Tetrahedral Transition Metal Chalcogenides as Functional Inorganic

Perspective pubs.acs.org/cm

Tetrahedral Transition Metal Chalcogenides as Functional Inorganic Materials† Xiuquan Zhou and Efrain E. Rodriguez* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States ABSTRACT: We provide a perspective on a series of materials that we have termed tetrahedral transition metal chalcogenides (TTMCs), which have a common layered structural motif that could carry novel functionalities on account of the d-orbital filling. While strong covalent bonding predominates within the TTMC layers, the layers themselves can be held together by van der Waals interactions, Coulombic forces, or even hydrogen bonding. Although similar to transition metal dichalcogenides (TMDs) in some respects, TTMCs have been less explored in their synthesis and materials properties. Unlike TMDs where the transition metal is typically tetravalent and in a 6-coordinate environment, TTMCs contain the transition metal in a tetrahedral environment and in a low valent state of I or II. Structurally, TTMCs crystallize in tetragonal or orthorhombic structures on account of the square lattice formed by the transition metal centers. We present their electronic structure and resulting properties, including superconductivity, metallic conductivity, and itinerant ferromagnetism. We briefly discuss their synthesis and the intercalation chemistry that can be performed to form new phases. Like TMDs, they also offer the tantalizing opportunity to be manipulated toward the formation of two-dimensional (2D) structures. Finally, we provide a future outlook on their development and the possibility that they could be integrated with other 2D materials to form novel heterostructures.

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INTRODUCTION In this perspective, we discuss materials we term tetrahedral transition metal chalcogenides (TTMCs) that display unique properties on account of their chemical compositions, unique bonding, and layered structures, which consist of the metal square lattice as the fundamental structural motif. These materials typically accommodate first row transition metals from group 7 to group 12. Since they contain 3d transition metals such as iron, cobalt, and manganese, TTMCs can therefore display correlated electron behavior leading to electronic instabilties that induce phase transitions and even superconductivity.1 Following the great discovery by Hosono et al. that the layered pnictide LaO1−xFxFeAs is an unconventional superconductor,2 TTMCs came into prominence in 2008 when several groups soon demonstrated that FeSe itself was superconducting without any need to charge dope it.1,3−7 The progression of the iron-based superconductors is a successful story in how directed synthesis following a discovery led to different families of superconducting compounds, and both the chalcogenides8−12 and pnictides13−16 helped push the field toward a deeper understanding of the possible mechanisms. Interestingly for the case of FeSe, the critical temperature Tc can be increased from 8 K up to 37 K with applied external pressure.17 Chemical techniques can supplant physical ones as well, and several groups found that

intercalating FeSe to form phases such as Ax(NH3)x(NH2)yFeSe through liquid ammonia reactions could raise the Tc even higher to 43 K.18−23 Although they have a name similar to the title compounds, transition metal dichalcogenides (TMDs) such as MoS2 or TaSe2 are not included in the category of TTMCs. However, it is useful to compare the two categories as they may display similar physical properties24−29 and may be similar in their syntheses, chemical reactivity, and propensity for intercalation chemistry.30−34 TMDs have experienced renewed interest recently on account of the extraordinary properties when isolated either as single layers or as nanosheets.35,36 Much like the resurgent research on TMDs, we hope that this perspective rekindles an interest in TTMCs and their exploration for properties other than superconductivity. We provide both a brief review and a future perspective on the development of TTMCs and the role that materials chemists could have in pushing new frontiers through their exploration. The TTMCs we will be covering in this perspective belong to those with transition metals late in the 3d series, beginning with manganese and ending with zinc. The coordination around the metal is strictly tetrahedral, although the true local symmetry is typically D2h rather than Td. To date, only iron37−39 and cobalt40 have been found to form the binary Received: April 15, 2017 Revised: June 19, 2017 Published: June 21, 2017

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This Perspective is part of the Up-and-Coming series. © 2017 American Chemical Society

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DOI: 10.1021/acs.chemmater.7b01561 Chem. Mater. 2017, 29, 5737−5752

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Chemistry of Materials chalcogenides that fit under the TTMC umbrella, but ternary and quaternary chalcogenides with the key structural motif also include Mn, Ni, Cu, and Zn.41−46 In order to best introduce TTMCs to a wider audience, we have decided to compare and contrast them with TMDs, which are well-known in the physics, chemistry, and materials communities. In addition to helping the reader understand the unique structure and bonding of TTMCs, the comparison to TMDs will allow us to predict future directions for TTMC materials. Figure 1 shows the sections of the d-block elements

layered TTMCs are not comprised of hexagonal or trigonal crystal structures. Instead, the structures are tetragonal or orthorhombic, and the major structural motif consists of edge sharing MCh4 tetahedra as opposed to MCh6 polyhedra as found in TMDs. As shown in Figure 2, the structure of MoSe2

Figure 1. Comparison of the different sections of the periodic table that transition metal dichalcogenides (TMDs) and tetrahedral transition metal chalcogenides (TTMCs) each claim. The most important structural motif of each materials class is also presented.

Figure 2. (a) Example crystal structure of a transition metal dichalcogenide (TMD). The structures consist of metal cations in either trigonal prismatic or octahedral coordination, and a van der Waals gap is found between the chalcogenide layers. The metal sublattice is a simple 2D hexagonal lattice. (b) An example crystal structure of a tetrahedral transition metal chalcogenide (TTMC). The structure consists of metal cations in tetrahedral coordination and a smaller van der Waals gap than that found in layered TMDs. The metal sublattice is a simple 2D square lattice but can be distorted from the ideal.

in the periodic table that are typically found in each of the two classes. Also shown are the key structural motifs of the layered TTMCs and TMDs. Several features become apparent in Figure 1. First, TMDs tend to favor the early transition metals, which require that the metal be close to the tetravalent state. Second, TTMCs favor the late transition metals and are therefore more electron rich than TMDs. On account of the small interstitial position of the tetrahedral site, to date only first-row transition metals have been found to form TTMCs. We have divided the sections of this perspective to cover different aspects on TTMCs. We start with a description of their crystal structures, which allows us to easily discuss their bonding and electronic structure. Given that electronic instabilities in the solid state can lead to new bonding, we have also included a section on how band filling of the square nets can lead to instabilities that favor bond formation. A discussion of physical properties naturally follows from our discussion of electronic structure and bonding. The reader will also learn about the preparative methods for synthesizing the bulk TTMCs, nanosheets of TTMCs, and their intercalated phases. Finally, we end with a brief discussion on how these materials could be further explored by materials chemists, physicists, and materials scientists.

is compared with that of of FeSe for their simple binaries. In crystal chemistry, FeSe in its β-phase (Figure 2) adopts what is known as the anti-PbO type structure. Some simple crystal chemistry arguments help partially explain why early transition metals do not form TTMCs. A tetrahedral interstice in a close packed arrangement of sulfide or selenide anions would dictate that the metal cation radius be close to 0.42−0.45 Å (rC/rA = 0.225). This interstice is still too small for 4-coordinate Fe2+ or Ni2+ cations to fit in a tetrahedral interstice, but they are nevertheless smaller than the early transition metal cations. In fact, early transition metals tend not to form such low valent cations in ionic solids as evidenced by the lack of Shannon and Prewitt radii for such species.47 The fact that TTMCs feature low-valent transition metals with radii larger than allowed in a tetrahedral interstice of a close packed structure suggest that electronic effects must also play a role in their stabilization. In the case of TMDs, polytypism is quite common and either rhombohedral or trigonal structures arise due to different anion packing sequences leading to the so-called 1T, 2H, or 4Hb (a mixture of 1T and 2H) structures.29 In the layered TTMCs, no such polytypism is available since the anions are in a cubic close packing sequence. However, upon changing the nature of the spacer between the layers in a TTMC, the stacking sequence of

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CRYSTAL STRUCTURES Our starting point is crystal structure since this will affect the dimensionality of the electronic interactions in these layered materials. Like TMDs, the layered TTMCs are also composed of a 2D array of transition metals sandwiched between two layers of chalcogenide anions. Unlike TMDs, however, the 5738


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Chemistry of Materials the metal cations can also be changed. For example, FeSe itself crystallizes in the space group P4/nmm (No. 129), but when it is found with a guest ion such as K+ between the layers, it can crystallize in the body-centered I4/mmm space group. TTMC compounds such as KCo2S2 have crystal structures that belong to either that of the ThCr2Si2- or BaZn2P2-type. Since the transition metals in TTMCs occupy the tetrahedral interstices, the choice for transition metals includes those that are divalent with a small cationic radius close to that of Fe2+ (≈ 0.63 Å for 4-coordinate). Therefore, the larger cations from the second and third row are likely too small to fit into the TTMC layers, and any ion to the left of Mn2+ (≈0.66 Å for 4-coordinate) is unlikely to be incorporated as well from simple crystal chemistry arguments. There are two major consequences for the bonding and geometry of layered TMDs and TTMCs on account of their crystal structures. First, in the case of the TMDs, regardless of the polytypism, the metal cations always form a hexagonal net that is 2D. In the case of the layered TTMCs, the metal cations comprise instead a square 2D lattice. Second, the coordination number for the metals in TMDs is always six and in TTMCs four. Both the hexagonal and square lattices have degeneracies in their electronic structure on account of their high symmetry. Of course, hexagonal vs square lattices will have different implications for the structure of the electronic band diagrams. For the differences arising from coordination, these are due to crystal field splitting energies from octahedral (or trigonal prismatic) and tetrahedral geometries for TMDs and TTMCs, respectively. The latter includes a smaller splitting energy, and the ranking of the d-orbitals in energy is therefore different between the two. Finally, we would like to include here TTMCs with more one-dimensional (1D) type character than those presented above and label them 1D-TTMCs to distinguish them. As shown in Figure 3, one can arrive at both single and double chain structures built of the similar edge-sharing MCh4

tetrahedra. These chains represent 1D slices from the 2D layers as shown in Figure 3, and such changes are stabilized by the presence of electropositive cations such as K+, Ba2+, or cationic sheets such as (LnO)+.48−55 So far, many of the same metals found for the layered TTMCs have been found for these chain compounds, especially iron. For example, superconductivity was recently found for the double chain compound BaFe2S3 when external pressure is applied.56 There are obvious similarities between the layered materials and the 1D chains such as edge-sharing MCh4 tetrahedra, and it has been suggested that similar physics may be present in both materials.52,55 Dimensionality is therefore the control parameter for the electronic and magnetic properties, and we will discuss those changes in the Physical Properties section.

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ELECTRONIC STRUCTURE While a metal can be defined as a material with filled electronic states at the Fermi level, the characteristics of those states can have profound impact on the resulting physical properties. One characterestic that we can use to further classify metals is the dimensionality of the extended solid; for example, a nearly twodimensional crystal structure can lead to the electronic states to exhibit highly anisotropic behavior. Early examples of such anisotropic metals include graphite where the layers interact through weak van der Waals interactions. In the extreme case, a single layer can be isolated (graphene) and exotic new states such as Dirac cones at the Fermi level (EF) realized. As materials chemists we can extend such categories to include other p-block elements and transition metal compounds57,58 and utilize their orbitals at the Fermi level to achieve new properties. The TTMCs offer a good platform for such exploration. The major differences between the electronic properties of TTMCs and those of TMDs arise from their differences in coordination geometry, metal oxidation state, and crystal structure. Whereas most TMDs are semiconductors, most TTMCs are metals or semimetals. Rough schematics of the electronic density of states (DOS) presented in Figure 4 help illustrate these differences. For the group 4 TMDs such as ZrS2 and HfSe2, the d0 electronic configuration leads to empty conduction bands that are of primarily d-character and the full valence bands of primarily chalcogen p-character (Figure 4). For the group 5 TMDs such as TaS2, a partially filled orbital does lead to metallicity, but this also causes an instability in the Fermi surface leading to a Peierls-type distortion29 and therefore semiconducting behavior once again (Figure 4). Unlike in TMDS, the d-block metal in TTMCs is formally in either the divalent or monovalent state and therefore more electronegative than the tetravalent metals in TMDs. Figure 4 provides a schematic of the DOS for a representative TTMC system. The lower band consists of σ-bonding between the transition metal and the chalcogen anion but is mostly composed of the chalcogen p-states. A pseudogap separates these mostly p-states from the nonbonding orbitals that consist of mostly metal d-orbitals, which traverse EF. The fact that EF lies in these predominately d-bands for for TTMCs containing metals of intermediate electronegativity gives them their interesting magnetic and electrical properties. At the high end of the conduction band, some of the p-states do mix with the metal’s d-states, which represent the σ-antibonding states between the metal and chalcogenide anions. This is especially true for TTMCs containing Cu(I) and Zn(II) in the layers, since the ionization potential is buried deep for these closed-

Figure 3. View of a single 2D sheet of a TTMC viewed down the caxis. If the sheet is fragmented along one direction, it can either form a single chain or double chain compound. An example single chain compound includes the oxidized KFeSe2 compound and example double chian compound is BaFe2S3, which has iron in the divalent state. 5739


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Figure 4. Schematics of the electronic density of states (DOS) for group 4 and 5 layered TMDs, layered TTMCs, and chain-like TTMCs. Only the bands near the Fermi level (EF) are presented to illustrate the main features where the metal d-bands (in blue) mix with the chalcogenide p-states (in yellow). The TMDs are typically semiconductors with a band gap (Eg), but the group 5 TMDs can be metallic on the filling of the lowest d-band. For the layered TTMCs, their interesting magnetism and superconducting properties arise from the fact that the EF crosses the d-band. The chainlike TTMCs are semiconducting due to a phenomenon known as orbital-selective Mott insulators, whereby only some of the d-orbitals are split in energy while the others remain broad and itinerant.

Figure 5. Illustrations of both band structures of representative TMD and TTMC: (a) The dispersion curves for a TMD, MoS2, both in the bulk and as a single layer, (b) the labeling of the Brillouin zones for a hexagonal system as often found for TMDs, (c) the dispersion curves for a TTMC, FeSe, and (d) the labeling of the Brillouin zone for the primitive tetragonal structure of FeSe. Dispersion curves in (a) and (c) are produced with permission from refs 60 and 59, respectively. Copyright 2008 and 2011 American Physical Society.

In the case of FeSe, the bands arising from the d-orbitals also form the conduction band, which has a similar bandwidth of 4 eV. However, the difference with the case of the MoS2 is that the Fermi level is located midway in the mostly d-band for FeSe as shown in Figure 5b. The most interesting features are with respect to the BZ directions, which include the Γ-, Z-, and Mpoints on the dispersion diagram. The curves from Γ to Z are flat (i.e., nearly dispersionless) close to EF so that the Fermi surface is therefore nearly 2D.59 The bands arising mostly from the dxz and dyz orbitals are degenerate near the Γ-point, and these are not completely filled so as to constitute the hole pockets of the 2D Fermi surface. At the M-point, the dxz, dyz, and dxy orbitals form bands that are just beneath EF so that they constitute the electron pockets of the Fermi surface. These 2D hole and electron pockets lead to the interesting magnetic and superconducting properties of the iron-based superconductors.

shell systems. For 1D-TTMCs, the further reduction of dimensionality from the 2D layers typically causes a band gap to open as shown in Figure 4 from electron correlation effects. The special properties of both TMDs and TTMCs requires some understanding of the dispersion of the electronic bands in energy with respect to crystal direction. Figure 5 displays a comparison of the dispersion curves for MoS2 and FeSe; to guide the reader on the significance of the several points in the dispersion curves, the Brillouin zones (BZ) for the hexagonal and tetragonal crystal systems are also presented. As shown in Figure 5a, the bands above the Fermi level consist mostly of the Mo d-orbitals and span approximately 4 eV in energy. Interestingly, the band gap is indirect in bulk MoS2 but goes to direct gap (at the K point) upon forming a monolayer due to the changes in the dispersion of the S pz orbitals 5740


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Figure 6. (a) Single MCh4 tetrahedron where M is a transition metal and Ch is a chalcogenide anion and a possible molecular orbital diagram for this building block of the TTMC. (b) Schematic of how those individual MOs from the single tetrahedron might be spread in energy as they form bands in an extended structure. (c) Electronic density of states of FeSe as an example from DFT calculations. The juxtaposition of the DOS next to the schematic of the band is to demonstrate that the Fermi level falls in a band and that the frontier orbitals are those in the t2* and e-manifolds of the Fe(II) metal centers.

a pseudogap in the DOS, then the compound is very stable and this would be the solid-state equivalent for satisfying the 18electron rule for a molecular complex. Therefore, how the electronic levels are occupied is important for stabilizing TTMCs, and the early part of the transition metal series is likely too electron poor to fill in sufficient DOS, so that other structure types (e.g., NiAs, pyrite, CdI2) are likely more stable. In extended solid structures, bond formation can be further induced by electronic instablities. The chemical bond is a realspace concept, and we typically imagine how electrons are distributed between atoms in a molecule; extended structures can provide a challenge to this concept since their electronic structure can be best understood in reciprocal space. Fortunately, the works of Hoffmann61 and Burdett62 provide some guidance on how to conceive of the chemical bonding in reciprocal space and vice versa. In particular, Hoffmann has set a theoretical basis for approaching bonding in 2D sublattices as a way to make universal descriptions across various prototypes such as the ThCr2Si2-, PbO-, and PbOCl-type structures.63−65 Furthermore, Hoffmann et al. demonstrated that electron rich (and hypervalent) compounds containing metalloids such as Sb are susceptible to electronic instabilities that lead to charge density waves (CDWs) or chemical bonding in extended 1D, 2D, and 3D structures.66,67 These theoretical concepts have been applied to other inorganic materials with square net motifs, and indeed electronic instabilities have been observed in them.68−70 It is interesting to try this approach toward electron rich metal chalcogenides and especially TTMCs since they feature such square motifs. A heuristic strategy for understanding bonding instabilities in infinite square nets is to consider the molecular version of a square lattice, cyclobutadiene.61,64 The molecular orbital (MO) diagram of π-type bonding in cyclobutadiene (Figure 7) shows that the doubly degenerate states are half occupied leading to a triplet spin state. By occupying this doubly degenerate state, cyclobutadiene is Jahn−Teller active can distort from D4h to D2h symmetry via a vibrational mode, maximizing bonding by occupying a lower lying state (Figure 7). Likewise, in a square planar lattice, occupation at the M-point (see section above) can cause an instability that would lead to bonding known as a

In the next section, we discuss the implications for chemical bonding in FeSe and related compounds from the occupation of these high-symmetry dispersion bands.

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CHEMICAL BONDING AND ELECTRONIC INSTABILITIES IN SQUARE NETS Before presenting the physical properties of TTMCs, a discussion of the bonding in the context of electronic structure will shed light on their interesting behavior. At least four different types of bonding occur in these materials: covalent, ionic, van der Waals, and hydrogen. The weaker forces such as van der Waals and hydrogen bonding play an interesting role in the intercalation chemistry of TTMCs and will be discussed in more detail in the Intercalation Chemistry section. The ionic forces are mostly important in compounds such as the ternary phases (e.g., KCo2Se2) and the 1D-TTMCs (e.g., KFeSe2). Here the cations located between the sheets or chains serve to both stabilize the structures and to electron dope into the tetrahedral motifs. More important for understanding the interesting magnetic and electrical behavior is the covalent bonding between the transition metal and the chalcogenide anion, and we discuss this next. To understand the covalent bonding in the 2D motifs such as in FeSe, it is important to return to the electronic density of states and their corresponding dispersion curves. First, the electronic DOS can be understood from a ligand field point of view. The most basic unit of a TTMC is the MCh4 tetrahedron, so we construct a simple molecular orbital (MO) diagram for a generic MCh4 unit as shown in Figure 6a. If we assume that the bonding between divalent iron and the Se2− anions is mostly of sigma-type, then the typical two-below-three ligand field splitting results. For an 18-electron molecular complex, the filled states and unfilled states would therefore be constituted by the e-manifold (dx2−y2 and dz2) and t2*-manifold (dxz, dyz, dxy). In Figure 6b, we spread these states into bands in order to explain the predominant character of the electronic DOS. If we take FeSe as the prototypical TTMC, we can see that for a Fe2+ with a 3d6 configuration the Fermi level crosses primarily the t2* states of the transition metal (Figure 6c). Since the EF fills in 5741


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Figure 8. Dispersion curves of the electronic states of FeSe focused on three high-symmetry points. The M-point is filled by electrons and the Γ-point by holes at the Fermi level. These electronic states are sketched out in real space by the representation of certain d-orbitals on a square lattice. The Γ-point corresponds to the most antibonding configuration and the M-point to the most bonding one.

dispersion curves shown in Figure 8 demonstrate, the M-point is therefore lower in energy than the Γ-point, leading to electrons occupying the most bonding configuration and holes the most antibonding one. Turning to the TTMCs, we can examine whether such CDWs manifest in any crystallographic distortion. Fe1+xTe undergoes a tetragonal-to-monoclinic phase transition near 70 K,71 which would support the zigzag chain picture of Figure 7. The extent of this deformation depends on how much electron doping occurs from the interstitial iron, given by x in Fe1+xTe, which also has a profound effect on the observed magnetic and electrical properties.72 Concominant with the transition near 70 K is the formation of ferromagnetic chains, which are themselves antiferromagnetically coupled to their nearest neighbor chains. In the selenide analogue, FeSe does not undergo the full monoclinic distortion but rather stops at an orthorhombic one,4,6 where it remains superconducting below this structural transition. Interestingly, Hoffmann predicted that the orthorhombic space group Pmmm would be the intermediate phase from tetragonal P4/mmm to monoclinic P21/m according to group−subgroup relationships.64 In mackinawite FeS, which has a much lower Tc of 4 K, no deformation from tetragonal symmetry is observed down to 4 K.73 More work on how electron doping controls the formation of either CDWs or superconductivity should remain a topic of great interest.

Figure 7. Electronic instability inherent in the square molecule cyclobutadiene. In the molecular case, the middle manifold for pibonding is doubly degenerate. This degeneracy leads to a Jahn−Teller instability and the molecule easily distorts to a parallelogram of D2h symmetry. Likewise, occupation of a highly degenerate state in an infinite square lattice is subject to an instability. Three distortions are predicted by Hoffmann et al. in such a case including tetramers, armchair chains, and zigzag chains.

Peirls distortionthe solid state version of Jahn−Teller. In the solid state, just as in the molecular case, one tries to maximize bonding by distorting the structure so as to spread the antibonding and bonding bands further apart.61 Three possible distortions that maximize bonding for square nets are shown in Figure 7, which include tetramers, zigzag chains, and armchair chains. As in a molecular complex, solids will also try to maximize bonding if the degeneracy is too high at the frontier orbitals. The dispersion curves provide more detail on the nature of the bonding within the 2D layers that may not be apparent from the electronic DOS alone. As explained previously, the dispersion of the d-bands is essentially flat between the Γ- and Z-points for FeSe, which implies that the covalent interactions are mostly constrained within the 2D sheets. Occupation of the M-point by electrons and holes in the Γ-points leads to nesting of the Fermi surface that could lead to various CDW instabilities (e.g., zigzag chains and tetramers) as mentioned previously. Figure 8 shows a simple real-space visualization of the frontier orbitals that helps explain the shape of the key dispersion curves. In the case of FeSe, we have focused on the dxz and dyz which should be degenerate for the square lattice and are occupied for the d6 Fe(II) centers. When these orbitals are in phase with each other they constitute the Γ-point, which is the most antibonding configuration. When the orbitals are 180° out of phase with their neartest neighbors, they constitute the M-point, which is the most bonding configuration. As the

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PHYSICAL PROPERTIES From an understanding of the electronic structure discussed in the previous section, we can now anticipate some of the physical properties that make the TTMCs and TMDs interesting to study in the first place. In the case of TMDs, as discussed previously many of the group 4 and 6 compounds are semiconductors with band gaps ranging from 1 to 2 eV. This has made them popular for the study of a wide ranging number of applications including photovoltaics, photocatalysts, optoelectronics, and components in transistor-type devices.35,36,57,58 As shown in Figure 5, when isolated as a single layer MoS2 becomes a direct band semiconductor with a larger band gap, which has implications for applications in solid state lighting, for example.60 The group 5 TMDs have displayed 5742


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Chemistry of Materials other phenemena by virtue of only half filling the dz2 band such as superconductivity and charge density waves.27,28,74−76 In TTMCs, several groups have discovered that the Tc of FeSe can be increased even further when isolated as a single layer on a SrTiO3 (STO) substrate.77−80 Indeed, the record thus far for the arsenide-based systems remains approximately 55 K in SmOFeAs, so the chalcogenides have now surpassed them. The report by Ge et al. found a Tc close to 100 K for FeSe/STO,80 while another study estimated the Tc to be closer to 65 K.79 The discrepancy between these works is important since the one reported by Ge et al. claims a Tc above 77 K, or the boiling point of N2, and would therefore constitute the only other material besides the high-Tc cuprates to break the liquid nitrogen barrier at ambient pressures. The topic as to the real Tc of single layer FeSe remains an open question, but what is evident is that the ability to prepare and measure the properties of single layer FeSe was instrumental to possibly raising the Tc into the vicinity of the high-Tc cuprates. The telluride analogue to FeSe has not been found to be superconducting but rather antiferromagnetic at low temperatures accompanied by a structural phase transition. In Fe1+xTe the x indicates an excess of interstitial iron located between the FeTe sheets and can vary between 3% to 16%.81 It is virtually impossible to prepare Fe1+xTe without some interstitial iron, which has implications for the magnetic and superconducting properties. By substituting either Se or S onto the Te site, Fe1+xTe itself can be made superconducting, but its Tc remains stubbornly fixed to 14 K regardless of the form of anion substitution.8,9,11,12,82 What is interesting about Fe1+xTe and its superconducting variants is that the nature of its magnetism and transport properties are quite informative about the iron chalcogenides in general. Under a certain threshold value of x, Fe1+xTe goes from a semiconductor to a metal around 70 K, and this is accompanied by an antiferromagnetic ordering that is described as a double stripe formation. Upon increasing x, the 70 K transition is suppressed to 50 K and the compound remains a semiconductor due to mostly scattering from spin states.72 Suppressing the coupled antiferromagnetic and crystallographic transitions induces superconductivity in Fe1+xTe, and in this manner it resembles the iron arsenide based superconductors. Very recently, Lai et al. demonstrated that the metastable sulfide FeS with the FeSe-type structure is also a superconductor.83 For some time, this metastable form of FeS also known as mackinawite was thought to be a ferromagnetic semiconductor.84,85 However, Lai et al. showed that FeS prepared via hydrothermal routes is indeed superconducting,83 while Borg et al. demonstrated that deintercalation of preformed KxFe2S2 crystals could also lead to superconducting FeS.73 The Tc remains low in this compound, between 4 and 5 K, and pressure actually works toward decreasing Tc rather than enhancing it as in FeSe.73 The sulfide system, however, shows promise in that intercalation chemistry may be another route toward raising Tc up to 8.5 K.86 The 1D-TTMCs have shown a variety of interesting magnetic and transport behavior. For example, BaFe2Se3 has been described as an orbital-selective Mott phase (OMP)53,87 while BaFe2S3 as a superconductor under extreme pressures.56 While the term OMP is recent, it is related to that of a Mott insulator in that the partially filled d-band is split due to Coulombic onsite repulsions. In the case of an OMP, only some of the d-orbitals are sufficiently affected by this Coulombic repulsion to split above and below EF. The rest

remain occupited at EF, and the compound therefore remains metallic, although it has often been termed a “bad metal”. Other 1D-TTMCs such as Ln2O2Fe2OSe2, are indeed found to be Mott insulators, although the band gap is quite small.55 On account of their correlated electron effects, these OMP and Mott-insulating 1D-TTMCs remain enticing parent compounds for superconductors. Although most of the focus has been on the iron-based materials, ternary TTMCs such as AxCo2Se2 where A is typically an alkali metal have been known even before the 2011 discovery of filamentry superconductivity in the A1−xFe2−ySe2 phases.90 Greenblatt et al. had found a majority of both sulfide and selenide phases for Co, Ni, Cu, and their solid solutions.41−43 The Co phase tends to be ferromagnets with Curie temperatures TF near 100 K,91,92 while the Ni phases are Pauli paramagnetic metals with some displaying superconductivity with a Tc < 1 K.46 As the electronegativity and number of valence electrons increases from Fe to Cu, EF continuously rises with respect to the valence band. Following a rigid band model, for Cu-based TTMCs the bands should be mostly filled except the top of the band near the Γ-point (Figure 8). This valence band contains 3d-states mixed in an antibonding configuration with the chalcogenide p states (Figure 4), and therefore many Cucontaining TTMCs are p-type semiconductors. Indeed, a large family of MOCuCh TTMCs (M = Bi or lanthanides), where PbO-type (M2O2)2+ and anti-PbO-type (Cu2Ch2)2− layers stack alternatively along the c-axis, are p-type semiconductors.88,89 In the case of LnOCuCh, EF is on the edge of the Γ-point, and the valence and conduction bands close to EF consist mainly of Cu 3d and 4s states and Ch p-states.88 The La-5d and O-2p are buried too deep in energy to contribute to the properties. Hence, the observed wide band gaps in the LnOCuCh series are mainly attributed to the nature of the Cu−Ch bonding in the (Cu2Ch2)2− layers as shown in the diagram of Figure 9. However, for the BiOCuCh case, the band gap is significantly narrowed due to the Bi 6s-states, which lower the conduction band edge.88 In general, as the hybridization of the Cu and Ch states increases from smaller Cu−Ch distances, the band gap is narrowed. This band gap engineering, which can be performed with the Cu-based TTMCs, is nicely summarized in the work of Clarke et al. shown in Figure 9.89 In contrast to the MOCuCh series, ThCr2Si2-type layered TTMCs such as BaCu2S2 display metallic conductivity.45,93 BaCu2S2 was found to be Pauli paramagnetic below 300 K,45 and its electrical resistivity decreases with temperature.93 However, this metallic behavior has been explained as arising from heavy hole-doping due to copper vacancies. Zhang et al.94 reported that the resistivity of BaCu2S2 was significantly lowered after partially replacing Ba2+ with K+ cations, effectively increasing the hole carrier concentration. Thus, the conductivity of K0.35Ba0.64Cu2S2 is an order of magnitude higher than that of K0.2Ba0.8Cu2S2. Interestingly, upon complete substitution of Ba2+ by K+ in the selenide analogue, the edgesharing CuSe4 tetrahedra distort and the symmetry is reduced from tetragonal (I4/mmm) to orthorhombic (Fmmm).95 The Cu−Cu distance decreases from 2.857 Å45 in BaCu2Se2 to 2.841 and 2.827 Å95 in KCu2Se2. Likely, the greater overlap between Cu-3d orbitals increases the instability of the Cu-based square lattice leading to a structural distortion not unlike that in other TTMCs described previously. 5743


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TTMCs are often carried out using Nb tubes or the double quartz ampule technique, where a sealed ampule containing reactants is enclosed in a bigger ampule. Zhang et al.94 have prepared KxBa1−xCu2S2 using mixed KSx and BaS fluxes, which allow single crystal growth at moderate temperature (450 °C). Greenblatt and co-workers41−43 have prepared solid-solutions of AM1M2Ch2 (A = alkali metal, M1 = +1 metal ion, and M2 = +2 metal ion) by mixing the respective metal carbonates or oxides and heating them to 800−900 °C under a stream of CS2 or H2S gas. For selenides, elemental Se is utilized instead due to the difficulty and hazardous nature of H2Se gas. In those reactions, elemental Se is added to the metal mixtures, and they are heated to 900−1000 °C under flow of 5% H2 and 95% N2 gas. For both S and Se, M1 can host Li, Cu, and Ag, while M2 can accommodate Mn, Fe, Co, Ni, and Zn.41−44,99 Although the syntheses of 122-type TTMCs are fairly straightforward using solid-state methods, low-temperature solution methods have not been as successful. Only Zhou et al.86 has reported synthesis of KxFe1−yS2 and KxFe1−ySe2 under hydrothermal conditions using concentrated KOH solutions. Low-temperature solution methods, however, may be crucial toward preparing new metastable 122-type TTMCs, and this point is better illustrated by synthesis of binary TTMCs. There are only five known anti-PbO type binaries, FeTe, FeSe, FeS, CoSe, and CoS.1,37,40,96 Among them, only FeTe and FeSe are thermodynamically stable and can be prepared by direct elemental reactions. Since FeTe melts congruently, single crystals can be grown by slow cooling from 950 °C. Similar to many TMDs, there is always interstitial Fe present, and it can be as high as 20%.71,72,81,100−102 and low as 5% by oxidative deintercalation using I2 vapor.81 Compared to FeTe, crystal growth of FeSe is much more difficult since its melting point is higher than the phase region of the anti-PbO-type phase. Therefore, instead of cooling from a melt, single crystals of FeSe are usually grown from LiCl/ CsCl salt flux103 or chemical vapor transport (CVT) using KCl/AlCl3.104 Larger crystals up to a few millimeters in the basal plane direction can be obtained using CVT (Figure 10). However, both methods require extended annealing between 300 and 400 °C followed by quenching. Several groups have utilized solution-type methods, however, such as solvothermal synthesis and refluxing in solution to prepare FeTe and FeSe.105,106 Tetragonal FeS is not thermodynamically stable in high temperature reactions, so it cannot be prepared by reacting elemental Fe and S under conventional solid state conditions. Although tetragonal FeS is found in a mineral named mackinawite, the naturally occurring minerals consist of Ni and Co impurities with a large number of vacancies to stabilize the anti-PbO-type structure.37,107 Previously, magnetic and semiconducting forms of mackinawite FeS were prepared by room temperature aqueous conditions.84,85,108 Superconducting FeS can only be made via hydrothermal reactions. Lai et al.83 first reported a Tc of 5 K in FeS powders prepared by reacting Fe powder and Na2S hydrothermally at mild temperatures between 100 and 140 °C. Borg et al.73 later reported single crystals of FeS prepared by topotactic conversion of KxFe1−yS2 single crystals previously grown from a melt (Figure 11). Under reductive hydrothermal conditions, Fe vacancies in KxFe1−yS2 crystal were replenished, so that the new FeS single crystals were superconducting whereas the precursor crystals were semiconducting.

Figure 9. (top) Schematic band structures of (a) BiCuOS and (b) LaCuOS built on the basis of ultraviolet photoelectron spectroscopy (UPS), optical measurements, and DFT calculations. AB, NB, and B denote antibonding, nonbonding, and bonding states, respectively. Relative positions of all the states are based on the vacuum level. Reprinted with permission from ref 88. Copyright 2008 The American Chemical Society. (bottom) Band gaps of representative MOCuCh TTMCs and related compounds as a function of the basal plane lattice parameters. Reprinted with permission from ref 89 . Copyright 2008 The American Chemical Society.

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SYNTHESIS AND CRYSTAL GROWTH Given the vastly different nature of thermodynamic stability among TTMC structural families, we group the syntheses of TTMCs into two categories: (1) preparation of materials with the ThCr2Si2-type (or 122-type) structure41−44 and (2) preparation of binary phases with the anti-PbO-type structure.1,37,40,96 The anti-PbO-type phases consist tetrahedral layers held by van der Waals interactions, whereas in the 122type TTMCs retain the 2D layers but now as anionic (MCh)δ− motifs held together by cations, typically from alkali or alkaline earth columns. The former usually crystallize in a primitive tetragonal setting, whereas the latter are stacked in a bodycentered fashion. As a result, the 122-type TTMCs usually exhibit larger cohesive energies and are therefore the thermodynamic product from high temperature reactions. In contrast, binary TTMCs more easily decompose or undergo structural reconfiguration at higher temperatures, and some are only metastable products.73,92 Since most 122-type TTMCs are thermodynamically stable, they can usually be prepared by direct reaction of the elements or alkali metals with a transition metal chalcogenide precursor. In addition, single crystal products can be obtained by slow cooling of homogeneous melts from high temperatures (often >1000 °C).90,97,98 However, because alkali metals are very corrosive to quartz ampules, single crystal growth of 122-type 5744


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conditions to avoid forming H2Se or H2S and modulate the reaction rate. By carrying out the reactions at room temperature, the amount of thermal energy was too low for a major structural reconfiguration from the tetragonal structure to the more common hexagonal phase with the NiAs-type structure. Hence, metastable and tetragonal forms of the binary CoCh phases were stabilized kinetically for the first time. Being analogues to the FeSe and FeS superconductors, the properties of CoSe and CoS are quite different as they displayed weak itinerant ferromagnetism with TF at about 10 K. Finally, we briefly discuss the safety hazards associated with the synthesis of TTMCs. Although the elemental forms of the chalcogens exhibit rather low toxicity due to their insolubility in water, their soluble compounds can be lethal.109−112 The LD50 is 15 mg/kg (body weight) for S and Se and 4.8−7.0 mg/kg for NaHS and Na2SeO3, respectively.109,111 The reported LD75 for Na2TeO3 and Na2TeO4 are about 2.25 and 20 mg Te/kg, respectively.113 Since NaHS can be formed by hydrolysis of Na2S, care should be taken during hydrothermal synthesis of sulfides. Selenite or tellurites can be formed by oxidation of their respective TTMCs. There are also hazards associated with the possible evolution of H2Ch gases during synthesis. Concentrations higher than 500−1000 ppm of H2S can cause death.109 H2Se is more toxic than H2S as it causes death in rats after 1 h of exposure of 10 ppm in air.111 Compared to H2S and H2Se, H2Te is less of a threat as it is extremely unstable under ambient conditions, decomposing to H2O and Te when exposed to air and moisture. To avoid exposure to toxic H2Ch gases, all chemistry including hydrothermal reactions should be carried out in a fume hood. Furthermore, strong basic conditions can be used to stabilize Ch−2 or HCh− species and avoid evolution of H2Ch. If pungent odors are detected, an oxidizer such as common household bleach can be sprayed into the environment, and H2Ch can be neutralized to the solid elemental forms.

Figure 10. Photograph of representative samples containing millimeter-sized platelet-shaped single crystals of tetragonal FeSe. The batch was prepared in a two-zone furnace with a smaller, constant temperature gradient of 350 to 390 °C over 2 weeks. As starting materials, Fe and Se powder (total mass ∼0.5 g) in a molar ratio of 1.1:1 were diluted in ∼5 g of a eutectic mix of KCl and AlCl3. Reprinted with permission from ref 104. Copyright 2016 American Physical Society.

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INTERCALATION CHEMISTRY Intercalation chemistry has been a longstanding technique for the manipulation of layered materials such as graphite and chalcogenides.114−116 Like TMDs, binary TTMCs can be hosts for intercalation chemistry due to their van der Waals gaps. Ever since Geballe and co-workers28,75,117,118 raised Tc of TMDs by inserting various bases, intercalation has been an effective method for studying superconductivity and related phenomena in layered materials. Here, we distinguish the intercalation chemistry from the solid-state methods used for 122-type TTMCs. As discussed previously, the 122-type TTMCs are usually formed from melts, and the formation of KxFe1−ySe2 with a large number of Fe vacancies coexists with several impurity phases.19,90,119 Therefore, our discussion will be focused on intercalation chemistry via chimie douce or soft chemical methods, where no drastic change occurs in the host materials. Clarke et al.18,21 successfully intercalated alkali metals (Li+, Na+, and K+) into premade FeSe powders using solutions of these metals in liquid ammonia. Interestingly, not only are the alkali metal cations inserted between the layers but also ammonia and metal amide moieties. Like AxFe1−ySe2, these new compounds also assume the ThCr2Si2-type structure. However, unlike in the AxFe1−ySe2 phases prepared by high temperature techniques, in the Li0.6(ND2)0.2(ND3)0.8Fe2Se2 superconductors the ammine occupies the A site while alkali cations occupy the interstitials sites as shown in Figure 12. Most interesting of all,

Figure 11. Rietveld refinement of XRD powder diffraction on ground single crystal samples. (a) Refinement of KxFe2−yS2 starting material, body-centered tetragonal unit cell (I4/mmm). (b) Refinement of FeS product, primitive tetragonal unit cell (P4/nmm). Fe (orange) is tetrahedrally coordinated to S (yellow), and K (purple) is coordinated between two FeS layers. The removal of potassium cations through hydrothermal deintercalation causes the unit cell to shift from bodycentered to primitive type. Tick marks corresponding to their respective phase are shown below the curve. Reprinted with permission from ref 73. Copyright 2016 American Physical Society.

Kinetically controlled methods for metastable TTMCs proved to be even more effective later as Zhou et al.40 prepared CoSe single crystal and CoS powders using a similar method. Since KxCo2Ch2 reacts with water to generate H2 gas at room temperature, the reactions were carried out under basic 5745


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between FeSe layers, increasing the interlayer distance from 5.52 Å5 up to 16.23 Å.125 Although the as-synthesized product only raised the Tc of FeSe to 38 K, postsynthetic annealing can further enhance the Tc to 41 K.128 Other than linear-chained amines, aromatic amines such as pyridine can also serve as a solvent for alkali metal intercalation, and the Tc can be enhanced up to 45 K.126 Interestingly, although the distances between FeSe layers are increased to about 10.37 Å to 16.23 Å by varying the amine adducts, the maximum enhanced Tc’s are fairly close. It is suggested that the optimal electron doping is more important than interlayer distances once a certain spacer threshold is met.129 In addition to polar basic solvents such as liquid ammonia and organic amines, water can also be utilized as a solvent for the preparation of intercalated TTMCs. Although not intuitively desirable for the intercalation of chalcogenides due to possible decomposition and oxidation in water, metal chalcogenides can be stabilized as long as the aqueous reactions are carried out under strongly basic conditions.130 Utilizing an excess of LiOH under hydrothermal conditions, Lu et al.131 managed to intercalate FeSe with neutral Li hydroxide layers. Remarkably, the Tc of FeSe was raised to 42 K, comparable to the intercalation of Li+ cations in liquid ammonia. Extensive high-resolution neutron and synchrotron X-ray diffraction studies later revealed that the correct stoichiometry of the hydroxide-intercalated superconductor is (Li1−xFexOH)FeSe. The hydrogen position was also determined by the neutron studies, which support hydrogen bonding as the mechanism holding these new phases together.92,132−134 Furthermore, the Fe2+ cations substituting for Li+ in the hydroxide layer are crucial to the enhanced superconductivity as they charge dopes the FeSe layer by approximately 15 to 18% per formula unit, consistent with intercalation of alkali cations in liquid ammonia and amines. In addition to its high-Tc superconductivity, (Li1−xFexOH)FeSe attracted significant attention for its magnetic properties as well. Lu et al.132 reported coexistence of antiferromagnetism and superconductivity from nuclear magnetic resonance (NMR) studies, although no long-range magnetic ordering was observed in their neutron diffraction data. Pachmayr et al.135 reported coexistence of ferromagnetic ordering and superconductivity in (Li1−xFexOH)FeSe, attributing this to a spontaneous vortex lattice from the proximity of the two types

Figure 12. Structure of Li0.6(ND2)0.2(ND3)0.8Fe2Se2 where the Li cations reside mostly between the Se2− anions and the ND3 and ND−2 moieties at the cell center. Reprinted with permission from ref 18. Copyright 2013 Nature Publishing.

upon intercalation, the Tc of FeSe increased from 8 K1 to 42− 44 K. This drastic rise in the Tc has been attributed to partial electron doping of the FeSe layer,18,20,21,120 which is key toward filling in the M-points of the band diagram (Figure 8). Using a similar liquid ammonia route, alkaline earth metals can also be intercalated into FeSe, despite achieving a slightly lower Tc than the alkali metals. For Sr and Ba intercalation, the Tc is found to be 35 K121,122 and 36 K,123 respectively. Nonetheless, the compounds are quite remarkable considering that they cannot be prepared by conventional solid-state methods. Other than liquid ammonia, organic amines such as traditional coordination ligands can also be utilized for cation insertion into FeSe.124−127 Ethylenediamine (EDA) is the most commonly used amine for this purpose, and Li- and Na-EDA intercalated FeSe also can raise Tc to 42−44 K.124,127 Even higher-order amines, such as hexamethylenediamine (HMDA), can cointercalate along with Li+ ions to form large spacers

Figure 13. Synthetic scheme for the intercalation chemistry of FeS with metal hydroxides and K+ cations via hydrothermal preparations. Reprinted with permission from ref 86. Copyright 2017 Royal Society of Chemistry. 5746


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Chemistry of Materials of layers. The vortex lattice was observed later by small angle neutron scattering (SANS) results by Lynn et al.,134 where a ferromagnetic ordering below 12.5 K was observed albeit for a small applied field. Although long-range magnetic ordering in (Li1−xFexOH)FeSe has yet to be found, it is suggested that the magnetic anomalies arise from the Fe2+ in the LiOH layer, which may be tunable by synthetic conditions.92,133,136,137 Similar to FeSe, FeS can also act as a host for guest species such as alkali cations with nonaqueous solvents. Guo et al.138 recently intercalated FeS with K+ in EDA. Unlike the its Se analogues, however, the EDA-intercalated sulfide is not superconducting. Rather it is a semiconductor exhibiting weak ferrimagnetism below 50 K. For intercalation under aqueous conditions, FeS appears to exhibit richer chemistry than FeSe (Figure 13), which can be attributed to its higher stability in concentrated base.86,139−141 (Li1−xFexOH)FeS can be prepared using a similar route for its Se analogue. Although first reported to be nonsuperconducting by several groups,139−141 Zhou et al.86 later found that it could be made superconducting through resistivity and magnetization measurements. As in its Se analogue, in (Li1−xFexOH)FeS the amount of Fe2+ in the hydroxide layer could affect the observed Tc. Consistent with intercalation studies of FeSe, higher electron doping (larger x) favors higher Tc in the FeS system, which could be enhanced up to 8 K. By carrying out the hydrothermal syntheses with NaOH instead of LiOH as the base, Zhou et al.86 managed to synthesize a new NaOH-intercalated FeS. This layered sulfide/ hydroxide with the composition of [(Na1−xFex)(OH)2]FeS resembles the natural mineral tochilinite (Figure 13),142 which consists of an iron square lattice interleaved with a hexagonal hydroxide lattice. Compared to (Li 1 − x Fe x OH)FeS, [(Na1−xFex)(OH)2]FeS is quite remarkable in that it is truly a layered heterostructure whereas the former contains two compatible square lattices for form an overall tetragonal structure. Furthermore, heterolayers of [(Na1−xFex)(OH)2]FeS can be incommensurate or commensurate, and they result in ferromagnetism below 16 K. These results demonstrate that upon intercalation new properties can emerge, and by carefully selecting these adducts, such properties can be controlled.

alities in areas such as catalysis (e.g., hydrogen evolution or desulfurization) on account of their metallic conductivity and electron rich states. • Due to the square metal sublattice being a fundamental motif, TTMCs are susceptible to electronic instabilities that can manifest into charge density waves. These electronic instabilities may compete with superconductivity observed in the iron-based TTMCs. Since the electrons occupy particular Brillouin zone boundaries, certain strategies for the rational design of superconductors could be pursued by these simple electronic arguments. • Ternary TTMCs such as KCo2Se2 and KFe2−yS2 are held by ionic forces and tend to be thermodynamically stable phases prepared by high temperature techniques. Thus, they can be prepared by high temperature solid state methods. Interestingly, these stable ternary phases can be utilized as precursors to new metastable phases via deintercalation of the A cations under basic aqueous conditions. • The binary TTMCs such as CoSe and FeS are held by van der Waals forces and are therefore susceptible to intercalation chemistry. Various strategies include intercalation of alkali metals in liquid ammonia and metal hydroxide insertion under hydrothermal conditions. Thus, a series of new phases could be prepared by insertion of guest species which act to both space the MCh host layers further apart and electron dope to tune the Fermi level. • A materials deign strategy for TTMCs can be pursued by combining suitable chalcogenide hosts, which act as the Lewis bases, and the guest species acting as Lewis acid. Such TTMCs are therefore acid−base adducts that can be stabilized at temperatures much lower than those required for typical solid state reactions. From these major points concerning the properties of TTMCs and our knowledge from recent developments of TMDs, we can make some predictions on possible future directions on their syntheses and properties. Future Directions: From Bulk TTMCs to Nanosheets. For TMDs the concept of nanosheets is not new, and almost all bulk TMDs can be broken down to the nanoscale by either mechanical or solution-mediated exfoliation.143−148 However, this synthetic strategy remains rather untapped for TTMCs. Considering both TMDs and binary TTMCs consist of 2D layers held by weak van der Waals forces, fabrication of TTMC nanosheets may be quite feasible utilizing similar techniques. When bulk materials are dimensionally confined, interesting quantum effects can emerge. For example, bulk MoS2 is an indirect band gap semiconductor, whereas monolayer MoS2 exhibits a direct band gap and displays orders of magnitude higher photoluminesce.148 Therefore, it is reasonable to assume that new properties may emerge from TTMC nanosheets. Indeed monolayered FeSe deposited on STO substrate has been reported to show a staggering high Tc varying from 55 to 110 K depending on the study.77,80,149 Its Tc is increased 10fold from bulk FeSe and higher than any known bulk Fe-based superconductors. Therefore, TTMC nanosheets are promising new materials yet to be explored. Since other binary TTMCs such as FeS, CoSe, and CoS are metastable, synthetic strategies other than those used for FeSe will be required. For example, chemical vapor deposition may be unfeasible due to the

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SUMMARY AND FUTURE DIRECTIONS Before providing a future perspective on the new lines of research with TTMCs, it is useful to highlight some of the major points concerning these materials. • The common structural motif in TTMCs includes all edge-sharing MCh4 tetrahra which form either 2D layers or 1D chains. For the layered TTMCs, the geometry of the tetragonal crystal structure dictates that the transition metals form a square lattice. • TTMCs tend to favor first-row and late transition metals in a low valent state. Size arguments suggest that such tetrahedral sites include smaller metal cations, and electronic reasons suggest that the transition metals have enough valence electrons to fill the bonding density of states. Therefore, most TTMCs are electron rich materials compared to TMDs. • The electronic structures of TTMCs reveal that they tend to be metals or semimetals with the Fermi level at the electronic DOS with predominant d-orbital character. Unlike semiconducting TMDs such as MoS2, these TTMCs have the possibility of offering new function5747


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Figure 14. Schematic representation of the layered components and the restacked material. (a) View of the [TaS2]−0.33 superconducting layer (Ta, blue spheres; S, yellow spheres). (b) View of the [Ni0.66Al0.33(OH)2]+0.33 magnetic layer (Ni, gray spheres; Al, white spheres; O, red spheres). (c) Representation of the restacked material along the c axis showing the alternating superconducting/magnetic layers. Reprinted with permission from ref 150. Copyright 2010 Nature Publishing.

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requisite higher temperatures. Instead of bottom-up syntheses, a top-down approach such as exfoliation of bulk materials could provide a more practical means to fabricate nanosheets of TTMCs such as CoSe. Future Directions: Integrating TTMCs into Novel Heterostructures. Another interesting direction for 2D materials is the building of new heterostructures by stacking them in particular sequences, as has been explored for TMDs.151 These heterostructures could offer unique functionalities on account of mixing different materials properties from their respective layers on an atomic scale. For example, Coronado et al.150 synthesized heterostructures whereby superconductivity coexists with ferromagnetism. To achieve this unique blend of properties, exfoliated TMDs such as TaS2 and exfoliated layered double hydroxides such as Ni0.66Al0.33(OH)2 were restacked and coprecipitated in solution (Figure 14). By applying similar techniques to exfoliated TTMC nanosheets, a great number of new heterolayered TTMCs could be fabricated. Because TTMCs are more robust superconductors compared to TMDs, by stacking them with insulating or magnetic layers, enhanced or magnetic superconductivity could be fabricated by design. Can the techniques used for TMD heterolayers be applied to TTMCs? Indeed, as discussed in the previous section, bulk heterolayered TTMCs such as Na-tochilinite, [(Na1−xFex)(OH)2]FeS, have already been prepared. This Na-tochilinite is metastable and can only be synthesized using a lower temperature route such as hydrothermal synthesis.86 Finding which specific heterostructures can be synthesized and the conditions under which one achieves either a commensurate or incommensurate structure would be a worthwhile endeavor for materials chemists. To promote TTMC materials by design, more binary hosts such as FeSe need to be synthesized. The recent discovery of two new anti-PbO type compounds, CoSe and CoS, expands the possibility for new families of intercalated TTMCs.40 For example, Zhou et al. reported a Li-EDA intercalated CoSe, the first new TTMC using CoSe as a host, which was prepared from KCo2Se2, a weak itinerant ferromagnet. Even though metastable, CoSe and CoS could be hosts for further intercalation chemistry. This multistep process in synthesizing Li-EDA-CoSe shows that novel TTMCs with targeted properties could be designed on account of their available chemistry.

AUTHOR INFORMATION

Corresponding Author

*(E.E.R.) E-mail: [email protected]. ORCID

Efrain E. Rodriguez: 0000-0001-6044-1543 Notes

The authors declare no competing financial interest. Biographies Xiuquan Zhou, originally from Yantai, China, received his B.S. in Materials Science and Engineering in 2007 at East China University of Science and Technology. He obtained his M.S. in Chemistry in 2013 from University of Toledo under the supervision of Prof. Cora Lind. Since then he has been pursuing his doctoral degree in chemistry at University of Maryland, College Park, in Prof. Efrain Rodriguez’s group. Efrain E. Rodriguez, originally from West Texas, received his B.S. from the Massachusetts Institute of Technology and Ph.D. in Materials at the University of California, Santa Barbara. Afterwards, he received a National Research Council Postdoctoral Fellowship at the NIST Center for Neutron Research. In 2012 he joined the Department of Chemistry and Biochemistry at the University of Maryland where he built a new solid-state chemistry laboratory for the preparation of functional inorganic materials. The group works at the dynamic intersection of materials chemistry and condensed matter physics with particular interests in the iron-based superconductors, magnetic materials, solid state synthesis, crystallography, and neutron scattering.

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ACKNOWLEDGMENTS Research at the University of Maryland was supported by the NSF Career DMR-1455118. We also acknowledge support from the Center for Nanophysics and Advanced Materials. The authors acknowledge the University of Maryland supercomputing resources (http://www.it.umd.edu/hpcc) made available for conducting the research reported in this paper.

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REFERENCES

(1) Hsu, F.-C.; Luo, J.-Y.; Yeh, K.-W.; Chen, T.-K.; Huang, T.-W.; Wu, P. M.; Lee, Y.-C.; Huang, Y.-L.; Chu, Y.-Y.; Yan, D.-C.; Wu, M.-K. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 14262−14264. (2) Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. Iron-Based Layered Superconductor LaO1−xFxFeAs (x = 0.05 − 0.12) with Tc = 26 K. J. Am. Chem. Soc. 2008, 130, 3296−3297. (3) Kotegawa, H.; Masaki, S.; Awai, Y.; Tou, H.; Mizuguchi, Y.; Takano, Y. Evidence for Unconventional Superconductivity in Arsenic-

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