexibility of the gold foil. Cuprous oxide as an inorganicsemiconductor that was epitaxially electrodeposited onto the gold foils exhibited a diodequality factor n of 1.6 (where n = 1.0 for an ideal diode), compared with a value of 3.1 for apolycrystalline deposit. Zinc oxide nanowires electrodeposited epitaxially on a gold foil alsoshowed flexibility, with the nanowires intact up to 500 bending cycles.
Single-crystal silicon (Si) is the bedrock ofsemiconductor devices; its high crystal-line perfection minimizes electron-holerecombination, and its dense SiOx nativeoxide minimizes surface states. There is
interest in moving beyond the planar structureof conventional Si-based chips to produce flex-ible electronic devices such as wearable solarcells, sensors, and flexible displays (1–5). Flexibledevices and beautiful architectures have beenproduced using ultrathin foils of Si (6–8). Con-ductive polymers (9), carbon nanotubes (10),graphene (11), and metal nanostructures (12, 13)have been used as transparent and flexible sub-strates for flexible electronics. Ultrathin (5 to30 nm) metal films (14, 15) have relatively highoptical transmittance, flexibility, improved deviceefficiency, and low sheet resistance. However,they usually are grown by vacuum evaporationor sputtering, which gives a polycrystalline ortextured deposit. Polycrystalline electronic mate-rials suffer from electron-hole recombinationat grain boundaries (16). To expand the paletteof electronic materials beyond planar Si, an in-expensive source of highly ordered material isneeded that can serve as an inert substrate for theepitaxial growth of grain boundary–free semicon-ductors, optical materials, and superconductors.We show that wafer-size transparent and
flexible single-crystal foils of gold (Au) can be
produced by a simple and inexpensive lift-offprocedure using single-crystal Si as the templatefor electrochemical epitaxial growth. The trans-parency of these single-crystal Au foils is exploitedto fabricate a flexible organic light-emitting diode(OLED) based on tris(bipyridyl)ruthenium(II). Thesingle-crystal nature of the Au is used to producean inorganic diode based on epitaxial cuprousoxide (Cu2O) that has more ideal diode char-acteristics than a diode based on polycrystal-line Cu2O.Epitaxial lift-off of films on single-crystal sub-
strates by dissolving a sacrificial adhesion layercan produce free-standing single-crystal foils(17). The epitaxial lift-off procedure for ultra-thin single-crystal foils of Au electrodepositedonto Si(111) substrate is shown in Fig. 1. Electro-deposition of epitaxial Au on a Si(111) substratewas carried out using the method developed byAllongue and co-workers (18, 19). Previously, weshowed that epitaxial electrodeposition of Auon Si(111), Si(100), and Si(110) substrates canserve as a proxy for bulk single-crystal Au (20).Single-crystal Si(111) with a 0.2° miscut toward½112� and a resistivity of 1.15 ohm·cm was usedas the substrate to grow the Au foils (Fig. 1A).The deposition was performed at room temper-ature in a solution containing 0.1 mM HAuCl4,1 mM KCl, 1 mM H2SO4, and 0.1 M K2SO4 witha Si electrode that was prepolarized at –1.9 Vversus Ag/AgCl before inserting it in the solution(Fig. 1B). Prepolarizing the electrode inhibited theformation of an amorphous native oxide layer onthe surface of Si and enabled epitaxial growth of Au.After the epitaxial growth of Au, lateral under-
growth of a sacrificial SiOx layer was achieved byphotoelectrochemically oxidizing Si under ir-
radiation of light at +0.75 V versus Ag/AgCl in0.5 MH2SO4 solution (Fig. 1C). Au/n-Si forms aSchottky (rectifying) junction, so irradiation oflight is required to generate holes at the Au/n-Siinterface to oxidize the Si. The epitaxy of the Aufoilwasmaintainedduring the lateral undergrowthof SiOx, analogous to the epitaxial lateral over-growth process in silicon-on-insulator technology(21). A polymer adhesive (tape or hot glue) wasapplied to the Au surface as a support and fa-cilitator for foil separation (Fig. 1D). The SiOx
layer was etched using dilute (5%) hydrofluoricacid to detach the Au foil from the Si substrate(Fig. 1E); this enabled an effortless separationof the foil (Fig. 1F). Both the tape and hot gluehave high optical transmittance in the visible range(400 to 800 nm) and are resistant to chemicaletching procedures. After the foil separation,the Si substrate was etched using a 0.6 M KI and0.1 M I2 solution to dissolve any residual Au andthen reused. Because only a 2- to 3-nm-thick layerof SiOx is removed during each fabrication cyclewithout extensive roughening, the Si should bereusable thousands of times.High-resolution transmission electron micros-
copy (HRTEM) was used to study the interfacialchanges during photoelectrochemical oxidationof Si. The as-deposited 30-min film (i.e., 30 minof deposition time) did not show an interfacialSiOx layer between the Si(111) and Au(111), asseen by the abrupt transition at the interface inFig. 2A. Electron diffraction patterns for the as-deposited layers of Au on Si showed a spot patternwith in-plane and out-of-plane order (fig. S1).After the photoelectrochemical oxidation of Si,an amorphous SiOx interface with a thicknessof 2.45 nm was evident in Fig. 2B. Native oxideformation on the Si surface in ambient air islimited to 0.5 to 1 nmbecause of thedense pinhole-free oxide layer that protects the Si from furtheroxidation. However, during photoelectrochemicaloxidation of Si, tunneling of electrons throughthe insulating oxide layer caused the SiOx layerto grow thicker. The tunneling of electrons com-pletely ceased when the oxide layer reached athickness of 2.0 to 2.5 nm and the photocurrentdropped nearly to zero (fig. S2). The electron dif-fraction pattern of Au on Si with an interfacialoxide layer showed a similar spot pattern, in-dicating that the epitaxy was maintained afterthe lateral undergrowth of SiOx (fig. S3). Bothmicrographs (Fig. 2, A and B) are viewed alongthe ½112� zone axis, and the measured d-spacingsfor Au and Si are consistent with bulk values.Scanning electron microscopy (SEM) was per-
formed to study the surface morphology and theinitial nucleation and growth of Au foils. All ofthe foils for SEMwere separatedwith an adhesivetape as the support layer. Figure 2C shows asegment of Au foil formed after 5 min with afractal morphology that had been reported forevaporated ultrathin films of Au (22, 23). Weattribute these fractal features to the diffusion-limited aggregation of Au on the surface of Siduring the initial nucleation period. This Au foilhas a coverage of 56% and appears to be at thepercolation threshold thickness (5 to 6 nm), in
RESEARCH
Mahenderkar et al., Science 355, 1203–1206 (2017) 17 March 2017 1 of 4
1Department of Materials Science and Engineering andGraduate Center for Materials Research, Missouri Universityof Science and Technology, Rolla, MO 65409, USA.2Department of Chemistry and Graduate Center for MaterialsResearch, Missouri University of Science and Technology,Rolla, MO 65409, USA. 3School of Engineering, BrownUniversity, Providence, RI 02912, USA.*Corresponding author. Email: [email protected]
agreement with the results of Hövel et al. (24).Beyond the percolation threshold, the fractal fea-tures are interconnected in a large networkmeshwith electrical connectivity across the entire foil.Figure 2D shows Au foils deposited for 10 minwith 70%coverage;Au foils deposited for 20minand30min, respectively, are shown in Fig. 2, E and F.The applicability of Au foils as a substrate was
shownby depositing and characterizing inorganicsemiconductor thin film and nanowires. Cu2Owas electrodeposited on a 30-min Au foil (Fig.2G) for study of the diode characteristics. Zincoxide (ZnO) nanowires were electrodeposited ona 10-min Au foil (Fig. 2H) and were subjected to500 bending cycles with a radius of curvature of3mm. The ZnO remained intact after the bendingcycles, which can be attributed to its nanowiremorphology and the flexibility of Au foil. Althoughthe 5- and 10-min Au foils appear porous, foilsbeyond the percolation threshold thickness (5 to6 nm) enable electrical contact over the entiresample area andmake electrodeposition feasible.We probed epitaxial Au on Si and free-standing
Au foils using x-rays to study the out-of-plane andin-plane orientations. Figure 3A shows the x-raydiffraction (XRD) pattern of a 30-min Au deposi-tion on Si, where Au follows the [111] orienta-tion of Si, indicating a high out-of-plane order.
The Au thickness on Si was measured from theinterference fringes in the XRD pattern (Fig.3B). Satellite peaks (Laue oscillations) around aBragg peak caused by constructive and destruc-tive interference of x-rays reflected from Si-Auand Au-air interfaces were used to precisely mea-sure the Au thickness (19). Figure 3B shows Laueoscillations around the Au(111) peak for Au filmson Si as a function of deposition time. The filmthickness was determined from the satellite peakpositions according to
t ¼ ðL1 − L2Þl2ðsin q1 − sin q2Þ
� �ð1Þ
where t is the film thickness, L is the satellite peakorder (numbering of peaks in Fig. 3B), l is thex-raywavelength (0.15418nm), and q is the satellitepeak angle. The calculated thicknesses from theLaue oscillations for the corresponding depositiontimes are listed in Fig. 3B. A linear dependence ofthickness with time was observed (fig. S4). Figure3C shows the XRD pattern of a segment of Au foiland electrodeposited Cu2O and ZnO on Au foil.Both the Au foil and the electrodeposited Cu2Ohad a strong [111] out-of-plane orientation. Thethickness of a 10-min deposition of Au on Siagreed closely with that of Au foil measured using
Laue oscillations (fig. S5). The ZnO also grew epi-taxially on Au foil but showed a strong [0001] out-of-plane orientation because of its hexagonal crystalstructure.The in-plane orientation and the epitaxial rela-
tion of the Au on Si, Au foil, Cu2O, and ZnOwasdetermined with x-ray pole figures. In a polefigure, planes other than those parallel to the sub-strate surface are probed while tilting and rotatingthe sample through a series of tilt and azimuthalangles (fig. S6). Figure 3D shows a (220) pole figureof Si(111), with three spots separated azimuthallyby 120° at a tilt angle of 35.5°, corresponding tothe three-fold symmetry of the (111) plane. A (220)pole figure of Au(111) on Si(111) is shown in fig. S7with three spots expected at a tilt angle of 35.5°;however, there are also an additional three spotsseparated azimuthally by 60°. The two sets of spotson the (220) pole figure of Au correspond to the180° in-plane rotation of parallel and antiparalleldomains. Figure 3E shows a (220) pole figure ofAu(111) foil, with the six spots at a tilt angle of35.5° separated azimuthally by 60°. This patternshows that the Au maintained its high in-planeand out-of-plane order after the foil separation.Figure 3F shows a (220) pole figure of Cu2O(111)electrodeposited on Au(111) foil with the expectedsix spots at a tilt angle of 35.5°. Figure 3G shows a
Mahenderkar et al., Science 355, 1203–1206 (2017) 17 March 2017 2 of 4
Fig. 1. Schematic for epitax-ial lift-off of single-crystalAu foil. (A) Miscut n-typeSi(111) wafer without thenative oxide layer. (B) Epitaxialelectrodeposition of Au onSi(111) from a 0.1 mM HAuCl4solution at –1.9 V versusAg/AgCl with prepolarizedelectrode. (C) Photoelectro-chemical oxidation of Si underirradiation of light in 0.5 MH2SO4 solution at 0.75 Vversus Ag/AgCl. (D) A polymeradhesive (tape/hot glue) isapplied to the surface of Au to aid the foil separation. (E) A sacrificial SiOx interlayer is etched using dilute (5%) hydrofluoric acid to separate the foil from the Sisubstrate. (F) Single-crystal Au foil completely detached from the Si surface.
Fig. 2. Electron micros-copy of the single-crystalAu, epitaxial Cu2O, andepitaxial ZnO. (A and B)High-resolution TEM crosssection of epitaxial Auon Si without the SiOx
interlayer for the as-deposited film (A) andwith the SiOx layer afterphotoelectrochemicaloxidation of Si (B). (C toF) Surface morphology ofAu foils deposited for5 min (7 nm) (C), 10 min(11 nm) (D), 20 min(21 nm) (E), and 30 min(28 nm) (F). (G) Electro-deposited epitaxial Cu2O on30-min Au foil. (H) Electrodeposited ZnO nanowires on 10-min Au foil subjected to 500 bending cycles.
(102) pole figure of ZnO(002) with six spots at atilt angle of 42.77° separated azimuthally by 60°.The six spots for ZnO are expected from the six-fold symmetry of the (0002) basal plane of thehexagonal crystal structure.To understand the epitaxial relation, it is crucial
to look at the lattice mismatch of the film and thesubstrate. The lattice mismatch can be defined as[(dfilm – dsubstrate)/dsubstrate], where d is the lattice
spacing of the plane parallel to the substrate. Auon Si has a lattice mismatch of –24.9%, which istoo high to produce epitaxial deposits. Therefore,the epitaxy in the Au-Si system can be explainedby the formationof coincidence site lattices (CSLs),in which four unit meshes of Au coincide withthree unit meshes of Si (20). These CSLs lower themismatch from –24.9% for a single unit cell to+0.13% for the CSL (fig. S8). Similarly, the lattice
mismatch for ZnO on Au was minimized from+12.7% to +0.16% for the CSL (fig. S9). The latticemismatch for Cu2O on Au is +4.7%, which is lowenough to produce cube-on-cube epitaxial filmswith reasonable in-plane and out-of-plane strainin the material (fig. S10).High optical transmittance and low sheet re-
sistance are imperative for Au foils to be used asflexible and transparent substrates. Figure 4Ashows the photograph of a wafer-size Au foil witha diameter of 50.8 mm. Figure 4B shows the op-tical transmittance of Au foils as a function ofthickness. All of the foils showed a maximum intransmittance around 500 nm and the peaksslightly red-shifted with an increase in thickness.The sheet resistance, in terms of ohms per square,for all of the foils (measured with a four-pointprobe) increased along with transmittance asthe Au foil thickness decreased. A 7-nm-thick Aufoil showed the highest transmittance at 85%,and the 28-nm-thick foil showed the lowest at25%. The maximum transmittance (~500 nm) asa function of thickness is in close agreement withprevious studies on evaporated gold thin films (25).The endurance of the Au foils as a function ofsheet resistance was measured by subjectingthe foils to as many as 4000 bending cycles (Fig.4C). Bending cycles for all of the foils were per-formed with a steel rod as a guide with a radiusof curvature of 3 mm. The sheet resistance of 28-,16-, and 11-nm-thick Au foils increased by 4%,6.3%, and 34%, respectively, after 4000 cycles ofbending.To evaluate the flexibility and transmittance
of Au foils for light emission, we spin-coatedan OLED based on tris(bipyridyl)ruthenium(II)(26, 27). The complex showed strong photo-luminescence with an excitation wavelength of455 nm and an emission of bright red-orangecolor around 660 nm (26). The complex was dis-solved in a 3% (w/v) polyvinyl alcohol solution,spin-coated onto a 28-nm-thick Au foil, and driedin air. An indium/gallium (InGa) eutectic was used
Mahenderkar et al., Science 355, 1203–1206 (2017) 17 March 2017 3 of 4
Fig. 3. X-ray diffraction and pole figures to study the in-plane and out-of-plane orientation. (A) Out-of-plane orientation of electrodeposited Au(111) on Si(111). (B) Out-of-plane x-ray diffraction showingsatellite peaks (Laue oscillations) caused by constructive and destructive interference. (C) Out-of-planeorientation of Au(111) foil, electrodeposited Cu2O on 30-min Au foil, and electrodeposited ZnO on 10-minAu foil. (D to G) In-plane orientation was determined using (220) pole figure of Si(111) (D), (220) polefigure of Au(111) foil (E), (220) pole figure of Cu2O(111) on Au(111) foil (F), and (102) pole figure of ZnO(002)on Au(111) foil (G). The radial lines in the pole figure correspond to 30° increments of the tilt angle.
Fig. 4. Transmittance, sheet resist-ance, and flexibility of Au foils withdiode and OLED fabrication. (A)Wafer-size Au foil with diameter of50.8 mm and thickness of 28 nm.(B) Transmittance and sheetresistance of Au foils as a functionof thickness. (C) Sheet resistance ofAu foils with thicknesses of 11, 16,and 28 nm as a function of bendingcycles with a bending curvature of3 mm. (D) Current-voltage responseof Au foil/RuII(bpy)3/InGa junction,showing rectifying behavior. Inset:Red-orange electrogeneratedchemiluminescence of RuII(bpy)3BF4OLED on flexible 28-nm-thick Au foil.(E) Current-voltage response of Cu2Odiode on Au foil (epitaxial) and stain-less steel (polycrystalline) substrates.(F) Dark saturation current density(Js) and diode quality factor (n) ofepitaxial and polycrystalline Cu2O diodes measured using log(J) versus V at forward bias.
as a back contact as a low–work function metal,whereas the Au foil acted as a high–work func-tion contact. Figure 4D shows the current-voltageresponse of Au foil/RuII(bpy)3/InGa junction witha diode (rectifying) behavior. The inset in Fig. 4Dshows a flexible Au foil with electrogeneratedchemiluminescence from the OLED at an appliedforward bias of 6 V.To study the single-crystal nature of Au foils,
we prepared an inorganic diode by using electro-deposited Cu2O on Au foil. InGa eutectic wasused to make a rectifying contact to the p-Cu2O,and the Au foil substrate served as the ohmiccontact. Polycrystalline Cu2Owas electrodepositedona stainless steel substrate from the samedeposi-tion solution at low overpotentials to produce asample with a randomorientation. Cu2O on boththe Au foil and the stainless steel were depositedfor a constant charge density to maintain similarthickness. The XRD pattern of Cu2O on stainlesssteel with a polycrystalline powder pattern is shownin fig. S11. Defects or grain boundaries in amaterialincrease the probability of electron-hole recombi-nation and lower the overall efficiency of the diodeor solar cell. In a single crystal, an ideal diodequality factor (n) of 1 indicates diffusion-controlledcurrents with no electron-hole recombination inthe material, but in polycrystalline materials, nvaries from 2 to 7 (28, 29). The n value for poly-crystalline Si also increases with decreasing grainsize (29). Figure 4E shows current-voltage re-sponses for a Cu2O diode onAu foil and stainlesssteel. The epitaxial Cu2O had an n of 1.6, whereasthe polycrystalline Cu2O had an n of 3.1 (Fig. 4F).The higher value of n for polycrystalline Cu2O isconsistent with previous results for films of Cu/Cu2O Schottky diode solar cells (30).
Single-crystal Au foils offer the order of tradi-tional semiconductors such as Si wafers withoutthe constraint of a rigid substrate. The foils areflexible and optically transparent, and show prom-ise for producing flexible and wearable displays,solar cells, and sensors. The epitaxial growth ofCu2O and ZnO that we have demonstrated canbe applied to a wide range of inorganic semi-conductors such as CdSe, CdTe, and ZnSe foruse in flexible solar cells. Because ZnO is both awide-bandgap semiconductor and a piezoelectricmaterial, it should bepossible to produce pressure-sensitive “electronic skin” and LEDs based on theZnO/Au system (31, 32). Also, Au is hypoallergenicand could serve as a platform for wearable sweatsensors for continuous health monitoring (5). Al-though this work focused on the production ofordered substrates for flexible electronics, theprocessing method can be used to provide aninexpensive source of large metallic single crys-tals. These could serve as ordered substrates forphotovoltaics, high-temperature superconductors,stress-freemicroelectromechanical systems(MEMS),catalysts, underpotential deposition, self-assembledmonolayers, and molecular electronics.
REFERENCES AND NOTES
1. B. D. Gates, Science 323, 1566–1567 (2009).2. J. C. Kenry, J. C. Yeo, C. T. Lim,Microsyst. Nanoeng. 2, 16043 (2016).3. D. Akinwande, N. Petrone, J. Hone, Nat. Commun. 5, 5678 (2014).4. J. A. Rogers, T. Someya, Y. Huang, Science 327, 1603–1607 (2010).5. W. Gao et al., Nature 529, 509–514 (2016).6. D. Shahrjerdi, S. W. Bedell, Nano Lett. 13, 315–320 (2013).7. J. Yoon et al., Nat. Mater. 7, 907–915 (2008).8. S. Xu et al., Science 347, 154–159 (2015).9. T. Cheng, Y.-Z. Zhang, J.-D. Zhang, W.-Y. Lai, W. Huang,
J. Mater. Chem. A 4, 10493–10499 (2016).10. D. M. Sun et al., Nat. Nanotechnol. 6, 156–161 (2011).11. S. Bae et al., Nat. Nanotechnol. 5, 574–578 (2010).
12. W.-K. Kim et al., Sci. Rep. 5, 10715 (2015).13. J. H. M. Maurer, L. González-García, B. Reiser, I. Kanelidis,
T. Kraus, Nano Lett. 16, 2921–2925 (2016).14. Y.-G. Bi et al., Nanoscale 8, 10010–10015 (2016).15. B. O’Connor, C. Haughn, K.-H. An, K. P. Pipe, M. Shtein, Appl.
Phys. Lett. 93, 223304 (2008).16. J. A. Koza, J. C. Hill, A. C. Demster, J. A. Switzer, Chem. Mater.
28, 399–405 (2016).17. J. W. Shin, A. Standley, E. Chason, Appl. Phys. Lett. 90, 261909
(2007).18. S. Warren et al., Surf. Sci. 603, 1212–1220 (2009).19. P. Prod’homme, F. Maroun, R. Cortès, P. Allongue, Appl. Phys.
Lett. 93, 171901 (2008).20. J. A. Switzer, J. C. Hill, N. K. Mahenderkar, Y.-C. Liu, ACS Appl.
Mater. Interfaces 8, 15828–15837 (2016).21. L. Jastrzebski, J. Cryst. Growth 63, 493–526 (1983).22. G. B. Smith, G. A. Niklasson, J. S. E. M. Svensson,
C. G. Granqvist, J. Appl. Phys. 59, 571–581 (1986).23. D. N. Jarrett, L. Ward, J. Phys. D 9, 1515–1527 (1976).24. M. Hövel, B. Gompf, M. Dressel, Phys. Rev. B 81, 035402 (2010).25. O. S. Heavens, Rep. Prog. Phys. 23, 1 (1960).26. F. G. Gao, A. J. Bard, J. Am. Chem. Soc. 122, 7426–7427 (2000).27. J. A. Broomhead, C. G. Young, P. Hood, Inorg. Synth. 28,
338–340 (2007).28. K. Sen, B. P. Tyagi, J. Appl. Phys. 56, 1240–1241 (1984).29. S. Kumari, N. K. Arora, G. C. Jain, Sol. Energy Mater. 5,
383–390 (1981).30. R. J. Iwanowski, D. Trivich, Solar Cells 13, 253–264 (1985).31. W. Wu, X. Wen, Z. L. Wang, Science 340, 952–957 (2013).32. C. Wang et al., Nano Energy 14, 364–371 (2015).
ACKNOWLEDGMENTS
The material is based on work supported by the U.S.Department of Energy, Office of Basic Sciences, Divisionof Materials Sciences and Engineering, under grantsDE-FG02-08ER46518 (J.A.S.) and DE-SC0008799 (E.C.).All data are presented in the main paper and supplement.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/355/6330/1203/suppl/DC1Materials and MethodsFigs. S1 to S11References (33–36)
12 December 2016; accepted 13 February 201710.1126/science.aam5830
Mahenderkar et al., Science 355, 1203–1206 (2017) 17 March 2017 4 of 4
Epitaxial lift-off of electrodeposited single-crystal gold foils for flexible electronicsNaveen K. Mahenderkar, Qingzhi Chen, Ying-Chau Liu, Alexander R. Duchild, Seth Hofheins, Eric Chason and Jay A. Switzer
, this issue p. 1203Sciencecycles. Flexible films of single-crystal cuprous oxide and of zinc oxide nanowires were then grown on the gold foils.adhesive tape. A 28-nm-thick gold foil showed a minimal increase in sheet electrical resistance after 4000 bending
withphotoelectrochemistry to undergrow a sacrificial silicon dioxide layer. This layer allowed the gold film to be peeled off grew gold films on the face of a silicon wafer and then usedet al.substrate for devices. Mahenderkar
A method for growing and removing single-crystal gold films can be used to create a flexible and transparentLifting off gold films
Terms of ServiceUse of this article is subject to the
is a registered trademark of AAAS.ScienceScience, 1200 New York Avenue NW, Washington, DC 20005. The title (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience
