Tunable ferroelectricity in hBN intercalated twisted double-layer graphene
Yibo Wang, Siqi Jiang, Jingkuan Xiao, Xiaofan Cai, Di Zhang, Ping Wang, Guodong Ma, Yaqing Han, Jiabei Huang, Kenji Watanabe, Takashi Taniguchi, Alexander S. Mayorov, Geliang Yu
TTunable ferroelectricity in hBN intercalated twisteddouble-layer graphene
Yibo Wang , Siqi Jiang , Jingkuan Xiao , Xiaofan Cai , Di Zhang , Ping Wang ,Guodong Ma , Yaqing Han , Jiabei Huang , Kenji Watanabe , Takashi Taniguchi ,Alexander S. Mayorov , and Geliang Yu National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093,People’s Republic of China. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044 Japan. Collaborative Innovation Centre of Advanced Microsctructures, Nanjing University, Nanjing 210093, People’sRepublic of China. * [email protected], [email protected], [email protected] + These authors contributed equally to this work.
ABSTRACT
Van der Waals (vdW) assembly of two-dimensional materials has been long recognized as a powerful tool to create uniquesystems with properties that cannot be found in natural compounds . However, among the variety of vdW heterostructures andtheir various properties, only a few have revealed metallic and ferroelectric behaviour signatures , . Here we show ferroelectricsemimetal made of double-gated double-layer graphene separated by an atomically thin crystal of hexagonal boron nitride,which demonstrating high room temperature mobility of the order of 10 m V − s − and exhibits robust ambipolar switching inresponse to the external electric field. The observed hysteresis is tunable, reversible and persists above room temperature. Ourfabrication method expands the family of ferroelectric vdW compounds and offers a route for developing novel phase-changingdevices. Introduction
Competing phases in condensed matter demonstrate the variety of physics phenomena: superconductivity and ferromagnetism,charge density wave and superconductivity to name a few. The recently investigated ferroelectric semimetal such as WTe , as an example of an unintuitive interplay between polarization and free charge, which seems at first glance, should screen theformer. Generally, metallic properties of a material do not favour macroscopic polarization, which means that ferroelectricityas a phenomenon is observed in many ferroelectrics of dielectric nature or semiconducting materials. Only a few examplesexist for metallic ferroelectrics and even less number of candidates to show hysteretic behaviour . However, there is oneexperimental evidence of natural 2D ferroelectric semimetal at room temperature: a few-layer WTe made by exfoliation,and another example is artificially made 2D polar metal based on superlattices BaTiO /SrTiO /LaTiO using advantage ofmolecular beam epitaxy growth . Both of these materials demonstrate a room-temperature ferroelectric effect. A ferroelectricstructural transition occurs in bulk (3D) LiOsO crystals at 140K . The interest in these materials is hidden in the possibility ofcreating new quantum states, including coexisting ferroelectric, ferromagnetic, and superconducting phases , use them forfunctional nanoelectronics applications . It is relevant to the combination of memory effects and conduction modulation, whichimproves transistor performance , .Graphene has high-mobility charge carriers at room temperature with acoustic phonon scattering as a limiting factor .Bilayer graphene demonstrates lower mobility at room temperature than monolayer graphene, but it is still much larger thanany other semiconductors, and semimetals known , , . Therefore, it could improve the transport properties of ferroelectricmetals based on vdW graphene heterostructures. Previously a breakthrough idea of changing properties of two-dimensionalmaterials by staking them in a different order is useful for constructing the new artificially made vdW heterostructures . For aferroelectric metal to exist, several criteria need to be: second-order phase transition, breaking inversion symmetry, polarizationswitchability .Here we explore this general idea to create a recently discovered class of materials that combine semimetallic andferroelectric properties in a single material . The effect of the moiré pattern reveals the possibility of forming a strongferroelectric characteristic for conventional 2D materials such as hexagonal boron nitride(hBN) and bilayer graphene .Breaking of the inversion symmetry occurs by mechanical stacking of two individual layers under small angle rotation, which1 a r X i v : . [ c ond - m a t . m e s - h a ll ] F e b pens a new path to create different physical effects in graphene, besides the well know proximity of graphene to ferromagnets,superconductors, 2D materials with spin-orbit interactions. Here we study electronic properties and reproduce the ferroelectriceffect in bilayer graphene intercalated with a monolayer hBN up to 325K and demonstrate a metal-to-insulator transitionferroelectric effect in graphene. Figure 1.
Hexagonal boron nitride-separated quasi-twisted bilayer graphene. a . Optical photograph of our Hall bar device.An encapsulated qTBG heterostructure is connected to metal leads (dull green) and endowed with gold top gate(bright green)and bottom silicon gate electrodes. b . Schematic of the triple-layer structure. Two MLG layers are twisted by a small twistangle. c . The schematic of the qTBG device with top and bottom gates. d and e . The device’s resistivity measured as a functionof V bg and V tg at 2.1K for V tg =
0V and V bg = Fabrication
Our device is a multiterminal Hall bar (Fig.1 a ) made of quasi-twisted bilayer graphene (qTBG) using standard dry transfertechnique . Graphene layers in such qTBG are separated by a monolayer of hexagonal boron nitride, allowing the graphenelayers to be tunnelled transparent. The sandwich is encapsulated between two relatively thick hexagonal boron nitride slabsprotecting graphene layers from the environment (Fig.1 b ). The qTBG heterostructure is connected to metal contacts andendowed with top and bottom gate electrodes allowing independent control over the carrier density in each layer ( n t and n b ,respectively) and relative displacement field between the layers. Results
Electrical transport measurements are carried out in a He variable temperature inset system using the standard low-frequencylock-in technique. Fig.1 c shows our qTBG sample’s resistivity as a function of back-gate voltage, V bg , measured at T = 2.1Kand zero top gate voltage, V tg = 0V. It exhibits familiar for high-mobility bilayer graphene behaviour with a sharp peak of about2k Ω corresponding to the charge neutrality point (CNP), followed by a rapid decrease with increasing V bg . When the gatevoltage’s sweep direction is reversed, the resistivity curve is shifted so that the CNP appears at 7 V, more than 10 V away fromits initial position. The observed hysteresis is robust and reproduces itself for numerous gate (top and bottom) voltage sweeploops without an apparent sign of degradation of the hysteretic behaviour. A similar hysteresis is observed for the top gatesweep measured at V bg = .The resistivity map is measured as a function of the back gate voltage which charges from -40V to 40V for the forward sweepFig.2 a and 40V to -40V for the backward sweep (Fig.2 b ). Both maps are measured at 2.1K, and during the sweep, the top gatevoltage is fixed. The top gate voltage changes from negative to positive values (from -5V to 4V). The dark region shows anoticeable difference in the resistivity peak position between the forward and backward sweeps. We plot Fig.2 c the maximumresistivity for the forward and backward back gate voltage sweeps to characterize the hysteresis. The hysteretic behaviourillustrates internal polarization in the heterostructure , which changes the charge carrier concentration in both graphene layers.The external electric field created by the back and top gate voltages can reverse this polarization by applying a large back gatevoltage ( | V bg | > V tg < − .
2V and V tg > . V bg from -9.1V igure 2. Ferroelectric hysteresis at 2.1K. a - b The sample’s resistivity as a function of the back gate and top gate voltages forthe forward a and backward b back gate voltage sweeps at fixed V tg . c . The maximum of the resistivity for the forward andbackward as a function of the top gate and back gate voltages. The dashed line is the best fit for the forward and backwardsweep’s resistivity maxima in the linear ferroelectric regime. d . Hall effect measured at B = .
5T for forward (solid blackcurve) and backward (dashed black curve) sweeps as a function of back gate voltage at V tg = e . Hall effect measured at B = .
5T for the forward (solid blackcurve) and backward (dashed black curve) sweeps as a function of top gate voltage at V bg = . The top gate’s efficiency with respect to the bottom gatecannot be determined from the top and bottom hBN thicknesses (42.6nm and 111nm, respectively) using a small dielectricconstant for hBN. If the relative dielectric constant for hBN is taken as 5, then the expected efficiency is about 11.6. However,the dashed lines in Fig.2 c correspond to the ratio of V bg / V tg = . ± .
1. Therefore, the dielectric environment of grapheneis significantly distorted by the ferroelectric effect. In the regions without hysteresis, the resistivity peak shift also does notcorrespond to the gate’s efficiency. It enters the nonlinear polarization regime of ferroelectric, which is commonly attributedto the paraelectric phase . In the paraelectric phase, the dielectric constant is not linear, and the resistivity of the structuredepends strongly on the properties of hBN layers. To the best of our knowledge, this observation has not been reported beforein 2D heterostructures.The average concentration in the system then can be determined by the Hall effect (see supplementary). Calculations ofthe Hall concentration as a function of back-gate voltage at V tg =
0V are shown in Fig.2 d , e . The position of the maximum isshifted between the forward and backward shift on the same amount when the efficiency of the top gate is taken into account.We notice here that the view of this hysteresis and the positions of maxima and minima do not change if top gate voltage sweepsare used at fixed V bg (See supplementary).Finally, we characterized the temperature dependence of the resistivity for the forward and backward sweeps. The peakresistivity is reducing with increasing temperature as expected for both monolayer and bilayer graphene, and the separationbetween peaks, which is related to spontaneous polarization in our structure, is decreasing with increasing temperature. Still, itdoes not disappear entirely, even at the maximum available temperature of 325K (Fig.3 b ). The full data set is shown in the upplementary. The cross-over density between metallic ( d ρ / dT <
0) and insulator ( d ρ / dT >
0) states is equal to 5 × cm − , which is in agreement with previous report . The corresponding mobility is found to be better than 10 m V − s − atroom temperature (Fig.3 d ), which is higher than in bilayer graphene and comparable with acoustic phonon limited mobilityin monolayer graphene . The charge carrier mobility reduces with increasing temperature, as shown in Fig.3 d linearly. Figure 3.
Transport properties at high temperatures. a . Temperature dependence of the resistivity as a function of back gatevoltage at V tg =
0V for the forward (solid style) and backward sweeps (dashed style) for four selected temperatures. b . Thevoltage difference between CNP positions of the backward and forward sweeps. The temperature changes from 2K to 325K. c .The temperature dependence of the resistivity for the forward sweeps for different electron concentrations. The smallestresistivity is multiplied by 5. d . The mobility of electron gas as a function of temperature measured at 3 × m − , 6 × m − and 9 × m − . The dashed lines are guides for eyes. Discussion
Previously a strong ferroelectric effect was observed in aligned and rotated by 30 ◦ hBN/bilayer graphene heterostructures . Theauthors argued that the different parts of a supercell could induce spontaneous polarization in such a structure. Here we study asimilar system without intentional alignment between hBN and graphene layers. The absence of any supercell at low energycould be justified by the gate voltage dependence of the resistance. In this case, the band structure should have more features atlow energy, which creates extra CNPs as was reported in . Also, Brown-Zak oscillations , which could be an indication ofalignment between graphene and hBN, are not observed in our device. Therefore, our system’s ferroelectric effect could not beof the same origin as in the intentionally aligned graphene/hBN structure . However, we could not exclude 30 ◦ rotation or the inversion symmetry breaking in the vertical graphene/hBN/graphene heterostructure as a cause of ferroelectricity. Thetwisted hBN layers can demonstrate the ferroelectric effect . This observation was reported in , . If we assume that themonolayer hBN can be easily twisted when placed between two graphene layers. In that case, the polarization can be created bythe top hBN and monolayer hBN or between monolayer hBN and bottom hBN layers. This effect needs further investigation.Mobility of the charge carriers in graphene at high temperature is limited by acoustic phonon scattering . The temperaturedependence of mobility is inversely proportional to temperature. This observation is valid for graphene and its bilayer . Thetemperature dependence of resistivity is proportional to ρ ∝ T in the case of twisted bilayer graphene, which was observedexperimentally and predicted theoretically for small angles of rotation. Our observations demonstrate inverse parabolic ependence of the mobility ρ ∝ T − as shown in Fig.3 d , which we attribute to the contribution of optical phonons in hBNsubstrate or polar optical phonons in agreement with ferroelectric nature of our structure. The low-temperature mobility islimited by scattering at the edges of the sample. Methods
The device was measured in an Oxford Instruments TeslatronPT cryogen-free superconducting magnet system equippedwith Oxford Instruments HelioxVT Sorption Pumped He Refrigerator insert (300mK,14T) and the magnetic field appliedperpendicular to the plane of the film. Stanford Research Systems SR830 lock-in is used to apply a AC bias current with a100MOhm bias resistor at a frequency of 13.333 Hz, and Keithley 2614B SourceMeters were used to apply DC current with a100MOhm bias resistor. Keithley 2400 SourceMeters were used to apply voltages to the gates.
AFM measurements are performed with a Bruker Dimension Fastscan system at tapping mode. Scan area of the bottom/tophBN are shown in the right/left red dotted boxes in the supplementary. The length to width ratio is 1.16. The width of thesample is 2.7 µ m. The top and bottom hBN thicknesses are equal to 42.6nm and 111nm, respectively.Room temperature Raman scattering is performed using a WITec/alpha 300R confocal microscope with a 532nm laserunder ambient conditions. The laser power was kept below 1mW to avoid damage or heating. The G and 2D peaks in theRaman spectra are fitted with Lorentzian. Typical Raman spectra of different positions of the heterostructure are plotted in theSupplementary. References Geim, A. K. & Grigorieva, I. V. Van der waals heterostructures.
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We thank B.G. Wang, K.S. Novoselov for useful discussions. This work is supported by the National Key R&D Program ofChina (grant nos. SQ2018YFA030066, SQ2018YFA030143), the National Natural Science Foundation of China (no. 11974169)and the Fundamental Research Funds for the Central Universities (nos. 020414380087, 020414913201), and the Basic ResearchProgram of Jiangsu Province (Grant No. BK20190283).
Author contributions statement
A.S.M. and G.Y. designed the project. Y.W. and fabricated the samples, S.J. and J.X. performed transport measurements, X.C.and G.M. did the AFM and Raman research, K.W. and T.T. provided hBN crystals. S. J., J.X., Y.W., and A.S.M performeddata analysis, D.Z., P.W., G.M., Y.H, J.H., and A.S.M provided the experimental support. Y.W., A.S.M and G.Y. wrote themanuscript. All authors participated in the discussions.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at <>.
Correspondence and requests for materials should be addressed to Y.W, A.S.M. or G.Y.should be addressed to Y.W, A.S.M. or G.Y.