CuInP2S6 Room Temperature Layered FerroelectricA. Belianinov,† Q. He,† A. Dziaugys,‡ P. Maksymovych,† E. Eliseev,§ A. Borisevich,† A. Morozovska,⊥
J. Banys,‡ Y. Vysochanskii,∥ and S. V. Kalinin*,†
†The Institute for Functional Imaging of Materials and the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory,Oak Ridge, Tennessee 37831, United States‡Faculty of Physics, Vilnius University, Vilnius, Lithuania LT-01513§Institute for Problems of Material Sciences and ⊥Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine 03028∥Institute of Solid State Physics and Chemistry, Uzhgorod University, Uzhgorod, Ukraine 88000
*S Supporting Information
ABSTRACT: We explore ferroelectric properties of cleaved 2-D flakes of copper indiumthiophosphate, CuInP2S6 (CITP), and probe size effects along with limits of ferroelectricphase stability, by ambient and ultra high vacuum scanning probe microscopy. CITPbelongs to the only material family known to display ferroelectric polarization in a van derWaals, layered crystal at room temperature and above. Our measurements directly revealstable, ferroelectric polarization as evidenced by domain structures, switchable polarization,and hysteresis loops. We found that at room temperature the domain structure of flakesthicker than 100 nm is similar to the cleaved bulk surfaces, whereas below 50 nmpolarization disappears. We ascribe this behavior to a well-known instability of polarizationdue to depolarization field. Furthermore, polarization switching at high bias is alsoassociated with ionic mobility, as evidenced both by macroscopic measurements and byformation of surface damage under the tip at a bias of 4 Vlikely due to copper reduction.Mobile Cu ions may therefore also contribute to internal screening mechanisms. Theexistence of stable polarization in a van-der-Waals crystal naturally points toward new strategies for ultimate scaling of polarmaterials, quasi-2D, and single-layer materials with advanced and nonlinear dielectric properties that are presently not found inany members of the growing “graphene family”.
KEYWORDS: Atomic force microscopy, layered materials, ferroelectricity, 2D crystals
The last several years have witnessed explosive growth offunctional 2D materials.1 Following graphene introduc-
tion as a unique, high mobility, zero-gap semiconductor, muchinterest is now aimed at 2D materials with other functionalities,such as insulators, semiconductors and correlated electronmaterials.2−5 Equally important is the burgeoning developmentof functional 3D heterostructures via stacking of 2D materialswith different functionalities, for example, semiconductors withinsulatorssuch as graphene and MoS2. Numerous interestingapplications and fundamental phenomena can be anticipated if2D materials can support ferroelectric and ferromagneticfunctionalities, opening the pathway for bi- and multistabledevices and functional electronics. Interfaces between polarmaterials and high-k dielectrics with 2D electronic materialsmay lead to enhanced mobility,6 strongly temperature-depend-ent electronic properties, memory effects,7,8 and noveloptoelectronic properties.9 Ferroelectric and multiferroicbased tunneling devices, which incorporate an ultrathinferroelectric oxides, gained significant interest.So far the realization of ultrathin ferroic materials has been
limited by instability of remnant polarization in 2Dstructures.6−9 At the same time, integration of ferroelectricand 2D electronic functions, as well as ultimate size effectspresently faces a challenge of defect-free surfaces and interfaces.
Currently most ferroics are 3D crystalline materials, thesesurfaces have dangling bonds and rich intrinsic and extrinsicdefect chemistry6 impeding the control and coupling acrossinterfaces10 An effective solution would be a van-der-Waalscrystal with ferroic properties, where the surface energy isdrastically reduced and there exists a clear pathway to a 2Dmaterial through a simple preparation method such asexfoliation.Here, we explore the ferroelectric properties of copper
indium thiophosphate, CuInIIIP2X6 (CITP), and expound onsize effects and presently achievable limits of ferroelectric phasestability. CITP and its selenium-substituted derivativesCuMIIIP2X6 (M = Cr, In; X = S, Se), have been extensivelystudied using neutron scattering, Raman, and other bulktechniques; however, this is the only report of local ferroelectricexploration of this structure.11−17 The structure CITP isdefined by sulfur framework where the octahedral voids arefilled with Cu and In cations, whereas P−P pairs form atriangular pattern within the interlinked sulfur cages Figure
Received: February 5, 2015Revised: April 18, 2015Published: May 1, 2015
1d,e.18 The symmetry reduction from the paraelectric to theferrielectric, at the first-order phase transition, occurs at Tc =315 K, and is driven by the ordering in the copper sublatticeand the displacement of cations from the centrosymmetricpositions in the indium sublattice (C2/c to Cc symmetry).Based on X-ray diffraction,15 the direction of the spontaneouspolarization vector, at the onset of phase transition into theferrielectric phase, is perpendicular to the layered planes.Analysis and Results. We report on the properties of
cleaved 2D ferroelectric flakes and direct measurement ofpolarization switching along with the presence ferroelectricdomains in this material for the first time.11 The ambienttopography and piezoresponse force microscopy (PFM) studyof bulk single crystals cleaved in ambient conditions areillustrated in Figure 1a−c. Single frequency results of the samearea are shown in Supporting Information (SI), Figure 1. Bothsingle frequency PFM and band excitation19 PFM (BE-PFM)show rich domain networks consisting of two types of domains.Domains vary in size but are on the order of 1−3 um indiameter. The phase of piezoresponse flips by π radians acrossdomain walls Figure 1c, which confirms out-of-plane polar-ization orientation. Moreover, domains remain continuous overmonolayer steps and step bunches over 30 nm tall (Figure 1b).The steps are marked by a black dashed line in Figure 1a andwhite solid lines in Figure 1b,c. This result directly confirmsthree-dimensional ordering of spontaneous polarization in thismaterial. In addition, the surface features evenly distributedpartial delaminates on the surface that are typically ∼200 nmacross and ∼0.5 nm deep, which lack PFM response entirely.These are marked by a black rectangular box in the bottomright of Figure 1a, and a white rectangular box in Figure 1b,c.The marked region is cut out and processed separately forcontrast enhancement. The role or origin of these extendeddefects is presently unknown and will be explored in the future.Polarization switching of a clean surface using the tip as the
poling electrode was performed by writing opposite polaritysquare domain on the surface at various voltages. At ±2 Vdcapplied polarization switching is visible, but the response muchmore pronounced at a positive bias, as shown in SI Figure 2. At
a higher bias ±4 Vdc switching is observed in both directions,albeit still incomplete SI Figure 3. Interestingly, this level ofpositive bias triggers a reversible topographic change in theform of small spherical particles forming on the surface. At ±5Vdc practically all experimental area is polarized in bothdirections, Figure 2c, f, and a rescindable particle formation onthe surface is once again observed, Figure 2d (topography priorto the writing is shown in Figure 2a, with the single frequencyPFM of the same area shown in SI Figure 4). Preliminaryresults suggest that the formed particle size increases with
Figure 1. CITP surface and bulk. (a) 10 × 10 μm topographic image obtained in contact mode AFM; (b) band excitation amplitude image of thearea shown in (a); (c) band excitation phase image of the area shown in (a); (d) CITP unit cell atomic model structure viewed along [001] axis (Cu,red; In, green; P, dark blue, and S, yellow); (e) Representative STEM-HAADF image of the CuInP2S6 crystal, viewed along [001] zone axis, with theunit cell structural model overlaid.
Figure 2. CITP surface at ambient polarization switching. a) 20 × 20um derivative image of the surface; (b) band excitation amplitudeimage of the area shown in (a,b) after writing a ±5 Vdc squaresequence; (c) contact AFM derivative image of the surface afterpolarization switching; (d) band excitation phase image of the areashown in (a,b) after writing a ±5 Vdc square sequence.
Nano Letters Letter
DOI: 10.1021/acs.nanolett.5b00491Nano Lett. 2015, 15, 3808−3814
increasing bias and is likely due to the reduction of copper onthe surface.To rule out the effect of the ambient environment in
generating the domain structure and the observed peculiaritiesof polarization switching, BE-PFM and band excitationpolarization switching spectroscopy (BEPS)19,20 experiments
have been carried out in ultrahigh vacuum (UHV) using anOmicron AFM/STM with a base pressure 1 × 10−10 mbar, orbetter. The samples were cleaved in situ and immediatelyimaged. BE-PFM images of the sample in UHV showalternating up-and-down polarization domain structures ofsimilar size ∼1−3 um persist, Figure 3d,e, as do the partially
Figure 3. UHV results of the CITP surface. (a) 2 × 2 um contact AFM image; (b) amplitude image of a single voltage slice in a ±10 V BEPSmeasurement; averaged (c) amplitude and (f) phase loops of the identically color coded green and black areas in b; (d) band excitation amplitudeimage of the area in a; (e) band excitation phase image of the area in a.
Figure 4. Exfoliated CITP surface results. (a) 20 × 20 um contact AFM image of an exfoliated piece of CITP on SiO2 support, inset shows a lineheight profile for the green line in the image; (b) band excitation amplitude image of the area in a; (c) histogram of the PFM signal strength as afunction of flake thickness for the image in a; (d) band excitation phase image of the area in a.
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cleaved, trapezoidal dead zones of no response in Figure 3a. Noobvious difference between ambient and vacuum BE-PFMimages can be detected, suggesting that external screeningprocesses do not affect domain formation.BEPS loop results in vacuum by are shown in Figure 3b,c,f.
For a BEPS data set taken at 1 Vac and ±10 Vdc the behavior ofthe loop in the −6.5 to +6.5 V range can be described as atypical ferroelectric. There is hysteresis in the amplitude signaland an accompanying 180° phase flip. In the −10 to −6.5 and6.5 to 10 V ranges, behavior changes drastically, with theamplitude and phase hysteresis suddenly collapsing as seen inFigure 3c,f. We attribute this behavior to the environmentaleffects on the nonlocal electrochemical process, i.e., accessibilityof the counter-reaction sites, as was explored in detail for theclassical electrochemical systems.21
Topographical changes observed in ambient environmentand these peculiar features of the vacuum hysteresis loopssuggest the role of ionically conducting behavior in thismaterial. It is now established that ionic conductivity of Cu ionshas a remarkably low activation energy of 0.73 eV in CITP.12
Given the large magnitude of local electric field, the activationof ionic motion in the probed volume may be expected. In thiscase, the switching threshold for ferroelectric switching mayprecede, overlap, or exceed that of ionic motion, making theobservation and interpretation of purely ferroelectric switchingvery challenging. However, these processes do not affect the as-formed domain structures and become relevant only at highfields, very similar to classical ferroelectric materials that oftenexhibit field-induced reactivity in ambient.22
We proceed further to explore the delaminated films of CITPunder ambient conditions. Thin film preparation technique hasbeen described in Materials and Methods section. All of thefilms were deposited on a Si/SiO2 substrate precludingconductive and switching PFM measurements (Figure 4a).The thickness of the flakes ranged from 50 nm to 0.5 μm; BE-PFM as well as single frequency PFM detected response inflakes at thicknesses of 50 nm, but beyond that we found theresponse too weak to be detected, or the Tc is suppressed, as inFigure 4b,d. Thin layer effects are particularly well observed incases where there is a thickness gradient to the sample. Regionsthat are 100 or more nanometers thick exhibit bulk behavior,with excellent domain contrast, and at 50 nm domains are stilldiscernible in amplitude and phase signals. BE-PFM correlationof sample thickness to the amplitude of PFM signal is shown inFigure 4c.The contrast of ambient and UHV studies on cleaved bulk
materials and observations of the size effect in the exfoliatedfree-standing films suggest that the pristine domain structure isnot affected by the chemical state of the surface. At the sametime, the free-standing films exhibit bulk-like domain behaviorsdown to 100 nm thickness, and rapid decrease of domain sizes(as seen in domain area histograms in SI Figure 5) anddisappearance of domain contrast at 50 nm and less. Thisbehavior is consistent with the internal screening of ferro-electric polarization by mobile ionic carriers in CITP sincechemical surface screening will sustain ferroelectricity to muchsmaller scales (and is incompatible with UHV observations).To quantify this behavior, we assume the applicability of a
Landau−Ginzburg−Devonshire (LGD) theory of a properferroelectric for CITP. As such, equilibrium one-dimensionaldistribution of the out-of-plane polarization component P3(z)in a single-domain ferroelectric film can be found from theEuler−Lagrange boundary problem shown in eq 1.
α β γ φ
λ
λ
+ + −∂∂
= − ∂∂
+∂∂
=
+∂∂
=
=+
=−
⎧
⎨
⎪⎪⎪⎪
⎩
⎪⎪⎪⎪
⎛⎝⎜
⎞⎠⎟
⎛⎝⎜
⎞⎠⎟
P P P gP
z z
PPz
PPz
0;
0
z L
z L
3 33
35
23
2
33
/2
33
/2 (1)
In eq 1, α, β, and γ are 2−6 LGD expansion coefficients onpolarization powers, g is the polarization gradient coefficient λis the extrapolation length,23,24 and L is the film thickness.Coefficient α = αT(T − TC), TC is Curie temperature, which islower that the phase transition temperature TFE = TC + (β2/(4γαT)). Electric potential φ could be found from the Poissonequation with the open-circuited boundary conditions for thedisplacement field D3 as shown in eq 2:
ε φε
ρ φ
ε ε φ
∂∂
=∂∂
−
| ≡ ∂∂
− =±±
⎜ ⎟
⎧
⎨⎪⎪
⎩⎪⎪
⎛⎝⎜
⎞⎠⎟
⎛⎝
⎞⎠
zPz
Dz
P
1( )
0LL
33b
2
20
3
3 /2 0 33b
3/2 (2)
In eq 2 ε33b is the background permittivity of a ferroelectric, and
ε0 is a universal dielectric constant.25 For a proper ferro-electric−semiconductor the space charge density is given byρ(φ) = 2en sinh(eφ/kBT)
26,27
To derive analytical expressions for the film critical thicknesscorresponding to the transition into the paraelectric phase, onecan use Debye approximation for the charge density in theimmediate vicinity of the transition, since the spontaneouspolarization and corresponding depolarization field becomevery small. In accordance with our numerical calculations basedon (eq 2), the Debye approximation is valid in the immediatevicinity of the phase transition into the paraelectric phase fromeither single-domain or polydomain ferroelectric states, becausethe condition of its validity, |eφ/kBT| ≪ 1, holds true. We usethe Debye approximation (|eφ/kBT| ≪ 1) in the Poissonequation, (∂2φ/∂z2) − (φ/Rd
2) = (1/ε0ε33b )(∂P3/∂z), where Rd
= [(kBT)/(2ε0ε33b e2n(T))]1/2 is the Debye radius, which we
estimate to range from a few nm to 20 nm at room temperaturefor the carrier concentration ranging from 1022 to 1025 m−3 (SIFigure 6a,b).26
The critical thickness of the film transition into a paraelectricphase can be found from the characteristic equation obtainedfrom the boundary problem (eq 1) and (eq 2) by differ-entiation, namely, from the system of equations listed in the SI.After cumbersome transformations we derived that it can befound in eq 3:
κα
κλ κ ξ
λξ
κξ
πκ= −*
+ + + ≈⎛⎝⎜
⎛⎝⎜
⎞⎠⎟⎛⎝⎜
⎞⎠⎟
⎞⎠⎟L
g2 arctan
1 11cr 2 2
(3)
On the right-hand side of eq 3 the value κ ≈ (Rd/(−ε33b ε0α)1/2)is the screening length and ξ = (ε33
b ε0g)1/2 is the correlation
length. Coefficient α* = αT(T − TFE* ) for the strainedferroelectric film, TFE* = TC + (β2/(4γαT)) + (um*/αT)(4Q12/(s11 + s12)) is the FE transition temperature, TFE = TC + (β2/(4γαT)), renormalized by the epitaxial misfit strain um = (a/c)− 1. Equation 3 is valid under the condition L ≫ ξ and havephysical meaning for α < 0. To derive bulk LGD potential for
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the CITP, we consider classical 1D 2−6 power GLD expansionfor the free energy, ΔF = (α/2)P3
2 + (β/4)P34 + (γ/6)P3
6 − P3E3,and assume that only the first coefficient depends ontemperature as α = (T − T0)/(ε0CCW). Using experimentallyobserved temperature dependence of dielectric permittivity andthe value of spontaneous polarization at room temperature(2.55 μC/cm2),11 we obtain that T0 = 292 K, CCW = 7.2 × 103
K, β = −2.3 × 1012 m5/(F × C2), and γ = 3.5 × 1015 m9/(F ×C4) (see SI Table 1 and Figure 5c).As seen from Figure 5a, depending on the material
parameters, the critical thickness (3) can vary in a wide range(50−1000 nm). It appears that the thickness (3) is almostindependent of the extrapolation lengths, i.e., the approx-imation Lcr ≈ πκ is rather rigorous for the open-circuitedboundary conditions. CIPS material parameters are listed in theSI Table 1. Figure 5b can be interpreted as a phase diagram incoordinates “film thickness−temperature”. The unusual shapeof the single-domain ferroelectric phase, followed by a polydomain state originated from the semiconductor propertiescontribution.The screening length can be rather high, while the
correlation length is typically smaller than the lattice constantfar from the ferroelectric transition point.28 To estimate, therelevant parameters for CITP, the experimentally measuredtemperature dependence of dielectric permittivity was fit withits equation derived from the bulk LGD potential, andassuming spontaneous polarization of 2.55 μC/cm2 (Figure5e). The estimate for the high carrier density of 10−25 m−3
yields the critical thickness for the monodomain polarized stateof ∼200 nm. Experimentally, we observe that a poly domainstate survives until ∼50 nm. In principle, even a poly domain
state could be suppressed at the ultrathin limit in the absence ofepitaxial strain. The latter renormalizes α* and further reducesthe critical thickness. Experimental verification of these trendsrequires a detailed investigation of the domain structure andpolarization switching as a function of temperature, as well ascomparison between local and macroscopic studies.
Conclusions. Our observations unambiguously confirmstable ferroelectric polarization in an intrinsically layered, van-der-Waals material system, as evidenced by domain structures,rewritable polarization, and hysteresis loops. Although depola-rizing effect seems to be suppressing polarization domainsbelow ∼50 nm in thickness, polarization may persist at lowertemperature or in a polydomain state that is below theresolution limit of our present techniques. These intriguingmaterials warrant further investigation, particularly with respectto the mechanisms of coupling of atomic ordering across thevan-der-Waals gapsresponsible for the 3D ferroic order, aswell as the extraordinarily large diversity of ionic substitutionsthat are accessible within the thiophosphate transition metalfamily. Most importantly, a clean polar surface has so far haslargely eluded nearly all ferroelectric materials. Our studiesconfirm that ferroic order persists on the surface and thesurface itself is air stable. This already opens up newopportunities to understand ferroic coupling and enablesheretofore unachievable van-der-Waals epitaxial coupling offerroic and electronic functionality.
Materials and Methods. Sample Growth. The crystallinesamples of CuInP2S6 system were prepared by chemicaltransportation reaction using the elements in the stoichiometricproportions. The resulting product has the form of thin plateswith c-axis normal to the surface. The plate-like lamellae,
Figure 5. (a) Critical thickness of the film transition from paraelectric to a single-domain ferroelectric phase vs carriers concentration calculated fromeq 3 at 210, 293, 250, and 200 K. (b) Phase diagram in coordinates “film thickness−temperature” for CIPS. PE and single-FE and poly-FE denoteparaelectric and single-domain and polydomain ferroelectric phase regions correspondingly. (c) Experimentally observed (symbols) and fitted (solidcurves) temperature dependence of dielectric permittivity.
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CuInP2S6 crystals were grown at Uzghorod University, Ukraine.The complex dielectric permittivity ε* of the mixed CuInP2S6crystals were investigated in the wide frequency range using aHP4284A capacitance bridge in the 20 Hz to 1 MHz band. Thedata in the frequency region from 1 MHz to 3 GHz werecollected by a coaxial dielectric spectrometer equipped with avector network analyzer Agilent 8714ET. At very lowfrequencies, dielectric measurements were carried out using aSolartron 1260 impedance analyzer with a 1296 dielectricinterface. The dipole glass phase was observed and microscopicparameters were extracted from the dielectric properties.29
Thin Film Preparation. Ultrathin flakes of the compoundswere obtained using the micromechanical cleavage, also knownas the “Scotch tape method”, which is widely used for preparing2D materials such as graphene.30 All operations wereperformed in a clean room; CITP was exfoliated 2−3 timesby the scotch tape, in order to obtain fresh surfaces of thecompound. Next the sample containing tape was firmly pressedagainst the preprocessed SiO2/Si wafer, and then the tape wasgently peeled away so that some exfoliated film flakes were lefton the substrate. The SiO2 layer on the wafer was ∼300 nm,which is originally optimized for visualizing single layergraphene with white light. The apparent color of the compoundwith different thickness was then calibrated with AFMmeasurement and used to quickly locate thin layers. In orderto obtain a specimen for TEM characterization, a Quantifoilholey carbon Au grid was attached to the target thin flake viathe solvent surface tension of Isopropyl alcohol (IPA), and thenisolated by etching away the underneath SiO2 layer by bufferedHF.Ambient Imaging. Ambient single frequency PFM and band
excitation imaging/polarization switching were performed on aCypher AFM provided by Asylum Research. Pt−Cr coatedMulti-75EG AFM cantilevers (NanoAndMore) have been usedfor all imaging modes. Typically a contact mode Set point of 1V was used. All single frequency images were collected at thethird excitation mode of the cantilever (300−400 kHz) usingthe IgorPro control software; additional image postprocessedwas done in WsXM31 and Matlab.CITP samples were mounted using silver paste (Ted Pella
Prod. No. 16035) metal disks (Ted Pella Prod. No. 16218),with a grounding wire soldered directly on to the disk. A cleansurface was prepared by the “Scotch tape method” widely usedfor graphene preparation.30
UHV Imaging. Ultra high vacuum contact AFM and bandexcitation imaging/polarization switching were performed onan Omicron AFM/STM, interfaced with a Nanonis controllerpackage. The base pressure was 1 × 10−10 mbar or better. Thesamples were mounted on a standard Omicron sample plateand affixed with a silver conductive epoxy (Epo-Tek EJ2189-LV). A clean surface was prepared in situ by attaching a steelrod to the CITP sample surface with the same conductiveepoxy in ambient conditions, curing at 100 °C andmechanically removing it in the vacuum chamber. The resultingsurface was clean with monatomic steps and large terracesranging from 10−100 um.
■ ASSOCIATED CONTENT*S Supporting InformationAdditional modeling details omitted in the main text.Additionally, single frequency PFM images of the cleanmaterial surface, as well as band excitation PFM images ofthe switched areas are shown. The Supporting Information is
available free of charge on the ACS Publications website atDOI: 10.1021/acs.nanolett.5b00491.
■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] ContributionsA.B., Q.H., A.D., J.B., and A.B. characterized the samples usingAFM, UHV-AFM, STEM, and microwave techniques.P.M.A.N.M. and S.V.K. proposed the way for theoreticaldescription and heavily contributed to the model formulationand discussion portion of the manuscript. A.N.M. developedthe theoretical model and derived analytical results. E.A.E.performed numerical calculations. Y. V. grew and provided thesamples studied.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSResearch was supported (A.B.) and partially conducted (AFM,UHV-AFM) at the Center for Nanophase Materials Sciences,which is sponsored at Oak Ridge National Laboratory by theScientific User Facilities Division, Office of Basic EnergySciences, US Department of Energy. This work was alsosupported (Q.H., A.B., P.M., S.V.K.) and partially conducted(STEM) by the U.S. Department of Energy, Basic EnergySciences, Materials Sciences and Engineering Division.
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