REPORT◥

2D MATERIALS

A liquid metal reaction environmentfor the room-temperature synthesisof atomically thin metal oxidesAli Zavabeti,1 Jian Zhen Ou,1 Benjamin J. Carey,1 Nitu Syed,1 Rebecca Orrell-Trigg,1

Edwin L. H. Mayes,2 Chenglong Xu,1 Omid Kavehei,1 Anthony P. O’Mullane,3

Richard B. Kaner,4 Kourosh Kalantar-zadeh,1* Torben Daeneke1*

Two-dimensional (2D) oxides have a wide variety of applications in electronics and othertechnologies. However, many oxides are not easy to synthesize as 2D materials throughconventional methods. We used nontoxic eutectic gallium-based alloys as a reactionsolvent and co-alloyed desired metals into the melt. On the basis of thermodynamicconsiderations, we predicted the composition of the self-limiting interfacial oxide.We isolatedthe surface oxide as a 2D layer, either on substrates or in suspension.This enabled us toproduce extremely thin subnanometer layers of HfO2, Al2O3, andGd2O3.The liquidmetal–basedreaction route can be used to create 2D materials that were previously inaccessible withpreexisting methods.The work introduces room-temperature liquid metals as a reactionenvironment for the synthesis of oxide nanomaterials with low dimensionality.

Room-temperature liquid metals have anumber of interesting surface and bulkproperties that make them intriguing fora variety of engineering applications, in-cluding flexible electronics andmicrofluidics

(1). Gallium-based eutectic alloys such as EGaIn(containing gallium and indium) and galinstan

(containing gallium, indium, and tin) are liquidat room temperature, are nontoxic, and featuremetallic bonds.Unlikemolecular and ionic liquids,liquid metals are rarely used as reaction sol-vents. Here, we used eutectic galliummelts as areaction environment to create a variety of low-dimensional metal oxides.

Most metals, including gallium-based alloys(2, 3), feature a self-limiting thin oxide layer underambient conditions at the metal-air interface(4). This atomically thin interfacial oxide is con-sidered a naturally occurring two-dimensional(2D)material. The concept of using a liquidmetalas a reaction environment is based on the obser-vation that the self-limiting oxide layers of gal-lium alloys, such as EGaIn and galinstan, areexclusively composed of gallium oxide, despiteindium content of 22 to 25 weight percent (wt %)and tin content of as much as 10 wt % in thesealloys (2, 3). Co-alloying suitable reactive metalsallows us to form the co-alloyed metal oxides atthemetal-air interface. We identified suitable co-alloyed metals on the basis of thermodynamicconsiderations, which require that the compo-sition of the surface oxide is determined by thereactivity of the individual metals within themelt. Thus, the oxide resulting in the greatestreduction of Gibbs free energy (DGf) will domi-nate the surface (Fig. 1A). If room-temperatureliquid gallium alloys are the solvent, all lantha-nide oxides and a sizable portion of the transi-tion metal and post–transition metal oxidesshould be accessible as 2D nanostructures (tableS1) (5, 6).

RESEARCH

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1School of Engineering, RMIT University, Melbourne, Victoria3001, Australia. 2School of Applied Sciences, RMITUniversity, Melbourne, Victoria 3001, Australia. 3School ofChemistry, Physics and Mechanical Engineering, QueenslandUniversity of Technology, Brisbane, Queensland 4001,Australia. 4Department of Chemistry and Biochemistry,Department of Materials Science and Engineering, andCalifornia NanoSystems Institute, University of California,Los Angeles, CA 90095, USA.*Corresponding author. Email: torben.daeneke@rmit.edu.au (T.D.);kourosh.kalantar-zadeh@rmit.edu.au (K.K.-z.)

Fig. 1. Fundamental principles and syntheticapproach. (A) Gibbs free energy of formationfor selected metal oxides (5, 6). Oxides tothe right of the red dashed line are expected todominate the interface. (B) A cross-sectionaldiagram of a liquid metal droplet, with possiblecrystal structures of thin layers of HfO2, Al2O3,and Gd2O3 as indicated. (C) Schematicrepresentation of the van der Waals exfoliationtechnique. The pristine liquid metal droplet isfirst exposed to an oxygen-containingenvironment. Touching the liquid metal with asuitable substrate allows transfer of theinterfacial oxide layer. An optical image is shownat the right. (D) Schematic representation ofthe gas injection method (left), photographs ofthe bubble bursting through the liquid metal(center), and an optical image of the resultingsheets drop-cast onto a SiO2/Si wafer (right). Seemovie S1.

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We synthesized three different 2D oxides usinggalinstan as a reaction environment. To show theversatility of the method, we selected oxides of atransition metal (HfO2), a post–transition metal(Al2O3), and a rare earth metal (Gd2O3) with highmelting points. Of these oxides, hafnium oxideis particularly important because of its use as aninsulating oxide in the electronics industry. Forgalinstan, we expected gallium oxide to form onthe metal surface because it features the mostnegative DGf of the possible oxides (3, 5). Wealloyed ~1 wt % of elemental hafnium, aluminum,or gadolinium into the liquid metal, which served

as the precursors for the formation of their re-spective oxides (fig. S1) (7). These oxides fulfilledthe requirement of featuring a lower DGf thangallium oxide, and as a result we expected themto form surface oxides on their respective alloys(Fig. 1, A and B) (5). All three oxides that wesynthesized as 2D materials have nonstratifiedcrystal structures and cannot be obtained usingstandard exfoliation techniques (8, 9). Nonetheless,either the investigated oxides or closely relatedstructures have been predicted to be stable as 2Dmonolayers (10–12). We prepared alloys containing~1 wt % copper and silver for control experiments.

We developed two synthesis approaches forthe isolation of the surface oxide. The first methodwas a van der Waals (vdW) exfoliation techniquesimilar to the method for obtaining monolayergraphene pioneered by Novoselov et al. (Fig. 1C)(13). The technique entails touching the liquidmetal droplet with a solid substrate. The liquidnature of the parent metal results in the absenceof macroscopic forces between the metal andits oxide skin, allowing clean delamination (14).This technique is suitable for the productionof high-quality thin oxide sheets on substrates.The second technique relies on the injection of

Zavabeti et al., Science 358, 332–335 (2017) 20 October 2017 2 of 4

Fig. 2. Characterization of materials derived from the exfoliation method.Left: AFM images, with thickness profile (inset) determined at the red line asindicated. Center: TEM characterization, with selected-area electron diffraction(SAED) (top inset) and HRTEM images (bottom inset; scale bar, 0.5 nm). Right:Elemental composition determined by XPS (24–29). (A) Results obtained froma eutectic gallium-indium-tin alloy. (B to D) Alloys containing approximately 1%of added hafnium, aluminum, and gadolinium, respectively. See (7) for char-

acterization of the alloys before use.The lattice parameters in SAED and HRTEMimages were indexed using literature reports (15–17).The lack of crystallinity inthegalliumoxide samplemight bebeam-induced.See (7) forXPSspectra used todetermine the oxide composition.The sample derived from pure galinstanfeaturesmetallic inclusions,which are visible as dark dots and elongated nodules.The other materials feature no inclusions.The lateral dimensions of the 2Dsheets are extraordinarily large and exceed the AFM field of view.

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pressurized air into the liquid metal (Fig. 1D andmovie S1) (7). We expected the metal oxide to formrapidly on the inside of air bubbles as they rosethrough the liquid metal. The released air bubblespassed through a layer of deionized water that wehad placed above the liquid metal, allowing theproduced oxide sheets to be dispersed into an

aqueous suspension. This technique is highly scal-able and is therefore suitable for creating high-yield suspensions of the target oxide nanosheets.To directly compare the different oxide nano-

sheets, we used the exfoliation method on allthe alloys we investigated. The optical imagesshow the large lateral dimensions of the pro-

ducts (Fig. 1, C and D). To further characterizethe nanosheets, we used atomic force microscopy(AFM) on samples deposited on top of SiO2/Siwafers and transmission electron microscopy(TEM) on samples directly deposited onto TEMgrids (Fig. 2). Materials produced by the exfo-liation method featured a smooth appearanceunder AFM imaging. Each 2D oxide sheet weproduced had a different thickness: 2.8 nm forgallium oxide, 0.6 nm for hafnium oxide, 1.1 nmfor aluminum oxide, and 0.5 nm for gadoliniumoxide. The ultrathin nature of the oxides is appar-ent from the translucent appearance and occa-sional wrinkles in our TEM images (Fig. 2).We found that the nanosheets derived from

unalloyed galinstan were amorphous when charac-terized by electron diffraction and high-resolutionTEM (HRTEM). By contrast, the 2D oxides pro-duced from alloys containing hafnium, aluminum,and gadolinium were polycrystalline, with charac-teristic lattice parameters of m-HfO2, a-Al2O3,and cubic Gd2O3 (Fig. 2) (15–17). Using x-rayphotoemission spectroscopy (XPS), we found thatthe exfoliated oxide of unalloyed galinstan is exclu-sively composed of gallium oxides (mostly Ga2O3

with a small Ga2O content) (2, 3). The 2Dmaterialsderived from our alloys were composed entirelyof the oxide of the added element (Fig. 2 and fig.S2) (7). Our control experiments with copper andsilver alloys resulted in nanosheets predominantlycomposed of gallium oxide (fig. S3) (7). The resultsconfirm our hypothesis that the oxide with thelowest DGf dominates the interfacial surface oxide.Galinstan and hafnium-containing galinstan

were investigated using the gas injection syn-thesis method. The aluminum- and gadolinium-containing alloys visibly reacted with water andwere not investigated further. Replacing waterwith an inert solvent was expected to enable thesynthesis of aluminum and gadolinium oxide

Zavabeti et al., Science 358, 332–335 (2017) 20 October 2017 3 of 4

Fig. 4. Characterization of HfO2 as a dielectric. (A) Schematic of the peak force tunneling AFM(PF-TUNA) setup. (B) AFM height (top) and current (bottom) maps for the edge region of anHfO2 sample directly deposited onto a Pt-coated wafer by the exfoliation method. Both maps are300 nm wide. The profiles at the right correspond to the regions indicated by the red lines. The HfO2

and Pt sides are labeled. (C) Current-voltage curve measured through the HfO2 layer. Inset: Fit to theSchottky emission model and the determined dielectric constant (7). (D) Plot of the low-loss EELSspectrum, which provides an estimate of the band gap. Inset: Analysis of the nature of the band gap,indicating a direct gap (7).

Fig. 3. Characterization of materials derived from the gas injectionmethod. Left: AFM images, with thickness profile (inset) determined atthe red line as indicated. Center: TEM characterization, with SAED (topinset) and HRTEM images (bottom inset; scale bar, 0.5 nm). Right: Raman

spectra of the resulting oxides. (A) Results obtained from a eutecticgallium-indium-tin alloy. (B) Alloy containing approximately 1% of addedhafnium. The Raman spectra match well with literature reports for Ga2O3

(A) (30) and HfO2 (B) (15).

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with this gas injection method. The materialswederived fromthe gas injection synthesismethodwere similar in appearance to the oxides we pro-duced with the exfoliation method (Fig. 3). Thematerial synthesized using unalloyed galinstanhad a thickness of ~5.2 nm, about double that ofthe exfoliation method. We found that the HfO2

nanosheets had a thickness of 0.5 nm, which issimilar to the thickness of exfoliated HfO2. Thegalliumoxide nanosheets contained small, spheri-cal metallic inclusions that we discovered duringTEM imaging. We confirmed the presence of~15-nm spherical inclusionswith high-resolutionAFM and TEM imaging (fig. S4) (7). The nano-sheets have low crystallinity, which we examinedusing HRTEM and electron diffraction measure-ments. This indicates that the gas injection syn-thesis may have a tendency to create amorphousoxides. The short reaction time, attributable to alimited residence time for the gas bubble withinthe liquid metal, may be the origin of the poorcrystallinity. The reaction time frame availablefor the exfoliationmethod is considerably longer,because the metal droplet may rest for severalminutes prior to the vdW transfer, allowing forthe reorganization of the crystal structure withinthe interfacial oxide layer.Using Raman spectroscopy, we found that

the nanosheets derived from the gas injectionmethod are also predominantly composed of theoxide with the lowest DGf (Fig. 3). The successfulgas injection synthesis of both gallium and haf-nium oxides demonstrates the suitability of theapproach for the high-throughput synthesis of2D oxides for applications where high crystal-linity is not a primary concern. The 2D materialswe synthesized hold promise for applications inenergy storage, such as supercapacitors and bat-teries that require large quantities of materialswith high ratios of surface area to volume (18).One major application of the materials we

developed is their use as ultrathin insulator di-electrics for the fabrication of field-effect tran-sistors. HfO2 is the material of choice for theelectronics industry, with a relative dielectricconstant of >20 and a band gap of >5 eV (19).Because of the lack of a stratified crystal struc-ture and the resulting absence of amenable ex-foliation techniques, HfO2-based dielectrics aretraditionally deposited using atomic layer depo-sition or other chemical and physical vapor-phasedeposition techniques (19). These techniques rely

on an island-mediated film growth mechanismthat results in a minimum film thickness of 3 to5 nm (20). Our liquid metal–derived HfO2 had aminimum thickness of approximately 0.5 nm.We directly deposited 2D HfO2 onto a wafer

coated with Pt to evaluate our exfoliation meth-odology for the synthesis of dielectrics. Usingconductive AFM characterization, we found thatthe deposited 2D HfO2 sheet is completely in-sulating and pinhole-free, despite being only~0.5 to 0.6 nm thick (Fig. 4, A and B) (7). Thebreakdown electric field, defined as the fieldat which the current rises above the noise level,is ~5.3 GV cm−1, as determined by current-voltage (I-V ) characterization of the 2D nano-sheet. This value is three orders of magnitudehigher than the breakthrough field for chem-ical vapor deposition (CVD)–grown multilayerh-BN (21, 22), highlighting the excellent qualityof the dielectric. Fitting the I-V curve to theSchottky emission model allowed us to determinea dielectric constant of ~39 for the oxide sheets(7). We analyzed the low-loss region of the elec-tron energy loss spectrum (EELS) of exfoliatedultrathin HfO2 sheets and found that the bandgap of the material is of a direct nature with a~6-eV gap (Fig. 4D) (7, 23).Our findings show that oxide layers on liquid

metals can be manipulated by selecting appro-priate alloying elements on the basis of the Gibbsfree energy for oxide formation. The twomethodswe used to recover the 2D nanosheets are bothscalable, do not require complex equipment, andprovide nanosheets either directly deposited ontosubstrates or as an aqueous suspension. Theliquid metal serves as a solvent during the re-action. Our methodology facilitated the isolationof atomically thin layers of metal oxides that donot naturally present themselves as stratified sys-tems, providing a synthetic pathway toward animportant class of 2Dmaterials thatwaspreviouslyinaccessible. The underlying principles suggestthat this should apply to a sizable fraction ofmetals,giving access to 2D crystals of many transitionmetal, post–transitionmetal, and rare earthmetaloxides. Several of these metal oxides are of excep-tional importance because of their various elec-tronic, magnetic, optical, and catalytic properties.

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ACKNOWLEDGMENTS

Supported by Australian Research Council Centre of ExcellenceFLEET (CE170100039). We thank RMIT University’s Microscopyand Microanalysis Facility, a linked laboratory of the AustralianMicroscopy and Microanalysis Research Facility, for scientific andtechnical assistance, and the RMIT MicroNano Research Facilityfor associated technical support. Additional data are availablein the supplementary materials. The authors declare no conflictof interest.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/358/6361/332/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S5Table S1Movie S1References (31–36)

19 July 2017; accepted 7 September 201710.1126/science.aao4249

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metal oxidesA liquid metal reaction environment for the room-temperature synthesis of atomically thin

Kavehei, Anthony P. O'Mullane, Richard B. Kaner, Kourosh Kalantar-zadeh and Torben DaenekeAli Zavabeti, Jian Zhen Ou, Benjamin J. Carey, Nitu Syed, Rebecca Orrell-Trigg, Edwin L. H. Mayes, Chenglong Xu, Omid

DOI: 10.1126/science.aao4249 (6361), 332-335.358Science 

, this issue p. 332Science2D oxides by using the same process.oxide that appears on the surface is the oxide with the lowest energy, suggesting that it should be possible to make otherappear as a surface layer in gallium-based liquid metals after the Hf, Gd, or Al is dissolved into the bulk alloy. The 2D

. The 2D sheets3O2, and Al3O2, Gd2, HfO3O2 exploited liquid metals to synthesize 2D Gaet al.crystals. Zavabeti certain compositions of 2D materials are difficult to obtain owing to the challenges in exfoliating thin sheets from bulk

Two-dimensional (2D) materials have a wide variety of potential applications in the electronics industry. However,Expanding the world of 2D materials

ARTICLE TOOLS http://science.sciencemag.org/content/358/6361/332

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/10/19/358.6361.332.DC1

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