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Ferroelectric Polarization Switching Dynamics and Domain Growth of Triglycine Sulfate and Imidazolium Perchlorate
He Ma , Wenxiu Gao , Junling Wang , * Tom Wu , Guoliang Yuan , * Junming Liu , and Zhiguo Liu
Dr. H. Ma, Dr. W. Gao, Prof. G. Yuan School of Materials Science and Engineering Nanjing University of Science and Technology Nanjing 210094 , P. R. China
E-mail: [email protected] Dr. W. Gao, Prof. J. Liu, Prof Z. Liu
National Laboratory of Solid State Microstructures Nanjing University
Nanjing 210093 , P. R. China Prof. J. Wang
School of Materials Science and Engineering Nanyang Technological University
Singapore 639798 , Singapore E-mail: [email protected] Prof. T. Wu
Physical Sciences and Engineering Division King Abdullah University of Science and Technology Thuwal 23955-6900 , Saudi Arabia
DOI: 10.1002/aelm.201600038
The exploration and design of organic or organic–inorganic molecular ferroelectrics should be of continuous importance not only in the academic sense but also for technological developments such as in fl exible and low-temperature processable ferroelectric components. [ 2,3 ] As expected from the generally soft nature of most molecular crystals, they show a quite high sensitivity of Curie temperature ( T c ) to an applied hydrostatic pressure, which would also greatly benefi t their high piezo- electricity. [ 4–7 ] One merit of low-molecular mass organic compounds is their applica- bility with a solution and/or their drying processes such as spin-coating, [ 8 ] spray-on, inkjet printing [ 9,10 ] and vapor-phase depo- sition [ 11 ] techniques. This would also be advantageous to the fabrication of fl exible, lightweight, large-area, low-cost organic devices. Many molecular ferroelectrics are more or less soluble in common organic solvents, and occasionally even in water. In addition, many of them can be thermally sublimed in vacuum.
A typical example is the vapor-phase deposition of vinylidene fl uoride oligomer, [ 11 ] a ferroelectric thin fi lm, in place of the nonvolatile polymers as demonstrated recently. [ 12–14 ] The lead- free organic ferroelectrics would also have the advantage of being environmentally friendly. [ 2 ] These features would stimu- late both academic and technological interests in molecular ferroelectrics as well as the innovative advances in the ever- growing fi eld of soft electronics. [ 2,15 ]
During the last several years, the study of molecular ferro- electrics has attained signifi cant advancement and their satu- rated ferroelectric polarization is now comparable to that of BaTiO 3 . [ 16–20 ] For example, the croconic acid crystal has shown a large spontaneous polarization of ≈20 µC cm −2 and a high ferroelectric Curie temperature ( T C ) of 127 °C. [ 16 ] The trigonal imidazolium perchlorate (C 3 N 2 H 5 ClO 4 , IM) crystal has a polari- zation of ≈8 µC cm −2 at room temperature, a high T C of 100 °C and a low coercive fi eld. [ 6,21–23 ] Besides, the motion of domain walls is critical to many applications involving ferroelectric materials, such as the ferroelectric memory, [ 24,25 ] domain wall diodes, [ 26 ] and spintronic devices. [ 27,28 ] However, even though the experimental measurements of domain growth and polari- zation switch in some well-established oxide ferroelectrics (e.g., PbTiO 3 and BaTiO 3 ) have been performed, the develop- ment of molecular ferroelectrics has been hampered for lack of a microscopic understanding of how domain walls move. [ 29 ] Although extensive studies have been carried out, we still The weak bond energy and large anisotropic domain wall energy induce
many special characteristics of the domain nucleation, growth, and polari- zation switch in triglycine sulfate (TGS) and imidazolium perchlorate (IM), two typical molecular ferroelectrics. Their domain nucleation and polariza- tion switch are rather slower than those of conventional oxide ferroelectrics, which may be due to the weaker bond energy of hydrogen bond or van der Waals bond than that of ionic bond. These chemical bonds dominate the elastic energy, with the latter being an important component of domain wall energy and playing an important role in domain nucleation and domain growth. The ratio of anisotropic domain wall energy to Gibbs free energy is large in TGS and IM, which allows a favorable domain shape and a special domain evolution under a certain electric fi eld. Therefore, this study not only sheds light on the physical nature but also indicates the application direction for molecular ferroelectrics.
1. Introduction
Ferroelectricity is a property of certain materials whose spon- taneous electric polarization can be reversed by the application of an external electric fi eld. Although the widely used ferro- electrics, such as PbZr 1− x Ti x O 3 and BaTiO 3 , belong to perovs- kite oxides, ferroelectricity was fi rst discovered in Rochelle salt (KNaC 4 H 4 O 6⋅4H 2O), a molecular ferroelectric, by Valasek in 1920. [ 1 ] However, until now, it is still vague on some specifi c differences between the oxide and molecular ferroelectrics.
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need to clarify the special phenomena of domain nucleation, growth, and the corresponding polarization switch in some molecular ferroelectrics, such as monoclinic triglycine sulfate ((NH 2 CH 2 COOH) 3 H 2 SO 4, TGS) and trigonal imidazolium perchlorate (C 3 N 2 H 5 ClO 4 , IM).
Here, we observed the slow polarization switch, diffi cult domain nucleation, and elaborate domain growth in TGS and IM, which may be due to the weaker chemical bonds and the smaller elastic energy in molecular ferroelectrics than those in oxide ferroelectrics.
2. Results and Discussion
2.1. Polarization Switching and Chemical Bond of Most Molecular Ferroelectrics
As illustrated in Figure 1 a, the universal polarization switching process microscopically involves domain nuclea- tion, forward domain growth, sideways domain growth, and domain merging when a ferroelectric sample is under an external fi eld ( E ex ) being much larger than its coercive fi eld ( E C). Domain nucleation becomes diffi cult though domain growths still continue when E ex is close to E c in some cases. [ 30 ] For conventional oxide ferroelectrics, the timescale of domain nucleation and forward domain growth is typically 1 ps to 1 ns, whereas for sideways domain growth it is from several ns to seconds or even longer depending on various intrinsic and extrinsic factors. The domain nucleation and the forward domain growth of a molecular ferroelectric would cost much longer time than those of oxide ferroelectrics. [ 31,32 ] In fact, the ferroelectric fatigue of oxide thin fi lms can be measured at
1 MHz while the saturated P-E loops of molecular ferroelec- trics are commonly measured at 1–500 Hz. [ 16–20 ] Besides, there are ionic bonds with a large bond energy (≈200–800 kJ mol −1 ) in oxide ferroelectrics, which results in a large elastic energy under a certain strain in Figure 1 b. By comparison, there is hydrogen bond (≈25–40 kJ mol −1) or van der Waals bond (≈0.4–50 kJ mol −1 ) with a smaller bond energy in molecular ferroelectrics, which accordingly results in a smaller elastic energy under the same strain. [ 33 ]
The characteristic time and the shortest time ( T S ) for fi nishing the macroscopic polarization switch of several typical ferroelectrics are compared in Table 1 . [ 23,33–48 ] The characteristic time (τ0 ) is inversely proportional to the fre- quency of crystal lattice vibration. [ 34,35 ] The τ 0 is estimated to be 1 × 10 −13 s in oxide ferroelectrics, [ 34–36 ] which is sev- eral order shorter than that in many molecular and organic ferroelectrics such as TGS, IM, and PVDF-TrFE. [ 37 ] The T S of most molecular ferroelectrics is much longer than that of most oxide ferroelectrics, [ 38–42 ] though the measured T S may depend on the domain wall creep process and varies in a wide range. In comparison, Table 1 also shows the bond energy ( E b ) of adjacent homopolar atoms, and the homopolar atom displacement (Δz ) between Temp << T C and Temp > T C where Temp is the measured temperature. [ 23,43–48 ] In this case, the bond type and the bond energy should be the intrinsic factors for the different characteristics between oxide and molecular ferroelectrics. Although how these chemical bonds infl uence polarization switch is still under further study, the elastic energy that depends on these chemical bonds is undoubtedly an important component of the domain wall energy, which again plays an important role in both domain nucleation and domain growth.
Figure 1. a) Domain nucleation and growth along an external electric fi eld and b) usual bond energy of conventional oxide ferroelectrics and molecular ferroelectrics.
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2.2. Polarization Switching Dynamics of TGS and IM
Both domain nucleation and domain growth of TGS and IM crystals are much slower than those of oxide ferroelectrics.
TGS belongs to monoclinic P2 1 space group and its b axis is its single polar axis, and IM owns trigonal R3m space group
and a single polar axis as well (Figure S1, Supporting Informa- tion). [ 6 ] The saturated P-E loops of b-axis-perpendicular TGS crystal were achieved with a maximum electric fi eld ( E max ) of 0.6 kV cm −1 with 0.2 Hz in Figure 2 a. Both the saturated polari- zation ( P S ) and remnant polarization ( P r ) become smaller with the measured frequency ( Fre ) increasing from 0.2 to 20 Hz and www.advelectronicmat.de
Table 1. The characteristic time ( τ 0 ), the T s derived from the macroscopic polarization switch, the homopolar atom displacement (Δz ), and the con- nective bond energy ( E b ) of adjacent homopolar atoms of several organic and oxide ferroelectrics, where the E b between chains is smaller than that inside the chains in PVDF 0.7 -TrFE 0.3 .
Materials τ 0 [s] T s [s] Δ z [Å] E b [kJ mol −1 ]
TGS 8 × 10 −6 1 × 10 −2 1 [ 43 ] 10–40 [ 33 ]
IM 5 × 10 −6 5 × 10 −3 0.7 [ 23 ] 10–40 [ 33 ]
PVDF 0.7 –TrFE 0.3 7 × 10 −9 [ 37 ] 5 × 10 −8 [ 38 ] – 10–40 [ 33 ] ,429.3 [ 47 ]
BaTiO 3 ≈1 × 10 −13 3.6 × 10 −9 [ 39 ] 0.109 [ 44 ] 653 [ 48 ] Pb(ZrTi)O 3 1 × 10 −13 [ 34 ] 8 × 10 −8 [ 40 ] 0.23–0.33 [ 44,45 ] 653–753 [ 48 ] BiFeO 3 1 × 10 −13 [ 36 ] 8 × 10 −7 [ 41 ] 0.135 [ 46 ] 397 [ 48 ] LiNbO 3 1 × 10 −13 [ 35 ] 5 × 10 −7 [ 42 ] 0.269 [ 44 ] 385 [ 48 ]
Figure 2. Ferroelectric polarization switching. Polarization versus electric fi eld curves and two corresponding current versus electric fi eld curves of a) TGS and d) IM, remnant polarization versus pulse width curves measured by the PUND method for b) TGS and e) IM, and typical waiting time ( T min and T max ) of domain nucleation versus electric fi eld curves of c) TGS and f) IM, where the inset of f) shows the four voltage pulses of PUND method.
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the P r is close to zero at Fre = 20–20000 Hz, which suggests that polarization hardly switches at E max = 0.6 kV cm −1 and Fre ≥ 20 Hz. The polarization current is the maximum near E C at 0.2 Hz, while it can be ignored at ≥20 Hz due to the diffi cult polarization switching (Figure S2, Supporting Information), where the leakage current is too small to obviously change the P-E loops above (Figure S3, Supporting Information). The P r was measured with a pulse ( T PU , from µs to s) electric fi eld by the positive-up-negative-down (PUND) polarization measure- ment (Figure 2 b). The P r begins to increase at 5 µs and then approaches a maximum of 4 pC cm −2 under 3.2 kV cm −1 for 0.003 s. This means that the fi rst domain nucleation would occur at 0–5 µs, and forward and sideways domain growths would mainly take place at 5–3000 µs under 3.2 kV cm −1 . The P r can reach a maximum at 0.1 s under 1.92 kV cm −1 , at 0.3 s under 1 kV cm −1 and at >1 s under 0.6 kV cm −1 . On the con- trary, the P r of the BaTiO 3 crystal changes immediately under an E ex larger than its E C within the limitation of the ferroelectric tester (10 −6 s), which is consistent with the fact that its polari- zation switch can occur at several ns (Figure S4, Supporting Information).
The dependence of P r on T PU is simulated to better under- stand domain nucleation and domain growth in Figure 2 . The essential difference between the Kolmogorov–Avrami–Ishibashi (KAI) model and the nucleation-limited switching (NLS) model is that the former focuses on the statistics of domain coales- cence, [ 49,50 ] whereas the latter considers the statistics of both domain nucleation and coalescence. In the case of 1D domain growth, the fraction of the ferroelectric volume switched by time t , p ( t ), is given as Equation ( 1) for the KAI model and Equation ( 2) for the NLS model.
p t( ) 1= −e−t T/ ( 1)
p t( ) 1= − <e−t T/ > ( 2) where T is the typical waiting time and 1/ T is the nucleation rate in each elementary region. [ 34,51 ] The T of the KAI model is constant while that of the NLS model varies from the minimum ( T min ) to the maximum ( T max ) values. The P r versus T PU curves are well fi tted with the NLS model, and the T min and T max are shown in Figure 2 b. Then from Figure 2 c,f, the domain nucle- ation hardly occurs at T < T min , while it is highly possible at T min < T < T max (Figure 2 c); the forward and sideways domain growths along the electric fi eld mainly occurs in the range of T to T S , where the local polarization becomes saturated at T S . The T min and T max of both TGS and IM decrease with the E max increasing. The T min of TGS is ≈10 −8 s under 3.2 kV cm −1 and that of IM is ≈10 −7 s under 3.3 kV cm −1 . The T max versus electric fi eld ( E ) can be well fi tted with the formula lg( T max ) = lg(τ 0 ) + ( E 0 / E ) n , where E 0 and n are two constants. [ 34 ] In accordance with TGS, the strong dependences of polarization on E max and Fre are also observed in the IM crystal. The polarization can be saturated under 4.9 kV cm −1 at 1–100 Hz, while the P r is close to zero at 1 and 2 kHz (Figure 2 d). At 0.66 kV cm −1 , the P r measured through the PUND method reaches a maximum at 0.2 s, the early domain nucleation occurs at 0.0008–0.01 s, and the main forward and sideways domain growths occur at 0.01–0.2 s (Figure 2 e). Furthermore, a 30 nm-diameter TGS surface was
polarized by a tip bias and then the dependence of phase angle on time is studied (Figure S5, Supporting Information). It turned out that the phase angle changes abruptly once an anti- domain is formed at the tip-contacting surface. The nanodo- main formation costs 0.3 s, 1.1 s, 8 s, 15 s, 25 s, and 40 s under the tip bias of 4 V, 3 V, 2.5 V, 2.2 V, 2 V, and 1.8 V, respectively.
These data prove that the domain nucleation of the TGS crystal is much slower than that of normal oxide ferroelectrics.
2.3. Domain Nucleation and Growth of TGS and IM
The in-plane polarization switching confi rms that the domain walls move integrally under a low poling fi eld and then the domain nucleation beyond electrodes becomes popular with the electric fi eld along b axis increasing in the TGS crystal ( Figure 3 ). At fi rst, the TGS crystal shows high resis- tivity according to the low leakage current measured with an Au/TGS/Au capacitor (Figure S3, Supporting Information).
The initial polarization is in the surface plane and points to the top electrode ( P up ) according to the in-plane (IP) and out- of-plane (OP) phase images of piezoelectric force microscopy (PFM) (Figure 3 a and Figure S6 in Supporting Information).
The domains with a downward polarization ( P down ) do not grow obviously after a 2.5 kV cm −1 between two in-plane electrodes was applied for 3500 s (Figure 3 b). Then the downward domains become larger at 11 500 s (Figure 3 c) and continually grow until they fi nally occupy over 95% area at 24700 s (Figure 3 d,e). In this period, the domain nucleation beyond electrodes is not observed and the integral move plays a key role in the polariza- tion switch, indicating that the domain walls’ integral move is much easier than the domain nucleation under a low electric fi eld such as 2.5 kV cm −1 . [ 30 ] In other words, the domain nuclea- tion beyond electrodes requires a higher electric fi eld than that of domain walls’ move. [ 24 ] On the contrary, some original anti- domains still exist and/or new domains easily nucleate due to the electric fi eld enhancement at electrodes. Figure 3 f–j shows the domain evolution under 5 kV cm −1 . Initially, the domain walls move bodily and then many new antidomains nucleate at 150 s (Figure 3 g). As more antidomains nucleate and grow, the domain walls still keep moving until the top domain walls meet the bottom electrode at 3150 s (Figure 3 h–j). Under 50 kV cm −1 , these conventional processes, i.e., domain nucleation, growth, and fi nal merging, now play a key role for the polariza- tion switch (Figure 3 k–o). Many antidomains nucleate at 2 s (Figure 3 l), and then more antidomains nucleate and grow until they merge with each other to fi nish the polarization switch (Figure 3 m–o). Yet in this process, the integral move of domain walls from the top electrode can still be vaguely differentiated.
From the above three domain evolutions, we can derive the domain wall velocity and the ratio of the switched area ( R SW ) to the total domain area (Figure S7, Supporting Information).
It shows that the velocity is not a constant under a certain electric fi eld here (Figure S7, Supporting Information). NLS model is discussed above. [ 34,37,51 ]
The domain evolution of the (0 1 0) TGS under a low elec- tric fi eld is special compared with those of many oxide ferro- electrics. In many oxide ferroelectrics, domain nucleation and growth can fi nish within several ns under an electric fi led
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larger than E C, and polarization switching exactly occurs at the area where the external electric fi eld is applied. [ 52,53 ] On the contrary, TGS shows a special domain evolution. On a smooth 15 × 15 µm 2 surface ( Figure 4 a), an ellipse-like white domain with an outward polarization ( P up) is embedded in a large brown domain with an inward-polarization ( P down ) (Figure 4 b). The ellipse-like shape is mainly due to the anisot- ropy of domain wall energy and elastic energy according to the elasticity theory. [ 54 ] Such a ≈15 µm length white domain com- pletely disappears after the central 5 × 5 µm 2 area was scanned by a tip with a 10 V bias (Figure 4 c). Importantly, the area of the annihilated domain is much larger than the polarized area and the polarization switch may be completed through the move- ment of domain walls. Then, a 4.5 µm length white domain nucleates and grows after a −10 V bias was applied to the same 5 × 5 µm 2 area (Figure 4 d). After an −18 V bias was upon the 5 × 5 µm 2 area, at least fi ve white domains nucleate, grow, and merge into a 10 µm length one (Figure 4 e). Then the domain decomposes into a ≈2 µm length domain inside the 10 V polar- ized area and a ≈3 µm length domain outside the polarized area (Figure 4 f). After the central 5 × 5 µm 2 area was polarized by an 18 V bias, these two domains disappear (Figure 4 g). Several new white domains appear outside the polarized area after a
−36 V bias and a 36 V bias were upon the 5 × 5 µm 2 area in sequence (Figure 4 h). Besides, the domain evolution under a point electric fi eld is studied in a 25 × 25 µm 2 region, where the PFM tip with 35 nm diameter was fi xed at the center of an ellip- tical domain with the long/short axis of 21.6/4.5 µm (Figure 4 i).
After a 2 V bias was applied to the center for 635 s and 1435 s, the elliptical domain shrinks to a smaller one with the long/
short axis of 17/2.4 µm (Figure 4 j) and 11.5/1 µm (Figure 4 k) respectively. Finally, this domain disappears at about 1635 s
(Figure 4 l). Meanwhile, the domain evolution on time under the A.C bias of the PFM tip is also studied for comparison (Figure S8, Supporting Information).
IM fi lms were grown on Pt-coated Si (Pt/Si) and quartz substrates, respectively, to study their polarization switch and domain evolution. The (1 0 −2) fi lm is about 2 µm thickness with a smooth surface ( Figure 5 a). There appear some equian- gular hexagonal domains, due to the 3 m point group and the uniaxial poling axis (Figure 5 b), which again result in a large anisotropic domain wall energy ( E wall ). [ 6,55 ] Here the white/
brown domain has a polarization against/toward the Pt/Si sub- strate, i.e., P up / P down . After the central 30 × 30 µm 2 area was scanned by a tip with −5 V bias, the introduced white domain becomes much larger than the polarized area (Figure 5 c). After the central 20 × 20 (Figure 5 d) and 10 × 10 µm 2 (Figure 5 e) areas were polarized by a 4 V bias in sequence, the central big domain becomes much smaller though it still keeps its shape.
This 4 V bias is not high enough to induce domain nuclea- tion in the polarized area; however it drives the movement of domain walls. The above domain evolution suggests that the domain walls’ movement of IM is commonly easier than the domain nucleation under a mild poling voltage. Besides, we also grew the (2 −1 0) IM fi lm on the insulated quarts sub- strate, and afterward we grew two parallel Pt electrodes, perpen- dicular to the in-plane polarization (Figure 5 f). Initially, there is only a brown domain. After a 15 V bias was applied between the in-plane Pt electrodes for 15 min (Figure 5 g) and 30 min (Figure 5 h), the domain walls near the left electrode bodily move toward the right electrode and the domain nucleation beyond electrodes is not observed either. The morphology of the measured area remains unchanged in these processes. It also proves that there is only domain walls’ movement during www.advelectronicmat.de
Figure 3. Domain evolution of a TGS crystal with the in-plane polar axis and electrodes. The IP-PFM images after 2.5 kV cm −1 was applied for a) 0 s, b) 3500 s, c) 11 500 s, d) 19 700 s, and e) 24 700 s, the images after 5 kV cm −1 was applied for f) 0 s, g) 150 s, h) 750 s, i) 1550 s, and j) 3150 s, and the images after 50 kV cm −1 was applied for k) 0 s, l) 2 s, m) 8 s, n) 20 s, and o) 120 s.
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Figure 5. Domain evolutions of two IM fi lms under different poling voltages. a) The morphology of the (1 0 −2) IM fi lm on the Pt/Si substrate, b) the initial OP-PFM image, and the images after the central region marked with red square was scanned by a tip with c) −5 V, d) 4 V, and e) 4 V in sequence.
f) The OP-PFM image of the (2 −1 0) IM fi lm on the quart substrate, and the image after it was polarized by 15 V between the two in-plane Pt electrodes for g) 15 min and h) 30 min, where the white arrow marks the in-plane polarization ( P IP ).
Figure 4. Domain evolutions of the (0 1 0) TGS crystal under different poling voltages. a) The surface morphology; b) the initial OP-PFM image; the OP-PFM images after its central 5 × 5 µm 2 area (red square) was scanned by a tip with c) 10 V, d) −10 V, e) −18 V, f) 10 V, g) 18 V, and h) −36 V, and 36 V. i) The OP-PFM image of the other region, and the images after its center (red dot) was polarized by a fi xed tip with 2 V for j) 635 s, k) 1435 s, and l) 1635 s.
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www.advelectronicmat.de the polarization switch process under a small electric fi eld. In
other words, the small electric fi eld can move the domain walls without domain nucleation beyond electrodes.
The special domain evolution of TGS or IM is due to the large anisotropic E wall compared with the Gibbs free energy ( E Gibbs), i.e., a large E wall / E Gibbs . Generally, for the ferroelectric phase, the E Gibbs can be written as:
E Gibbs = E G + 0.5aP 2 + 0.25bP 4 + E elec + E elas + E wall , where P , E elas, and E elec are ferroelectric polarization, elastic energy, and elec- trostatic energy, and E G , a , and b are three constants. When tiny anti-domains nucleate and grow, the E wall increases and the E elec decreases. If there are too many tiny domains in the crystal, the E wall will greatly increase so that E Gibbs gets higher.
Therefore, there exists a favorable domain size and shape as well as a corresponding E elec , E elas , and E wall , in order to obtain a minimum E Gibbs and a stable state. The large anisotropic E wall seriously limits the domain nucleation, since tiny domains have higher-density domain walls and larger E wall per unit volume than those of large domains. Previous research indicates the E wall (≈6–9 mJ m −2) of TGS is smaller than that of BaTiO 3 (15–18 mJ m −2 ), [ 55,56 ] and thus the large E wall / E Gibbs rather than the only E wall should be the main reason for the special domain evolution observed in TGS and IM (Figures 4 and 5 ). However, since these chemical bonds infl uence E wall and E Gibbs in a complex way, it still needs further studies concerning their dependences.
3. Conclusion
Many special characteristics are observed during the domain nucleation, growth, and polarization switch processes of TGS and IM. Their domain nucleation and polarization switch are rather slower than those of conventional oxide ferroelectrics, which may be due to the smaller bond energy of hydrogen bond or van der Waals bond than that of ionic bond. These chemical bonds greatly affect the elastic energy, an important component of domain wall energy which again seriously infl u- ences both domain nucleation and domain growth. Besides, the anisotropic E wall and large E wall / E Gibbs together give rise to a favorable domain shape and a special domain evolution under a certain electric fi eld in TGS and IM. Therefore, this study sheds light on the special characteristics of both polarization switch and domain nucleation in many molecular ferroelectrics.
4. Experimental Section
Sample Preparation : The TGS and IM single crystals were grown in an aqueous solution at ambient temperature. For the TGS single crystal, some glycine was mixed with the diluted sulfate acid in the ratio of 3:1, and then they were recrystallized to get high-purity TGS crystals. For the IM crystal, it was obtained through the evaporation of an aqueous solution containing equal molar amounts of imidazolium chloride and perchloric acid. We prepared and polished the TGS crystals with the polar b-axis being perpendicular or parallel to the surface, and the IM crystal with a (1 0 −2) surface plane.
For the IM fi lm growth, fi rst, the single-phase IM grains were dissolved in purifi ed water to form a saturated solution with the solubility of about 60%, and then the Si/SiO 2 /Ti/Pt and quarts substrates were placed in this saturated solution to induce a single-crystal-like thin fi lm. It turned out that the fi lm can show a (2 −1 0) or (1 0 −2) surface corresponding
to the XRD 2 θ diffraction angle of 21.98° or 24.32°. The IM fi lm thickness is about 2 µm through the measurement of a man-made gap with AFM.
Experimental Section : The morphology and piezoelectric force microcopy (PFM) studies were conducted on an atomic force microscope (Multimode 8, Bruker Co.). First, an A. C. voltage of 2 V at 41 kHz was applied to the Co/Cr-coated conductive tip (MESP-RC) in order to obtain the domain images. Then, the dependence of ferroelectric phase angle on the poling time was measured with a tip contacting the sample surface. Furthermore, two 200 µm diameter Pt electrodes were deposited on the (0 1 0) TGS and the (1 0 −2) IM fi lm respectively by a pulsed laser deposition system, and subsequently the macroscopic polarization was measured by using a commercial ferroelectric tester (Radiant multiferroic). Finally, the two parallel Pt electrodes were grown on both the TGS with an in plane b axis and the (2 −1 0) IM fi lm so as to study their respective in-plane domain evolution under the poling electric fi eld by PFM. Besides, the τ 0 of TGS and IM were also acquired through the simulation of T max − E curves according to lg( T max ) = lg(τ0 ) + ( E 0 / E ) n .
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The work is supported by the National Natural Science Foundation of China (11134004, 11234005, and 51472118), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. J. W. acknowledges support from National Research Foundation of Singapore under project NRF-CRP5-2009-04.
The authors also thank Prof. Ren-Gen Xiong from Southeast University, Nanjing, China for providing all ferroelectric crystals.
Received: February 2, 2016 Revised: March 14, 2016 Published online: April 8, 2016
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