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Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1084−1090
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Strong Polarity Asymmetry and Abnormal Mechanical Electroresistance Effect in the Organic Monolayer Tunnel Junction Gelei Jiang,†,‡ Yun Chen,†,‡ Ye Ji,†,‡ Weijin Chen,*,†,‡,§ Xiaoyue Zhang,*,†,‡,∥ and Yue Zheng*,†,‡ †
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, ‡Micro and Nano Physics and Mechanics Research Laboratory, School of Physics, and §School of Materials, Sun Yat-sen University, Guangzhou 510275, China ∥ Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
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S Supporting Information *
ABSTRACT: The performance of organic polar tunnel junctions under external electrical/mechanical stimuli is crucial for their great promise in developing flexible and highperformance sensor and memristive devices. Here, we prepared a single P(VDF-TrFE) monolayer on an Au(111) atomic-level surface by using a Langmuir−Blodgett (LB) technique. The polarity stability and electronic transport behavior of the monolayer were investigated via piezoresponse force microscopy (PFM) and conductive atomic force microscopy (c-AFM) measurements. We observed a strong polarity asymmetry in the P(VDF-TrFE) monolayer tunnel junction. Electrical bias and mechanical force exerted by the tip both can switch the monolayer from the metastable state (“H−Au” state, down polarity) to the stable state (“F−Au” state, up polarity). Moreover, the conductance of the monolayer tunnel junction can be controlled not only by the electrical bias but also by the tip force. Interestingly, a decrease of conductance from the ON state to OFF state is caused by the tip force, which is against our common sense that the larger the tip force, the larger the tunnel conductance. This abnormal mechanical electroresistance effect is attributed to the strong asymmetric interfacial barriers and is useful for the design of novel mechanical sensor and memristive devices. KEYWORDS: tunnel junction, P(VDF-TrFE), polarity asymmetry, mechanical electroresistance effect, Langmuir−Blodgett technique, atomic force microscopy
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design of flexible electronics. With regard to this field, the mechanical effect on PVDF films, especially on their polarization stability and transport performance, has been explored.22−26 Particularly, Chen et al.26 showed the stability of ferroelectric domains in P(VDF-TrFE) ribbons (with a thickness of 900 nm) under large mechanical deformation (>20%). When the thickness drops down to nanoscale, mechanical switching of ferroelectric domains27,28 was realized in 30 nm thick P(VDF-TrFE) films.25 These works indicate the great thickness dependence of the mechanical effect in PVDF films, and such an effect is expected to be even stronger in PVDF tunnel junctions. However, exploration on the mechanical effect on PVDF films at the required thickness (≤3 nm) for tunnel junctions remains exclusive. Moreover, inspired by the fact that the GER ratio in inorganic FTJs can be manipulated by mechanical loadings, termed the giant piezoelectric resistance (GPR) effect,29,30 it is important to
INTRODUCTION Poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (TrFE) thin films have attracted intensive research interest due to their flexibility and the exhibition of ferroelectricity down to the monolayer.1,2 With the observation of diode-like transport behavior in P(VDF-TrFE) thin film with a few monolayers,3 PVDF and P(VDF-TrFE) monolayers are considered as promising candidates in developing organic ferroelectric tunnel junctions (FTJs)4−9 and multiferroic tunnel junctions (MFTJs).10−12 Similar to that of inorganic tunnel junctions,13−20 the performance of these organic tunnel junctions is highly correlated to the asymmetric polarization stability and the interfaces as well. For example, by utilizing ntype semiconductor Nb:SrTiO3(STO) as the bottom electrode, Majumdar et al.6 exploited an extra barrier over the space charge region at the interface of P(VDF-TrFE)/Nb:STO and raised the OFF/ON ratio up to 10000. Additionally, ab initio total energy calculations demonstrated that conductive substrate can play a crucial role in orientating the PVDF dipole direction.21 The combination of polarity-tunable transport and flexibility of PVDF tunnel junctions makes them very suitable for the © 2019 American Chemical Society
Received: January 22, 2019 Accepted: May 20, 2019 Published: May 22, 2019 1084
DOI: 10.1021/acsaelm.9b00039 ACS Appl. Electron. Mater. 2019, 1, 1084−1090
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ACS Applied Electronic Materials ask whether mechanical force also has a great effect on the conductance of organic FTJs. In this work, we prepared a single P(VDF-TrFE) monolayer on an Au(111) atomic-level surface by using a Langmuir− Blodgett (LB) technique. Piezoresponse force microscopy (PFM) and conductive atomic force microscopy (c-AFM) measurements with the use of Ti/Ir-coated tips were performed to characterize the polarity and electronic transport behavior in the P(VDF-TrFE) monolayer. We observe a strong preference in the polarity stability of P(VDF-TrFE) monolayer and a tip-force-induced switching of the polarity in the monolayer. Moreover, a switching of conductance from the ON state to OFF state is shown and can be induced either by the electric bias or by the mechanical force. The conductance of the negatively biased monolayer is observed to decrease gradually as the applied tip force increases, although the contact resistance or the barrier width should decrease. This abnormal mechanical electroresistance effect is attributed to the strong polarity-dependent asymmetric interfacial barriers.
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Figure 1. (a) Schematic of a P(VDF-TrFE) monolayer on Au (111) substrate subjected to a bias voltage and/or a tip force. In the current characterization via c-AFM, the bias is applied from the substrate to the sample. (b) The monolayer of P(VDF-TrFE) on Au exhibits two states: the stable state of “F−Au” (up polarity) as grown and the metastable state of “H−Au” (down polarity).
METHODS
Fabrication. The atomic-level Au (111) surface was prepared by burning a rinsed gold wire in oxyhydrogen flame and annealing the acquired golden sphere several times until the atomic-level gold surface appears. The ferroelectric copolymer P(VDF-TrFE) solvent was prepared by dissolving 75% vinylidene fluoride and 25% trifluoroethylene (Piezotech Co.) in dimethyl sulfoxide with a concentration of 0.1 wt %. The solvent was stirred at 60 °C for 2 h. Then this polymer solvent was dispersed, and the P(VDF-TrFE) molecules were arranged on the surface of deionized water in a KSV LB trough (Nima 611). Before the compression, we waited for 20 min to let the dimethyl sulfoxide evaporate. During the preparation process, the compression speed was 10 mm/min. A single monolayer of P(VDF-TrFE) was transferred onto the Au (111) surface via the LB technique vertically at a surface pressure of 5 mN m−1 at room temperature with a transfer speed of 4 mm/min (Figure S1). The pressure−area isotherm of P(VDF-TrFE) at room temperature is shown in Figure S2. Characterization. The bonding information about the as-grown P(VDF-TrFE) monolayer was analyzed by X-ray photoemission spectroscopy (XPS) (ESCALAB 250Xi, ThermoFisher). The morphology of the monolayer was characterized by atomic force microscopy (AFM) (Asylum MFP-Infinity). The polarity and electronic transport behavior of the P(VDF-TrFE) film were characterized in the PFM and c-AFM modes. The conductive tip coated with Ti/Ir was used as the top electrode as shown in Figure 1a. The bias voltage is applied from the gold substrate to the sample in this measurement mode. The AFM measurements were done in a clean room with a constant humidity of 40%. The effect of H2O absorption layer on the conductance at this humidity condition is believed to be negligible. No obvious H2O absorption layer is indicated from the IR spectra AFM-IR measurement (nanoIR2-FS by Anasys Instruments) under ambient conditions as shown in Figure S3.
area including the removed part was measured by using tapping mode. As shown in Figure 2b, the 2 × 2 μm2 square in the center corresponds to the exposed Au (111) substrate after removal of the P(VDF-TrFE) film. An altitude intercept of ∼5 Å is obtained in the film thickness profile in Figure 2c along the red line between points A and B, which agrees with the thickness of an atomic layer of P(VDF-TrFE). Note that the thickness of a LB film is highly dependent on the roughness of substrate and the deposition process.1 Our monolayer sample prepared on the Au (111) atomic-level surface is much thinner than those of the previous works.4−7 The P(VDF-TrFE) monolayer was then analyzed by XPS. As shown in Figure 2d, the PVDF C 1s spectrum mainly consists of three components, including peaks at 291 and 286.5 eV which correspond to carbon bonded to two fluorines and carbon bonded to two hydrogens in the polymer, respectively. The phase hysteresis loop and amplitude curve are characterized by PFM in DART mode and are shown in Figures 2e and 2f. It should be noted that the voltage here is applied to the sample from the tip, which is different from the subsequent measurements. The average remnant vertical piezoresponse amplitude of the P(VDF-TrFE) monolayer obtained is 0.3 ± 0.06 nm, and the coercive voltage is 3 ± 1 V. These two curves are asymmetric and indicate that the state with negative polarity is unstable. Polarity Change of the P(VDF-TrFE) Monolayer under Electrical and Mechanical Stimuli. The polarity of P(VDFTrFE) monolayer originates from the arrangement of molecular dipole moments formed by the −CH2− groups and the −CF2− groups in each molecule. The orientation of the molecular dipole moment can be changed and even reversed by a bias voltage. There are two methods to characterize the polarization state via PFM. One is the scanning method by imaging the polarization distribution of the sample, that is, the domain pattern. However, the phase contrast of our P(VDF-TrFE) monolayer under using such a scanning method is rather weak due to the strong polarity
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RESULTS AND DISCUSSION Basic Property Characterization of the P(VDF-TrFE) Monolayer. The morphology of the as-grown P(VDF-TrFE) monolayer characterized by AFM in Figure 2a reveals a typical topographical feature of the Au (111) surface with 60° triangles. A root-mean-square roughness of ∼200 pm obtained from AFM indicates that the film is rather smooth. Crosssectional analysis of the topographic AFM image was performed to determine the thickness of the monolayer. Because the prepared P(VDF-TrFE) monolayer is soft, a part of it can be easily removed by the AFM tip using contact mode with a relatively large force. Then the morphology of a larger 1085
DOI: 10.1021/acsaelm.9b00039 ACS Appl. Electron. Mater. 2019, 1, 1084−1090
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Figure 2. (a) AFM morphology image of the as-grown P(VDF-TrFE) monolayer. (b) AFM morphology image of the P(VDF-TrFE) monolayer after removal of a 2 × 2 μm2 square area of P(VDF-TrFE) in the center. The square area in the center corresponds to the exposed Au (111) substrate. (c) The film thickness profile along the red line between points A and B as indicated in (b). (d) XPS spectra of the P(VDF-TrFE) monolayer. The (e) phase hysteresis loop and (f) amplitude curve characterized by PFM in DART mode.
Figure 3. Change of PFM phase and amplitude of P(VDF-TrFE) monolayer under time-varying electrical bias and/or mechanical tip force. Three situations are considered: (a) applying a triangular wave bias voltage (0 V ∼ −2.7 V ∼ 0 V ∼ 2.7 V ∼ 0 V) at zero tip force; (b) applying a constant bias voltage (−2 V) with a triangular wave tip force (0 nN ∼ 200 nN ∼ 0 nN); (c) applying a two-step bias voltage (−2 V ∼ 0 V) with a triangular wave tip force (0 nN ∼ 200 nN ∼ 0 nN).
step to −2.7 V, then increasing from −2.7 to 2.7 V, and finally being reduced to 0 V). The evolution curves of PFM amplitude and phase with the bias voltage are shown in Figure 3a. Note that because of the large number of data points (pink dots), we have drawn a smoother line in red by averaging every 500 data points to present the change of phase more clearly. It can be deduced that the polarity magnitude of the P(VDF-TrFE) monolayer is positively correlated to the bias voltage regardless of its direction. Meanwhile, the phase reacts in a different way. In the process of applying a positive bias voltage (0 V ∼ 2.7 V
asymmetry (Figure S4). Alternatively, one can use the method by detecting the vibration amplitude and phase locally at a certain fixed point on the sample, which can give both the magnitude and direction of the polarity, respectively. In the following, we used the second method to explore the influence of electrical bias and mechanical force on the orientation of the P(VDF-TrFE) monolayer. We first characterize the polarity of a point on the monolayer without mechanical force but subjected to a triangular wave of bias voltage (decreasing from 0 V step by 1086
DOI: 10.1021/acsaelm.9b00039 ACS Appl. Electron. Mater. 2019, 1, 1084−1090
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Figure 4. (a) Current change of the P(VDF-TrFE) monolayer under a sequence of “write”−“read” processes. A triangular wave voltage of 3 V was first applied (+write), followed by four smaller triangular wave voltage of 1 V to measure the conductance (read) of the sample; then a reverse triangular wave voltage of −3 V (−write) was applied and again followed by four smaller triangular wave voltage of 1 V. (b) Schematic illustration of the possible state change of the P(VDF-TrFE) monolayer during various subprocesses.
∼ 0 V), there is no obvious fluctuation with the curve of phase. Furthermore, the initial phase (at zero bias) is exactly the phase under positive bias. This indicates that the monolayer sample prepared by the LB technique has an initial stable orientation state that has the same polarity (up polarity) of the state after a positive bias voltage is applied. Moreover, when a negative bias voltage (0 V ∼ −2.7 V ∼ 0 V) is applied, a phase change of nearly 180° is observed at around −2 V bias, indicating a switching of the polarity into downward direction. This down-polarity state can only be maintained at a relatively large negative bias and will return back to the initial state as the negative bias is reduced. This result clearly reveals that the negative bias voltage can cause an unsustainable reverse of the polarity of the P(VDF-TrFE) monolayer, and the polarity is highly asymmetric in the two directions, in consistence with Figure 2d. From the above results, one can see that the bistability of the polarity of P(VDF-TrFE) monolayer on Au (111) substrate has been destroyed. The molecules of P(VDF-TrFE) on gold have two orientation states: the “F−Au” (up polarity) state with the F atoms in contact with the Au surface and the “H− Au” (down polarity) state with H atoms in contact with Au surface. As reported previously,31 the CF2 groups of the PVDF chain tend to form hydrogen bonds with the O−H groups of water, causing the F atoms to face downward and the H atoms to face upward at the water surface (see Figure S1c). As we lift the Au substrate up from the water into the air, the downward F atoms tend to be in contact with the Au substrate. This LB process leads to the formation of an F−Au interface on the Au substrate of the as-grown PVDF monolayer. Additionally, if the monolayer grows on a conductive substrate, the substrate effect has to be considered.21,32 To compare the stability of these two states, we calculated the energy of Au/PVDF/Ir structures in both the “F−Au” state and “H−Au” state by density functional theory (DFT) (see the Supporting Information for details of the DFT calculations parameter). The results in Table S1 show that the free energy of the structure in the “F−Au” state is 0.16 eV lower than that of the “H−Au” state. This is consistent with previous work, in which the binding strength of the F atoms on Au (111) (−2.77 eV on the fcc site) is found to be larger than that of H atoms (−2.18 eV on the fcc site).33 Therefore, the
system of the P(VDF-TrFE) monolayer/Au (111) tends to form a more stable “F−Au” state than a metastable “H−Au” state, and switching from the metastable “H−Au” state to the stable “F−Au” state is energetically favored under external stimuli (heat, electric field, mechanical load, etc.). To reveal the relationship between mechanical tip force and the orientation of the P(VDF-TrFE) monolayer, we further measured the change of PFM phase with a triangular wave of tip force (increasing from 0 nN step by step to 200 nN, then decreasing from 200 nN to 0 nN) under a fixed bias voltage. Note that there is an adhesive force between ferroelectric monolayers and AFM tip when it is getting close to the sample. Nevertheless, the adhesive force is quite small once the contact is stable and can be neglected (see Figure S5). As shown in Figure 3b, when the bias voltage is fixed to −2 V, the phase undergoes a 180° change as the tip force increases from the minimum (0 nN) to maximum (200 nN). A reverse of polarity occurs around a tip force of 100 nN. In contrast, the PFM phase remains almost unchanged when the bias voltages is fixed to be 0 or 2 V, with only the PFM amplitude increasing slightly with the loading force (Figure S6a,b). This indicates that only the “H−Au” state (stabilized by the negative bias) can be switched by the mechanical load. When the external force is reduced back to 100 nN or less, the phase undergoes a 180° change, and the orientation of the monolayer returns to “H−Au” state under the fixed bias of −2 V (Figure 3b). Because there are two orientations of polarity with distinct stability in this asymmetric monolayer junction, the system tends to “slide” from the metastable “H−Au” state to the more stable “F−Au” state under the stimulation of tip force, as illustrated in schematic Figure 1b. To verify whether this forceinduced switching is energetically favored, we removed the negative bias after the switching, as shown in Figure 3c. After switching, the phase indeed maintains the same level without the negative bias when the mechanical load decreases, indicating no further switching occurs. Polarity-Dependent Electronic Transport Behavior of the P(VDF-TrFE) Monolayer Tunnel Junction. For inorganic FTJs, it is well-known that the different bonding environments at the two interfaces would lead to different work function steps at the two interfaces and different 1087
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Figure 5. (a) Current change of the P(VDF-TrFE) monolayer under various tip forces. A tip force increases from 10 to 160 nN in steps of 50 nN to see the tip force effect. A “write”−“read” process was performed by applying a negative triangular wave write voltage of −10 V followed by a sequence of triangular wave read voltage in range of −1 V ∼ 1 V. (b) Schematic illustration of the possible state change of the P(VDF-TrFE) monolayer during various subprocesses.
screening potentials brought by the polarization, leading to asymmetric polarity stability and polarity-dependent electroresistance.20,34,35 While the bonding environments at the two interfaces would not change much by the polarity switching (the ferroelectric distortion only changes slightly the bonding length), the screening potentials would change by the polarity switching, leading to a polarity dependence of the tunnel barrier profile and the electroresistance. Similar interfacial effects on the polarity stability and electroresistance are expected in our P(VDF-TrFE) monolayer tunnel junction. The difference is that in P(VDF-TrFE) tunnel junction both the bonding environments at the two interfaces (e.g., from Au−F bonding to H−Au bonding at the bottom interface) and screening potentials would change significantly by the polarity switching. This may lead to an even stronger polaritydependent tunnel electroresistance. To see this, now we study the electronic transport behavior of the P(VDF-TrFE) monolayer tunnel junction. The current− voltage measurements were performed by c-AFM, as sketched in Figure 1a. Note that the conductive tip coated with Ti/Ir is used as top electrode, and the bias voltage is applied from the gold substrate in Orca mode. To first reveal the effect of bias voltage on the conductance of the P(VDF-TrFE) monolayer, “write”−“read” processes were performed: a triangular wave voltage of 3 V was first applied as write voltage (+write), followed by four smaller triangular wave voltage of 1 V to measure the conductance (read) of the sample; then a reverse triangular wave voltage of −3 V (−write) was applied and again followed by four smaller triangular wave voltage of 1 V. It is found that after the positive write voltage is applied in advance the read current is significantly smaller than that with negative preset bias voltage (Figure 4). This shows that the conductance of the P(VDF-TrFE) monolayer can be switched
between OFF and ON states by reversing the write voltage, which is believed to change the orientation of the molecular dipole moments. For the P(VDF-TrFE) monolayer tunnel junction, the positive write voltage keeps the molecular orientation with the initial “F−Au” state, and the current is relatively smaller (OFF state), while the negative write voltage could turn the P(VDF-TrFE) molecules to the “H−Au” state, and therefore the tunneling current becomes larger (ON state). This phenomenon is analogous to the GER effect caused by the switching polarization in the asymmetric FTJs. DFT calculations have been performed on the transmission of the “F−Au” and “H−Au” state. The results are quantitatively in accordance with the experiments (Figure S7). Note that after each negative preset bias voltage the peak values of read current have a decay with time. This is attributed to the relaxation of the metastable “H−Au” state to the “F− Au” state and the possible charge relaxation processes during the “write”−“read” process. Mechanical Electroresistance Effect. As shown previously, the mechanical tip force can directly affect the orientation of P(VDF-TrFE) molecules. We speculate that mechanical tip force should also affect the electrical transport behavior of the P(VDF-TrFE) monolayer tunnel junction. To verify this, we measured via c-AFM the current−voltage curves of the P(VDF-TrFE) monolayer under various tip forces. The result is shown in Figure 5. Here, a tip force increases from 10 to 160 nN in steps of 50 nN to see the tip force effect. Similar to the previous case, a “write”−“read” process was performed by applying a negative triangular wave write voltage of −10 V followed by a sequence of triangular wave read voltage in range of −1 V ∼ 1 V. Here we define an OFF state when the current is less than 10% of that of the initial state (ON state) without applying tip force, as noted in Figure 5. The result shows that 1088
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as the tip force increases, the read current decreases. The conductance of tunnel junction switches from the ON state to OFF state when the tip force reaches 160 nN. This trend of current change is counterintuitive. Usually, as the tip force increases, the contact resistance and the thickness of tunnel junction (barrier width) would both decrease, leading to an enhancement of the electron tunneling effect. With a positive preset voltage or without a write voltage, the tunnel current through the monolayer will indeed increase with the tip force (Figure S8). The abnormal mechanical electroresistance effect observed at negative preset voltage indicates that direct tunneling between tip and Au substrate excluding P(VDF-TrFE) monolayer is not likely to occur (or at least not dominant in the overall conductance). Combining these results and the previous result shown in Figure 3, such an abnormal mechanical electroresistance effect is believed to be caused by the tip-induced polarity switching and the strong asymmetry of the polarity-dependent interfacial barriers. The possible change of the P(VDF-TrFE) monolayer state in the four loading stages in Figure 5a is schematically shown in Figure 5b. In stages 1 and 2, the orientation of the monolayer is mainly maintained as the “H−Au” state. A significant drop of current occurs at stage 3. We suspect that the metastable “H−Au” state is switched to the more stable “F−Au” state at this point as the force is up to 110 nN. This is consistent with the result in Figure 3b. While the current exhibits a small decrease with time during stage 2, and a slight increase with time during stages 3 and 4, indicating complicate relaxation process occur in the monolayer, the mechanical electroresistance effect is significant.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.Z.). *E-mail:
[email protected] (W.J.C.). *E-mail:
[email protected] (X.Y.Z.). ORCID
Yun Chen: 0000-0002-0415-6593 Yue Zheng: 0000-0002-2165-7859 Author Contributions
Y.Z. initiated and performed this work and manuscript. W.J.C. and X.Y.Z conceived and designed the basic idea and structures. G.J., Y.C., and X.Y.Z performed the experiments. G.J. and W.J. C. performed the simulations. G.J., W.J.C., and Y.Z. cowrote the manuscript. Y.J. gave the suggestion on the data processing. All authors contributed to the discussion and reviewed the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the NSFC (Nos. 11602310 and 11672339), the Guangzhou science and technology project (No. 201707020002), the Fundamental Research Funds for the Central Universities, and the National Key Basic Research Program of China (No. 2015CB351905). Y.Z. acknowledges the support from the Special Program for Applied Research on Super Computation of the NSFC Guangdong Joint Fund (the second phase), the Fok Ying Tung Foundation, the Guangdong Natural Science Funds for Distinguished Young Scholar, and the China Scholarship Council.
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CONCLUSIONS In conclusion, we have prepared a single P(VDF-TrFE) monolayer on the Au (111) surface utilizing the LB technique. PFM measurements show that there is a direction preference in the polarity of the Au/P(VDF-TrFE)/Ir tunnel junction. The as-grown monolayer has an initial state with the F atoms contacting with the Au substrate, while the junction can be set to the metastable state of “H−Au” by negative bias. The mechanical load as well as a positive bias both can switch the “H−Au” state back to the stable “F−Au” state. The c-AFM measurements demonstrate a GER-like effect in the P(VDFTrFE) monolayer tunnel junction. That is, the write bias voltage can change the orientation of the P(VDF-TrFE) molecules; thereby the conductance state is modified. More importantly, for the first time an mechanical controllability on the conductance of P(VDF-TrFE) monolayer is observed. When the junction is initially set to the state of “H−Au”, the tunnel current is found to decrease with an increasing tip force. A switching of conductance from the ON state to OFF state induced by mechanical load is realized. Such an abnormal mechanical electroresistance effect is believed to be caused by the tip-induced polarity switching and the strong asymmetry of the polarity-dependent interfacial barriers and should be useful for developing novel mechanical sensor and memristive devices.
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Article
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00039. Additional experimental and DFT calculation results (PDF) 1089
DOI: 10.1021/acsaelm.9b00039 ACS Appl. Electron. Mater. 2019, 1, 1084−1090
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DOI: 10.1021/acsaelm.9b00039 ACS Appl. Electron. Mater. 2019, 1, 1084−1090
