As a versatile dielectric material that can be used to fabricate 2D/quasi-2D electronic devices, thiophosphate and selenium phosphate van der Waal crystals have recently attracted a lot of attention. Among them, CuInP2S6 as a two-dimensional van der Waals ferroelectric material, its ionic conductivity has been widely studied, and is considered to be the key to controlling its rich dielectric and ferroelectric functionality. However, intuitive evidence of ion conduction at the nanoscale of CuInP2S6 has yet to be discovered, and the specific migration characteristics of its ions under the electric field have yet to be studied. The control of the conductive properties of CuInP2S6 ions at the nanoscale is of great significance for the manufacture of new van der Waals heterogeneous devices.
Recently, researchers And collaborators from the Jan Seidel Research Group at the University of New South Wales in Australia and the Marin Alexe Research Group at the University of Warwick in the United Kingdom found direct evidence of anisotropic copper ion transitions in copper indium thiophosphates (CuInP2S6) through detailed scanning probe microscopy results and energy dispersive X-ray spectroscopy (EDS) at the nanoscale, and discussed in detail the physical mechanism of material ion conduction for copper ion transitions. The research results were published in the internationally renowned academic journal Nano Letters under the title "Anisotropic Ion Migration and Electronic Conduction in van der Waals Ferroelectric CuInP2S6".
In recent years, as Moore's Law has gradually reached its limit, the construction of advanced electronic information devices has become one of the hot spots in the post-Moore era. Among them, electronic devices based on the heterogeneous structure of two-dimensional van der Waals have received widespread attention. The rapid development of two-dimensional semiconductor devices has also driven the search for functional dielectric materials that can be combined with two-dimensional materials such as graphene and transition metal sulphides. CuInP2S6 (CIPS) is well compatible with the two-dimensional van der Waal heterostructure and miniaturization of the corresponding devices because it has no suspension key and can still maintain stable ferroelectricity in the thickness of several atomic layers, and has been widely used in recent years to build functional logic/storage devices: such as negative capacitance field effect transistors, ferroelectric tunneling knots and ferroelectric field effect transistors. Among these devices, the local response of CIPS at electric field has a critical impact on the overall performance of the device. In addition, the macroscopic ion conductivity properties under the electric field in CIPS monocrystalline blocks have been extensively studied, and their coupling to ferroelectric properties can lead to novel electrochemical phenomena such as the nonlinear electrostatic coupling of ions and spontaneous polarization caused by ferroelectric ion states. At the same time, the nanoscale ionic electron conductivity of this material has yet to be studied. Based on this, this work combines scanning probe microscopy technology with energy dispersion X-ray spectroscopy at the nanoscale to analyze and characterize the ion conductivity caused by controlled copper ion migration under the electric field.
The sample composition used in the study was Cu0.2In1.26P2S6 copper indium thiophosphate, and chemical phase separation occurred in copper-missing CIPS, presenting a coexisting In4/3P2S6 (IPS) nonferroelectric phase and CuInP2S6 (CIPS) ferroelectric phase. Piezoelectric microscopy (PFM) shows a clear piezoelectric signal contrast between the two phases, with the dendritic CIPS phase exhibiting a strong piezoelectric signal and 180° domain, and the IPS phase presenting a near-zero piezoelectric signal (as shown in Figure 1).
Figure 1: Crystal structure, sample composition of CuInP2S6, and test results of the CIPS phase and IPS phase in the sample under a piezoelectric microscope (PFM).
In addition, conductive atomic force microscopy studies the electrical properties of these two phases at the nanoscale. As shown in Figure 2, unlike the insulation properties of the IPS phase, the CIPS phase shows obvious conductive behavior, and similar to other ion conductors that have been reported, such as KTiOPO4, its conductivity shows a significant "activation" phenomenon, that is, when reducing the scanning speed or increasing the number of cycles, there is a significant increase in the current, this is because the low scanning speed or multiple cycles can make the electric field act on the ions in time to increase, thereby increasing the concentration of local carriers to further produce a higher current. When the scanning speed is reduced from 2 mm/s to 0.5 mm/s, the current increases by more than 60 times. The current-voltage curves on the CIPS phase and the IPS phase also show very different electrical properties of the two. The monotonic increase in current value on the CIPS phase can also be seen from the current change curve over time.
Figure 2: Electrical properties of nanoscale CIPS phases and IPS phases.
The process of ion migration is often accompanied by reversible or irreversible morphological changes, so how the long-range migration of copper ions affects the macroscopic structure of the material is worth exploring in depth. Based on this, the electroion migration and its corresponding morphological changes under different atmospheres (air and nitrogen) were studied in depth by conductive atomic force microscopy. When the probe applies a voltage to the CIPS phase for a certain period of time, the surface morphology corresponding to the CIPS occurs locally, and the depth and area of the depression show a positive correlation relationship with time and the size of the applied voltage. This topographic change may be related to local structural changes caused by the redistribution of the CIPS/IPS phase. This change in surface morphology, which increases with increasing voltage, is observed in both air and nitrogen atmospheres, which excludes the possibility that this is due to metal oxides. This observed change in morphology further confirms the long-range movement of copper ions at room temperature.
Fig. 3: Electromigration of copper ions: Micrograph of conductive atomic force of CIPS phases under different voltages in air and nitrogen atmospheres and their corresponding surface morphology.
The chemical composition of this electrochemical surface morphology change may provide intuitive evidence of long-range migration of copper ions. Based on this, the chemical composition of the local depression of CIPS was characterized by SEM-EDS, and it was found that the surface copper element distribution showed a signal enhancement in CIPS, while the distribution of other elements EDS elements was relatively uniform, and the possibility of oxide formation could also be excluded through the uniform elemental distribution of oxygen. In addition, the signal strength of copper elements in EDS in CIPS is positively correlated with voltage processing time, and no enhanced copper signal is detected in the place of unvolted polarization, which proves the time-related electrotropic copper ion migration in the sample.
In the out-of-plane direction of the sample, the nature of copper ions transitioning between intralayer and interlayer sites, and even across the van der Waals gap between several layers, has been theoretically predicted, and the displacement instability of this active copper ion is also considered to be the root cause of the conduction of CIPS ions. In addition, according to previous reports, the activation energy of copper ion transition and copper ion conduction in the layer is lower than that outside the layer, and there are six crystallographic equivalent positions in the sulfur octahedron in the inner direction of the surface for copper ions to migrate, so there is also a possibility of copper ions migrating in the direction of the surface. Based on this, the authors conducted a local current over time test in the CIPS phase. Three same voltage turn-on times (-8 V, 60 s) Different voltage off times (-8 V, 30 s; -8 V, 1 s; The voltage on/off repeat cycle experiment of -8 V, 60 s) shows a consistent trend, that is, when the voltage is turned on, the current remains increasing, and once the voltage is turned off, the current immediately falls back to zero, and when the next cycle starts the voltage is turned on, the current will immediately increase to a high value. Where Fig. 4 (g) enlarges the current signal in the time interval of 230 s to 330 s in Fig. 4 (d), it can be seen that when the voltage is turned on, the current rises rapidly to -12 pA within 5 s of time, and the current changes with time at a large slope and is counted as phase I; then the current goes through a stage in which the growth rate of phase I slows down, which is recorded as stage II.
Such two-step current growth is likely to be related to the two migration pathways of copper ions, which have been reported to be Ea=0.85 eV/0.23 eV (out-of-plane/in-plane) and ion conductive activation energy EA=1.16 eV/0.55 eV (out-of-plane/in-plane, below curie temperature). Obviously, the activation energy of the copper ion transition and the ion conduction in the driving surface is much smaller than that outside the surface. Therefore, for stage I, the increase in current may mainly come from the migration of copper ions in the surface, and the sudden increase in current comes from the opening of the last cycle voltage, and a large number of copper ions that have gathered near the probe are rapidly re-aggregated when the voltage is turned on. For Phase II, the increase in current mainly comes from the transition of copper ions in the out-of-plane direction, and the transition of ions between layers takes longer, which leads to a slowdown in the growth rate of the current. Thus, through detailed local current characterization, a two-step transition pathway for copper ions is proposed, and Figure 4 (h) gives a schematic diagram of the copper ion transition.
Figure 4: Evidence of EDS for copper ion migration at the nanoscale and time-dependent current tests show different migration routes for copper ions.
The work reported on the ion conduction of CIPS materials at the nanoscale through conductive atomic force microscopy, and believed that this was related to the two-step transition of copper ions driven by electric fields. The morphological changes of the CIPS phase caused by copper ion migration, and the increase in the strength of copper in the EDS element analysis in this region confirms the migration of copper ions under the action of electric field, and then the local current analysis test discovers the possible migration pathway inside and outside the front surface of copper ions. The coexistence and interaction of this ferroelectric and ionic conductivity in CIPS and the regulation of its electrical properties at the nanoscale provide new ideas for the multifunctionality of two-dimensional van der Waals ferroelectric/semiconductor devices.
Da Wei Zhang, PhD student at the University of New South Wales, Australia, and Dr. Zhengdong Luo, a researcher at the University of Warwick in the United Kingdom (now a researcher at IMIC in Belgium), are the co-first authors of the paper. Dr. Zhengdong Luo and Professor Jan Seidel are the corresponding authors of the paper. Also involved in the work were Dr Yin Yao, Dr Peggy Schoenherr, Dr Sha Chuhan, Dr Pan Ying, Dr Pankaj Sharma, and Professor Marin Alexe of the University of Warwick, UK.
Thesis Link:
https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c04023
Team Profile:
In recent years, the research group led by Professor Marin Alexe of the University of Warwick has mainly conducted research on new physics in bulk photovoltaic and piezoelectric materials and neuromorphic computing devices based on ferroelectric materials, and the relevant results have been published in high-level journals such as Nature, Science, Advanced Materials, ACS Nano, Nano Letters and so on.
Professor Marin Alexe Homepage:
https://warwick.ac.uk/fac/sci/physics/staff/academic/marinalexe/
The research group led by Professor Jan Seidel of the University of New South Wales in Australia mainly uses scanning probe microscopy (SPM) to study the electrical, optical and magnetic properties of oxides, especially interface and topological materials (domain walls, smyminds, vortexes) at the nanoscale, as well as perovskite solar cell materials. The results have been published in high-level journals such as Science, Nature Materials, Nature Physics, Science Advances, Nature Communications and so on.
Jan Seidel's homepage:
http://spm.materials.unsw.edu.au/