Engineering Magnetism and Superconductivity in van der Waals Materials via Organic‐Ion Intercalation

Intercalation is the insertion of guest species between the planes of a host van der Waals layered crystal. The process is accompanied by a significant change of the charge carrier density and by the expansion of the interlayer distance, overall leading to a modification of the electronic band structure of the layered material. This perspective focuses on the possibilities offered by the intercalation of organic ions toward finely tuning the physical properties of van der Waals materials, in particular their magnetism and superconductivity. How the intercalation of organic ions offers several advantages over conventional guest species such as alkali metals is highlighted, since a careful choice of the molecular intercalant opens the possibility to tailor the interlayer distance and the charge carrier density. Moreover, specific properties of the molecular guest can be transferred to the host material, as recently demonstrated by the intercalation of thermo‐responsive and chiral molecules. It is anticipated that other functional organic ions can be incorporated in van der Waals materials to provide additional optical and magnetic capabilities, with the potential to enable an optical control of magnetism and superconductivity.


Introduction
The isolation of graphene in 2004 [1] marked the birth of the research in 2D materials, which quickly became one of the trending topics in condensed matter physics and materials science. Nowadays, the family of layered van der Waals (vdW) materials which can be exfoliated down to monolayer limit encompasses different compounds characterized by very diverse physical properties, determined by their specific composition and crystal structure. Already one decade ago it was realized that, in terms of electronic band structure, vdW crystals cover the entire spectrum from metals to wide bandgap insulators. This great variety of properties makes them an ideal materials platform for optoelectronics. [2] Moreover, certain layered compounds displaying other physical phenomena, such as superconductivity [3] and magnetism, [4,5] have recently attracted considerable interest, as they have enabled the fabrication of all-2D devices for quantum computing [6,7] and spintronics. [8][9][10][11] Despite showing very different physical properties, all vdW materials are characterized by a common trait, which is their extreme surface sensitivity when exfoliated to the ultrathin limit. This sensitivity, which is closely related to the reduced dimensionality, offers the unique opportunity to modulate their intrinsic properties in a controllable way by modifying their environment. For instance, electrostatic gating in vdW materials has been widely employed as a powerful tool to tune not only their conductivity, [1] as in conventional semiconductors, but also their magnetic properties [12][13][14][15] or their superconductive transition. [16] Interfacing 2D materials with functional molecules (including dopant, photo-active and magnetically active molecules) represents another widely explored strategy to controllably modify the physical properties of 2D materials, [17,18] including superconductivity [19] and magnetism. [20] Moreover, thanks to recent technical advances it is possible to mechanically stack different 2D materials with high precision forming so-called van der Waals heterostructures, [21] in which the properties of a single layer can be imparted to the adjacent sheets via proximity effects. In these systems, the lattice mismatch when two different sheets are stacked together at the right angle produces a Moiré superlattice which in some cases induces novel and unexpected physical properties. [22] The layered crystal structure of vdW materials offers a radically different possibility to tailor the physical properties not only of ultrathin exfoliated crystals, but also of bulk compounds. The vdW gap between adjacent sheets of a host material can be filled with guest species in a process called intercalation. [23] The resulting intercalated compounds can be thought of as a layered periodic structure in which interfaces between an inorganic sheet (host) and a layer of guest species repeat all along the crystal structure, generating a superlattice which extends in the out-of-plane direction (differently from the in-plane periodicity found in the Moiré superlattices aforementioned). Importantly, the intercalation process is typically accompanied by 1) a strong modification of the charge carrier density of the host material due to charge transfer from the guest species to the layered material and 2) the expansion of the interlayer parameters, overall leading to a modification of the electronic band structure of the intercalated materials. [24][25][26] Therefore, the insertion of guest species, molecules or atoms, into a bulk layered host material produces new intercalation compounds (ICs) with completely different physical and chemical properties compared to the original host material.
The intercalation of vdW materials has been investigated for several decades, focusing on both inorganic and organic compounds as guest species. Some of the first examples proving the effectiveness of this procedure to modify the intrinsic properties of layered compounds were based on the insertion of metallic ions. [27][28][29] Among the most striking effects discovered almost 50 years ago, we find the appearance of superconductivity in Kintercalated graphite (C 8 K). [27] However, a renewed interest in in-tercalation has risen in the context of the 2D materials research. On the one hand, intercalation chemistry can assist the exfoliation of layered crystals to obtain single layers in solution. [30] On the other hand, it has been explored as a method to tailor not only bulk layered crystals, but also pre-exfoliated flakes. [31,32] Moreover, the recent interest in magnetic and superconductive 2D materials has motivated an intense research effort to engineer their intrinsic properties, for instance to increase their transition temperature.
In this perspective article, we focus on the possibilities offered by the intercalation of organic molecules to finely tune the intrinsic physical properties of vdW materials, in particular their magnetism and superconductivity (Figure 1). We will show that intercalation can be employed not only to engineer preexisting magnetic and superconductive properties, but also to induce these phenomena in non-magnetic and non-superconductive vdW materials. While metals such as alkali metals to this day constitute the most used guest species and hold impressive results of intercalation-induced superconductivity, [33][34][35][36][37][38] here we will focus on the intercalation of organic molecules, which remain underrepresented despite their superior versatility when compared to more conventional metallic ions. Indeed, organic molecules offer multiple options due to their very different size, charge, electrical dipole, magnetic spin and optical properties. For instance, a few recent works demonstrate that the interlayer distance and the doping level of ICs can be tuned very effectively by choosing molecular guest species with different volume, with direct www.advancedsciencenews.com www.advphysicsres.com effects on their electronic and magnetic properties. [24][25][26] After discussing these works, we will mention that the intercalation of functional organic compounds may introduce new capabilities to ICs, which have been only partially explored. Besides, little is known about the molecular ordering in the confined space between the layers. Mastering the molecular self-assembly may provide an additional control knob to achieve further control over the ICs band structure.
Finally, we highlight that while organic-ion intercalation offers many interesting options, it is not a well controllable and fully predictable process. In fact, it is generally difficult to foresee whether a given layered material is prone to be intercalated by a specific molecule, and which intercalation method should be used for the selected host/guest combination. For instance, electrochemical intercalation is often employed to introduce organic (and inorganic) cations in the vdW gap of layered materials, [25] but the result of the process is markedly material-dependent. Within this approach, a layered material is attached to an electrode in an electrochemical cell, and typically biased at a negative potential. Therefore, the cations in the electrolyte are attracted toward the vicinity of the layered compound, and they are introduced in the vdW gap if the applied voltage is sufficiently high. This method provides a fast intercalation process which can be controlled using the electrochemical parameters and can be used for bulk crystals and exfoliated flakes contacted by microscopic electrodes. Electrochemical intercalation has been reported for many layered materials, including graphite, MoS 2 , [32,39] NbSe 2 , [40] black phosphorous, [31] and NiPS 3 [41] among others. However, the procedure tends to be rather aggressive, so that electrochemically intercalated crystals typically display broad X-ray diffraction peaks, indicative of structural disorder, and sometimes, the materials are even exfoliated in solution. Moreover, this method limits the type of guest/host compounds available for intercalation to electrochemically stable cations and electrically conductive layered materials. Besides, some (semi)conductive layered materials cannot be electrochemically intercalated, as they degrade before accepting intercalated guest species.
In other cases, the intercalation occurs spontaneously, by inserting a certain layered crystal in a solution containing a specific guest species (molecule or ion). [25] This process is driven by the donation of electron density from the guest to the host, in some cases accompanied by a redox phenomenon. [42][43][44][45] This type of intercalation is a much gentler process and therefore it can be applied to both thin flakes and bulk crystals.

Engineering Superconductivity in Layered Materials
Superconductivity is perhaps one of the most important physical phenomena which motivated the interest in ICs. Several studies show how intercalation, apart from modifying the superconducting transition in vdW superconductors, also allows to create superconductivity in compounds that do not exhibit this phenomenon in their pristine state. For instance, after the first study on the superconductivity of K-intercalated graphite, [27] several works have shown how the intercalation of different metals in the van der Waals gap of graphite leads to a superconductive state, with the highest critical temperature (11.5 K) reported in 2005 in the highly unstable compound C 6 Ca. [46] The intercalation of alkali metals in superconducting layered materials is still the focus of intense research activities. For instance, a decade ago it was shown that intercalating Li ions via ionic gating introduces superconductivity in MoS 2 [47] and ZrNCl flakes, [48] and more recently superconductivity and a new electronic phase were found in alkali-ion-intercalated WTe 2 . [37] The studies on superconductivity in ICs indicate how the superconductive transition is determined by a complex interplay between charge carrier concentration and interlayer distance. [49][50][51][52] While the intercalation of organic and inorganic species provides a similar control over the charge carrier density, choosing molecular guests with different size provides a unique tunability on the interlayer distance. In this section, we mention a few studies which take advantage of this tunability to tailor superconductivity in layered materials. Figure 2a shows how organic ion intercalation allows to change the charge carrier doping while maintaining the same interlayer distance. In particular, the superconductive transition was recorded for 2H-TaS 2 intercalated with a different number of cetyltrimethylammonium (CTA) cations. [49] Since each of the intercalated cations adds an extra electron to the host, the stoichiometric index x in the (CTA) x TaS 2 intercalated crystals is proportional to the charge carrier concentration, as the electron density increases at larger x.
Conversely, the interlayer distance in (CTA) x TaS 2 remains unaltered in the range from x = 0.1 to x = 0.9, as in all cases the linear alkyl chains of CTA lay oriented orthogonally to the TaS 2 basal plane, as schematically sketched in the inset in Figure 2a. We highlight that the IC structure sketched here and in all other figures of this article is highly schematic and not to scale. A more realistic representation of the structure of ICs can be found in ref. [53] The authors found that the intercalated (CTA) x TaS 2 crystals show higher critical temperature (T c ) than the pristine material for all the tested stoichiometries (Figure 2a, inset). Moreover, the crystals intercalated with different amount of CTA molecules display a slightly different T c (Figure 2a, inset), showing a dome-like behavior with a maximum T c at a x = 0.6 molecular concentration. It is worth noting that the variation between the onset of the superconductive transition measured in (CTA) x TaS 2 is in the range 2.7-3.7 K for the different x, whereas the T c of the pristine compound is 0.8 K. This indicates that for this compound the variation in the interlayer distance affects the T c more than the variation in the doping. We highlight that a similar study could be carried out using inorganic cations, which also allows to tune the charge carrier density without changing the interlayer distance. However, the large interlayer distance obtained in the (CTA) x TaS 2 , which amounts to almost 3 nm, introduces a layerdecoupling effect that cannot be achieved using inorganic guest species.
The advantages of using organic molecules as guest species are also clearly demonstrated by the data shown in Figure 2b. [50] In this case, a fine control over the interlayer spacing is not achieved by directly intercalating organic cations, but rather by co-intercalating solvent molecules with an alkali metal. In particular, the authors focus on the Li intercalation in SnSe 2 , which introduces a large amount of charge carriers in SnSe 2 and turns it into a superconductive metal. When only Li ions are intercalated without solvent molecules (Figure 2b, top panel), a T c of Figure 2. a) Intercalation of long quaternary alkylammonium chains, such as cetyltrimethylammonium cations (CTA), into the van der Waals gap of TaS 2 modifies the carrier density and interlayer distance leading to the enhancement of the critical temperature from 0.8 up to 2.8 K. Reproduced with permission. [49] Copyright 2018, IOP Publishing. b) Top panel-Intercalation of lithium cations in SnSe 2 beyond a given threshold does not increase the superconductive critical temperature (T c ). Bottom panel-Co-intercalation of organic solvents such as tetrahydrofuran (THF) and propylene carbonate (PC) with lithium achieves a higher T c compared to the T c of the Li-intercalate due to the increased interlayer distance, achievable thanks to the presence of the organic solvents. Reproduced with permission. [50] Copyright 2019, IOP Publishing. c) The enlargement of the van der Waals gap in FeSe triggered by the intercalation of 1-ethyl-3-methylimidazolium cations (EMIM) leads to the increase in the T c of FeSe from 8 to 44.4 K at ambient pressure. Reproduced with permission. [51] Copyright 2021, IOP Publishing. d) Tetrabutylammonium cations (TBA) inserted in the interbasal planes of FeSe transfer charge to the host crystal leading to a record-breaking T c of 50 K in FeSe. The sketches representing the crystal structures are schematic and not to scale. Reproduced with permission. [52] Copyright 2018, IOP Publishing.
≈4 K is measured, irrespectively of the amount of intercalated Li. On the contrary, a higher T c is reached by co-intercalating an organic solvent together with the lithium-ion. The incorporation of the organic solvent overtakes the effect of lithium ions alone, increasing the critical temperature from 4 K up to 7 K and 7.5 K when using tetrahydrofuran and propylene carbonate as solvent, respectively (Figure 2b, bottom panel). The authors correlate the observed increased T c with the larger interlayer distance, which is exclusively achievable by the presence of the organic solvents in the van der Waals gap.
Organic cations can also be intercalated as standalone guest species to tune superconductivity in layered materials, as shown in Figure 2c,d. In particular, Wang et al. [51] showed that FeSe intercalated with 1-ethyl-3-methylimidazolium cations displays a T c of 44.4 K, a similar value to the one obtained by Rendenbach et al. (43 K) using tetramethylammonium (TMA) instead of EMIM. [53] Once again, the effect is explained on the basis of the enlargement of the interlayer separation and the concomitant charge doping introduced by the organic guest species (Figure 2c). In another work, Shi et al. report a T c = 50 K in tetrabutylammonium (TBA)-intercalated FeSe, which is the highest T c achieved up to now for FeSe-based ICs. [52] The authors suggest that the reason behind the exceptionally large value, which is considerably higher than that obtained via pure metallic ion intercalation, is the weak interaction between the hosting layered material and the guest organic species. In particular, the disordered distribution and orientation of the TBA ions in the organic layers prevents both the formation of defects in the atomic planes of FeSe and the formation of impurity phases, which are detrimental to the superconducting phenomenon and more prone to appear when metallic ions are present.
We highlight that while all these studies focus on the superconductivity of bulk ICs, the potential use of ICs in van der Waals heterostructures and nanodevices will require the use of smaller crystals, such as micrometric exfoliated flakes. However, the superconductivity in intercalated flakes remains largely unexplored. A first step in this direction was taken by Pereira et al. [39] In this work, the authors studied the superconductivity in organic-ion intercalated MoS 2 . While it is well established that MoS 2 can be made superconductor via ionic-liquid gating [47,63] and through the intercalation of alkali metal ions, [28,64] the intercalation of organic cations in MoS 2 had not been previously explored for this purpose. Figure 3a shows that MoS 2 bulk crystals intercalated with tetraethylammonium (TEA) cations display a superconductive transition. [39] Additionally, the electronic properties of a flake exfoliated from a TEA-intercalated bulk crystal were also investigated. In this case, while a resistance drop is measured at low temperature, the zero-resistance state was not reached (Figure 3b). This behavior was ascribed to a spatially varying charge carrier density caused by an inhomogeneous distribution of the guest species. [39] This work points out the importance of studying the physical properties of ICs in small flakes to address the real potential of these materials for technological applications.

Engineering Magnetism in Layered Materials
The demonstration of layered-dependent ferromagnetism in ultrathin CrI 3 and CrGeTe 3 (CGT) flakes sparked a sudden interest toward magnetic vdW compounds. [4,5] Following these works, several magnetic vdW materials have been recently exfoliated and characterized, ranging from ferromagnetic metals such as Fe 3 GeTe 2 [65] to antiferromagnetic insulators, including several transition metal di-and trihalides. [66] However, most of these materials are not stable in atmosphere, and their pristine magnetic transition is below room temperature (below 77 K in several cases). [67] The intercalation of organic ions offers the possibility to tailor the magnetic properties in the same way it modulates the superconductivity, potentially representing one of the most versatile and effective routes to achieve above-room-temperature magnetism and air stability.
As compared to the vast literature focusing on the superconductivity of ICs, the study of magnetism in ICs remains relatively unexplored for both organic and inorganic guest species.
Pioneering works on the intercalation of the MPX 3 family, where M = metal and X = S, Se, were carried out since the 1970s, mostly by the group of R. Clement. [68] According to these studies, MPX 3 crystals, which are typically antiferromagnets in their pristine state, [42] can be turned into ferrimagnets by the intercalation of organic cations. Therefore, a spontaneous magnetization is recorded for most intercalated MPX 3 , even if the magnetic moment per transition metal atom typically remains rather low (on the order of 10 −2 B per atom for NiPS 3 [69] and 10 −1 B per atom for FePS 3 [70] ). A notable exception is represented by MnPS 3 intercalates, [68] in which an ion-exchange process results in the substitution of Mn atoms with organic cations. [71] In this case, ordered Mn vacancies preferentially located in one of the two magnetic hexagonal sublattices, [72] leave unpaired spins which couple ferromagnetically, resulting in relatively high moment per atom (which may overcome 1 B per atom). This process was reported for a variety of guest species, including Spyropirans, [73] Stilbazolium, [74] metal-organic complexes, [75][76][77] and other molecular cations. [78][79][80][81] More recently, the effectiveness of intercalation was demonstrated by Wang et al. who were able to remarkably increase the Curie temperature for the paramagnetic-ferromagnetic transition of pristine CGT and regulate its magnetic properties [82] through organic intercalation. In this work, they showed that TBA ions can be successfully inserted into the vdW gaps of a bulk CGT crystal via electrochemical intercalation. The process caused the expansion of the interlayer spacing from 6.8 to 16.48 Å after intercalation (Figure 4a), in agreement with the insertion of the 10-Å-size TBA cation and without any substantial change for the other unit cell's parameters. As it can be observed in the temperature dependence of the magnetization (Figure 4b), the Curie temperature is boosted from 67 K (red curve, pristine) to 208 K (black curve, intercalated CGT), and its magnetic easy-axis turns from out-of-plane in pristine CGT to in-plane in the hybrid superlattice TBA-CGT. In addition, its electrical properties were also altered: pristine CGT displays a usual semiconductor-to-insulator transition while the TBA-CGT intercalate exhibits a metallic behavior. Theoretical calculations indicated that heavy electron doping is beneath this dramatic change in its magnetic and electrical properties.
Among the effects on the magnetism of a 2D layered material resulting from organic intercalation, we also find the possibility of a complete rearrangement of the spins, giving rise to a magnetic phase transition which differs from the pristine one. This phenomenon was observed for the layered NiPS 3 , whose native XXZ-type antiferromagnetism (Neel temperature T N = 155 K) was suppressed upon electrochemical intercalation of the TBA organic ion in favor of a molecule-dependent ferrimagnetic state, as demonstrated by Tezze et al. [41] In particular, this work shows how the TBA ions can be electrochemically intercalated in TBA-NiPS 3 , and then easily exchanged by Cobaltocenium ions (Co(Cp) 2 ) in solution, leading spontaneously to the more thermodynamical stable Co(Cp) 2 -NiPS 3 intercalate (Figure 4c).
An increased spontaneous magnetization below 98 K was reported for the Co(Cp) 2 -NiPS 3 bulk intercalate, compared to the one found around 78 K for the TBA-NiPS 3 intercalate. Both organic-intercalates were characterized by a molecular-specific hysteric behavior with finite remanence and coercivity (Figure 4d). Since the nominal doping is set upon the initial electrochemical intercalation process and the cation exchange is a non-redox process, the doping for both intercalates is the same. Consequently, this two-step process could allow, in principle, to study a molecular dependence of the magnetic properties at a constant doping level. Moreover, the authors performed Raman spectroscopy to demonstrate that the same intercalation and ionexchange processes can be replicated for a NiPS 3 flake stamped on a SiO 2 /Si substrate, opening the way to the integration of intercalated magnetic materials into devices.
The same work shows how the parameters of the electrochemical intercalation process (electrolyte concentration and current) are crucial to finely control the introduced doping and therefore, the outcoming magnetic properties for both intercalates. The spontaneous magnetization shown in Figure 3d is detected for the crystallographic TBA 0.25 NiPS 3 phase and the [Co(Cp) 2 ] 0.25 NiPS 3 phase. For other phase compositions, such as TBA 0.32 NiPS 3 or [Co(Cp) 2 ] 0.32 NiPS 3 ), a paramagnetic behavior exhibiting no spontaneous magnetization was observed. Each crystallographic phase, characterized by its own interlayer distance, differs in the doping level due to the orientation-dependent molecular packing factor, leading to different magnetic properties.
Simultaneously, Mi et al. performed a similar study on the evolution of the magnetism of NiPS 3 upon electrochemical intercalation of different alkylammonium cations. [83] Guest species exhibiting different sizes were employed for establishing a connection between the magnetism and the molecular size. They discovered that the doping level, controlled mainly by the steric hindrance of the guest molecule, is one of the key points to modulate the magnetic ordering between Ni ions.
In particular, they observed a spontaneous magnetization arising below 100 K and a hysteretic behavior only for THAintercalated NiPS 3 , which is characterized by the lowest doping among all the intercalates. Interestingly, the reported TBA-NiPS 3 Figure 5. a) The chiral molecules R--methylbenzylamine and S--methylbenzylamine are intercalated into TaS 2 to make it a spin filter. b) Scheme of the vertical device used to measure the spin filtering effect, composed of the intercalated TaS 2 sandwiched between the magnetic material Cr 3 Te 4 and Au. Variation of the electrical current measured in the intercalated TaS 2 device as a function of the magnetic field. Specular results are measured for TaS 2 intercalated with c) left-handed and d) right-handed molecules. e) Intercalation of 4-(4-anilinophenylazo)benzenesulfonate in CoAl-layered double hydroxide yields an IC in which the interlayer distance can be modulated through a thermal stimuli. a-e) Reproduced with permission. [45] Copyright 2022, Nature Publishing Group. f) The thermally activated breathing-like property of this IC grants access to tune the magnetic properties of the layered compound. Side view of the representations of the organic ICs not to scale. Reproduced with permission. [91] Copyright 2015, Royal Society of Chemistry.
in this work does not show ferrimagnetism, contrarily to what was found in the study of Tezze et al. This apparent inconsistency can be attributed to the different parameters followed in each research process, which results in different doping levels even when the same guest molecules are adopted.
While these works indicate how sensitive the magnetic properties are to the conditions of the intercalation process, they also demonstrate that by mastering the intercalation parameters it is possible to finely tune the magnetic properties of the hybrid superlattice.
Finally, we would like to highlight that the increase in Curie temperature recently recorded in ionic-gated flakes of 2D magnets [14,15] is related not only to the electric field effect, but also to the intercalation of the (inorganic) cations present in the polymer electrolyte.

Outlook
The examples discussed in the previous sections show how the intercalation of organic molecules can introduce new physical properties in layered vdW materials. In all the exhibited cases the different molecules always have the same role, as they are used as i) dopants to modulate the charge carrier density and ii) spacers to increase the interlayer distance. However, the chemical flexibility of molecular synthesis offers the possibility to design guest species incorporating functional groups displaying additional capabilities, such as optical or magnetic, that could be conferred to the resulting ICs. We are convinced that this approach represents a viable route to provide vdW compounds with unique capabilities typically exclusive to molecular compounds, and to obtain novel hybrid compounds with on-demand physical properties. While this approach remains largely unexplored, there are a few works that demonstrate its potential. This section, based on these papers, aims to arise the curiosity on the prospect of this approach to design materials.
As a first example, we highlight the intercalation of chiral organic molecules in TaS 2 , which yields an IC with spin filtering properties. [45] Chiral compounds have been studied in the context of spin transport since 1999, [84] when it was first demonstrated that chiral molecules favor the transport of electrons possessing a spin oriented along a specific direction, blocking the electrons with spins in the opposite direction. The favored spin direction depends on the molecular handedness, and it is opposite for leftand right-handed molecules. This effect is often referred to as chiral-induced spin selectivity (CISS), [85][86][87] and it has been explored in several works focusing on both organic molecules [88,89] and inorganic materials. [90] Qian et al. [45] have recently shown how the same spinfiltering effect could be provided to a vdW material by intercalating the chiral molecules R--methylbenzylamine and S--methylbenzylamine into TaS 2 (Figure 5a,b). [45] This was demonstrated by measuring a device composed of TaS 2 flakes intercalated with left-and right-handed molecules sandwiched between a gold top contact and Cr 3 Te 4 as a ferromagnetic bottom www.advancedsciencenews.com www.advphysicsres.com electrode (Figure 5c). Figure 5d,e displays the current through the vertical stack as a function of an applied magnetic field. When the magnetization of Cr 3 Te 4 is switched, the spin polarization of the current injected in the chiral-molecule-intercalated TaS 2 is reversed. This causes a change in the current flowing in the device, demonstrating that the intercalated TaS 2 favors the transport of spins oriented along a certain direction, in other words, it acts as a spin filter. The high-and low-resistance states are opposite for stacks incorporating TaS 2 intercalated with right and left handed molecules, confirming the dependence of the spin-filtering effect on the chirality of the IC [45] (Figure 5d,e). This work demonstrates how a specific molecular capability-in this case the spin filtering related to the structural chirality-can be provided to a layered material via intercalation of organic compounds.
Another example of molecule-derived functionality in an IC is shown in Figure 5f. In this case, a layered ferromagnetic CoAl double hydroxide is intercalated with a thermoresponsive molecule which can be switched between two isomers by changing the temperature. [91] It was found that this thermally triggered tautomerization causes not only a reversible change in the interlayer distance of the layered material, resulting from the different size of each isomer, but also a modification of the magnetic properties (Figure 5f). In this way, the thermal responsivity of the guest molecule is transferred to the magnetism of the layered compound. Following a similar approach, a previous work showed that the optical response of a photochromic molecule intercalated into MnPS 3 can be provided to its magnetic properties. [73] In particular, when irradiated with light of specific wavelengths, photochromic molecules switch between two different isomers, yielding a light-controllable change in the magnetic properties of the intercalate MnPS 3 . [73] While these works show the potential of the intercalation of functional organic molecules in layered materials, we think that other molecular capabilities (e.g., magnetic [92,93] ) can be targeted and transferred to layered materials.
We would like to highlight that another area that warrants further exploration isthe self-assembly of the organic intercalants in the vdW gap. In spite of the relevance it may have in determining the properties of the ICs, this aspect remains largely unexplored. Indeed, the molecular intercalants are charged species which lie in close contact with the inorganic sheets in the layered structure. If orderly arranged, these cations could generate a periodic modulation of the electrostatic landscape of the layered material, which could give rise to in-plane superlattice effects. We emphasize that ICs are often called superlattices, [31,45] to stress that organic and inorganic layers are alternated in a periodic way in the direction orthogonal to the basal plane of the crystal. However, the superlattice effects introduced by the self-assembled intercalants would be in the plane, coming from the atomically precise alignment of the inorganic layers and the organic self-assembled structures. [94] In this regard, the in-plane superlattice effect may have a similar effect to that reported for Moiré patterns formed in vdW heterostructures, [22,95] which have been shown to deeply modify the physical properties of layered materials. [96,97] Moreover, the molecular self-assembly can be controlled by the experimental conditions employed during the intercalation process and by selecting the intercalant cation, using well-established concepts of supramolecular chemistry.
While the effect of molecular superlattices in ICs has not yet been explored, there are a few works demonstrating that molecular cations form ordered self-assembled structures in the vdW gap. For instance, Liu et al. [98] recently explored the molecular arrangement of cobaltocene (Co(Cp) 2 ) intercalated into SnSe 2 . Through scanning tunneling microscopy, they found that Co(Cp) 2 molecules lay on the surface of the atomic sheets preferentially arranged with the metal-to-ring axes parallel to the SnSe 2 planes . Their findings indicate that the organic intercalants generate an ordered superstructure overlaid to the SnSe 2 basal plane.
Finally, we should highlight that despite its interest for the materials science community, the intercalation of organic molecules is not a chemically well controllable process. So far, it is very difficult to predict whether a specific host material is prone to be intercalated by a given molecular guest, as the thermodynamic and kinetic details that rule the process remain not fully understood. Therefore, the search for novel physical phenomena in intercalated compounds is typically guided by the vast literature available on the topic rather than by predictive concepts derived from thermodynamical considerations. Additionally, ICs are typically characterized by a specific stoichiometry of the organic guest species, which is difficult to predict and modify on purpose. In some cases, a control over the crystal structure and stoichiometry of ICs has been demonstrated by acting on the intercalation parameters, [41] but such control is achieved thanks to the chemical intuition of materials scientists more than to a priori arguments. We think that the recent interest in the molecular intercalation will motivate a more detailed study of the thermodynamic and kinetic factors at play, possibly leading to a higher degree of control over the process.
In conclusion, this perspective reviews selected works supporting the potential of organic-ion intercalation as a tool to obtain functional materials by design. In particular, we have focused on those studies which employ this approach to tailor superconductivity and magnetism. Additionally, we have highlighted how organic ion intercalation can be used to impart molecular functionalities to layered materials and to generate organic-inorganic superlattices. While these possibilities remain largely unexplored, they hold the potential to introduce and control a wide range of physical phenomena in vdW materials.
www.advancedsciencenews.com www.advphysicsres.com Luis E. Hueso is an Ikerbasque Research Professor, leader of the Nanodevices Group and scientific director of the Unit of Excellence "Maria de MXiztu" at CIC nanoGUNE. Since his PhD in 2002 he has worked at the University of Cambridge (UK), the Italian National Research Council and the University of Leeds (UK). His current research interests are very wide, including but not limited to 2D and organic electronics and spintronic devices, as well as advanced nanofabrication.