Expanding the Concept of van der Waals Heterostructures to

Sep 8, 2017 - Several members of a new family of heterostructures [(LaSe)1.17]1Vn(1+y)+1Se2n+2 with n = 1, 2, and 3 were prepared using a diffusion co...
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Expanding the Concept of van der Waals Heterostructures to Interwoven 3D Structures Noel S. Gunning,# Torben Dankwort,‡ Matthias Falmbigl,#,¶ Ulrich Ross,† Gavin Mitchson,# Danielle M. Hamann,# Andriy Lotnyk,† Lorenz Kienle,‡ and David C. Johnson*,# #

Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403, United States Institute for Materials Science, Faculty of Engineering, University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany † Leibniz Institute of Surface Modification (IOM), Permoserstrasse 15, 04318 Leipzig, Germany ‡

S Supporting Information *

ABSTRACT: Several members of a new family of heterostructures [(LaSe)1.17]1Vn(1+y)+1Se2n+2 with n = 1, 2, and 3 were prepared using a diffusion constrained, kinetically controlled synthesis approach. Specular diffraction patterns collected as a function of annealing temperature show the evolution of designed precursors into highly ordered heterostructures. Scanning transmission electron microscopy (STEM) images reveal that the structure of n = 3 consists of rock salt structured LaSe bilayers alternating with vanadium selenide layers of varying thickness, which are structurally closely related to V3Se4. Interplanar distances obtained from the STEM images were successfully used as the starting point for Rietveld refinements of the specular diffraction patterns of these crystallographically aligned compounds. Utilizing this unorthodox combined approach to extract detailed structural information unambiguously, we demonstrated that these thin film compounds are the first examples of chalcogenide-based heterostructures, where the bulk structures of both building blocks lack a van der Waals gap, yet a nonepitaxial incommensurate interface forms. Moreover, the refinement results of the n = 2 and 3 heterostructures suggest that the structure of the V−Se layers can be varied ranging from VSe2 to VSe depending on the film composition. The electrical resistivity of the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures changes systematically from semiconducting toward metallic behavior with increasing n, showcasing the ability to tune physical properties by precisely controlling the layer sequence in these heterostructures.

N

and the degree of coherency at the interfaces.14 Nanoarchitecture in heterostructures, the identity of layers, the sequence of layers, and their thicknesses, is clearly a powerful tool to create new materials.12,15 A major limitation in this area is the synthesis of targeted heterostructures. For the top-down “scotch tape” assembly approach, a van der Waals gap in the parent compound is required to cleave or exfoliate layers; it is difficult to precisely control layer thicknesses, and it is challenging to reproducibly stack different structural units. Approaches to prepare monolayers on different substrates have reduced the challenges in obtaining monolayers for stacking, but a van der Waals gap is still compulsory. A constituent with a three-dimensional bulk structure, such as FeSe, is therefore inaccessible by top-down methods, and direct growth causes severe challenges to precisely control the thickness of thin films over large areas.9 We have demonstrated that a bottom-up approach based on diselenide building blocks with different crystal structures provides monolayer thickness control while minimizing cation

ew materials containing alternating layers of different 2D compounds have attracted significant attention due to emergent properties and the ability, at least conceptually, to prepare designed stacking sequences and combinations to both obtain new properties and tune existing ones through interactions between layers. For example, graphene films on hexagonal boron nitride substrates have much larger carrier mobility compared to films deposited on silicon oxide substrates.1−3 FeSe monolayer films deposited on a variety of substrates4−7 and FeSe films coated with potassium8,9 have higher superconducting critical temperatures due to charge transfer from the substrate and the top coating of potassium, respectively. Borophene monolayers on a silver substrate exhibit metallic behavior unique from the semiconducting behavior of other boron-based allotropes.10 The transition metal dichalcogenides have been recognized as a particularly attractive family of constituents for novel heterostructures with targeted properties due to the wide range of properties from semiconducting to superconducting.11,12 These properties are sensitive to both layer thickness and neighboring constituents. For example, a single layer VSe2 exhibits a distinctly different charge density wave behavior in heterostructures compared to thicker VSe2 layers,13 and the superconductivity of NbSe2 is a sensitive function of its thickness, the identity of adjacent layers, © 2017 American Chemical Society

Received: June 23, 2017 Revised: September 7, 2017 Published: September 8, 2017 8292

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

Article

Chemistry of Materials

Figure 1. (a) Schematic of fragments of the structures of LaSe, VSe2, V3Se4, and VSe. (b) Structural schematic of a [(LaSe)1.17]Vn(1+y)+1Se2n+2 heterostructure with y = 0.5 and n = 3.

and a family of reflections appeared and increased in intensity as the annealing temperature was increased from 500 to 700 °C. The La/V ratio was adjusted by changing the V layer thickness to optimize the intensity and minimize the line widths of the (00l)-reflections. Compositions in the range [(LaSe)1.1−1.2][V1.3−1.5Se]n resulted in the maximum intensity of the reflections after annealing at 700 °C, indicating that y is close to 0.5 as expected for V3Se4. Compounds could only be formed when the number of V−Se bilayers was larger than the number of La−Se bilayers in the precursor. In particular, the compound with n = 0, which would be a structural analogue to the misfit layered compound (La0.95Se)1.21VSe2,18 could not be prepared. In Figure 2, the specular diffraction patterns of the

intermixing, but at least one of the constituents must have a layered structure with van der Waals gaps.16 At present, the prerequisite of a van der Waals gap dramatically limits the number of heterostructures that can be prepared. Herein, we report the synthesis of a family of heterostructures containing blocks of constituents with structures that do not contain van der Waals gaps, [(LaSe)1.17]1Vn(1+y)+1Se2n+2 with n = 1, 2, and 3 (Figure 1a). The heterostructures were prepared using a kinetically controlled bottom-up synthesis approach that precisely controls constituent layer thicknesses in a precursor. The LaSe bilayer in our heterostructures has a distorted rock salt structure. The Vn(1+y)+1Se2n+2 layer consists of different thicknesses of fragments of V1+ySe2 (see Figure 1b for y = 0.5 and n = 3), which is a member of a family of structurally related transition metal chalcogenides, M1+yX2 (0 ≤ y ≤ 1) with M = Ti, V, Cr; X = chalcogen. If y = 1, the compounds typically have a NiAs-type structure, and if y = 0, they crystallize in a CdI2-type structure. For intermediate values of y, vacancyordered variants form.17 The transition metal atoms occupy octahedral vacancies within the hexagonal anionic sublattice. The metal occupancy governs the structure, and only dichalcogenides with y = 0 have alternating fully occupied and unoccupied metal layers, with van der Waals gaps across the empty layers.17 However, all members of this M1+yX2 family of compounds can be sliced into slabs having a Se−M−Se trilayer as the outermost structural unit on both sides (Figure 1a). This structural dissection can be applied to other M1+yX2 and My′M1X2 compounds and perhaps more broadly to compounds of spinel-type and other 3D structures. In this work, we use a LaSe bilayer to stabilize Vn(1+y)+1Se2n+2 slabs, as illustrated in Figure 1b for the vacancy-ordered variant where y = 0.5. This work demonstrates that heterostructures can be prepared with 3D structural building blocks that lack a van der Waals gap, which significantly broadens the playground for materials scientists in this field. The films of [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures were prepared by the modulated elemental reactant method, which uses precursors with compositional modulation resembling the desired products to kinetically trap the targeted heterostructures. Briefly, the precursors were prepared by depositing layers of elements on a nominally room temperature substrate in a vacuum. We first calibrated the deposition parameters for binary films containing alternating layers of La (or V) and Se corresponding to the targeted LaSe and V−Se constituent stoichiometries, with excess of Se to compensate for losses that occur during annealing. La−Se bilayers with a 1:1 ratio of La−Se and n V−Se bilayers with an excess of Se were sequentially deposited to create films with total thicknesses of 50 to 100 nm. On annealing, the films decreased in thickness

Figure 2. Specular X-ray diffraction patterns of [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures with y = 0.5 and n = 1, 2, and 3. Selected indices for (00l) reflections are provided. # marks reflections from the Si substrate.

three [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures with n = 1, 2, and 3 are depicted. Heterostructures containing dichalcogenides are typically described in terms of a- and b-axis parameters spanning the basal planes of the lattice and c-axis parameter perpendicular to the basal plane along the superlattice direction. Due to the underlying symmetry of the constituent layers, these directions thus correspond to the crystallographic a-, b-, and c-directions of a distorted rock-salt structure for LaSe and to the nonstandardized setting of bulk V3Se4 in space group I12/m1. The authors like to note that this is a modest deviation from the monoclinic distortion of the V3Se4 cell (β-angle = 91.73(5)°), which is not taken into account for the present heterostructures. The maxima in each pattern can be indexed as (00l) reflections, yielding c-axis lattice 8293

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

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Table 1. Lattice Parameters (in nm) of [(LaSe)1.17]1Vn(1+y)+1Se2n+2 Heterostructures with n = 1, 2, and 3 along the Superlattice Direction (c-Axis) and in the Basal Plane for Each Constituenta a

b

a

b

ρ

n

c

(LaSe)

(LaSe)

(Vn(1+y)+1Se2n+2)

(Vn(1+y)+1Se2n+2)

(285 K) μΩm

ρ25 K/ρ285 K

(285 K) 10−4 cm3/C

1 2 3

1.75(1) 2.34(1) 2.93(1)

0.602(2) 0.598(1) 0.602(1)

0.614(2) 0.609(2) 0.611(1)

0.606(1) 0.602(2) 0.603(2)

0.356(1) 0.354(1) 0.357(1)

32.8 30.1 15.2

1.57 1.11 1.00

0.14 3.41 2.22

RH

a In the last three columns are the electrical resistivity at 285 K (ρ), ratio between the resistivity at 25 and 285 K, and Hall coefficient (RH) at 285 K for all three samples.

misfit layer compound, 1.17 versus 1.21, results from an increase of the basal-plane area of the LaSe constituent, which might arise from the absence of La vacancies in [(LaSe) 1.17 ] 1 V n(1+y)+1 Se 2n+2 . In the misfit layer compound (La0.95Se)1.21VSe2, a monoclinic distortion of both constituents results in one commensurate in-plane axis.18 The LaSe and V1+ySe2 building blocks of the reported heterostructures have very similar a-axis lattice parameters. To investigate the local structure and spatial relationships between the constituent layers, scanning transmission electron microscopy was used (Figure 4). As expected from the specular XRD patterns, the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 films were strongly c-axis oriented and the adjacent highly crystalline building blocks form in-commensurate interfaces and exhibit rotational disorder typical for ferecrystals.22 A closer inspection of the Vn(1+y)+1Se2n+2 units for the n = 3 compound revealed alternating filled and half-filled vanadium layers closely

parameters (Table 1) that systematically increase by 0.59(1) nm as n is increased. This value is in excellent agreement with the expected thickness increase for one additional V1+ySe2 layer ranging between 0.597 and 0.610 nm for the bulk structures of V1+ySe2.23 The in-plane diffraction scans, shown in Figure 3, can all be indexed to (hk0) reflections for the individual constituents,

Figure 3. In-plane grazing incidence X-ray diffraction patterns of [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures with n = 1, 2, and 3. The indexing of the (hk0) Bragg reflections for the LaSe and Vn(1+y)+1Se2n+2 constituent layers are provided above the n = 2 and n = 3 patterns, respectively.

yielding in-plane lattice parameters summarized in Table 1. These results demonstrate that the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures are typical ferecrystalline compounds, where the highly aligned c-axis oriented superlattice perpendicular to the substrate surface exhibits diffraction features comparable to single crystals, whereas the rotational disorder among the individual constituents, namely, LaSe and V1+ySe2, in the basal plane displays diffraction patterns expected for polycrystalline compounds.20,21 However, in contrast to all other known ferecrystalline compounds, the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures have distortions in both constituents. LaSe exhibits a rectangular distortion, which decreases slightly from 2% to 1.5% as n increases (Table 1) and is smaller than the 3% distortion found in (La0.95Se)1.21VSe2.18 The Vn(1+y)+1Se2n+2 constituents have a-axis and b-axis lattice parameters that are within the range reported for bulk V1+ySe2 (0.5807 ≤ a ≤ 0.6445 nm, 0.3374 ≤ b ≤ 0.3721 nm, note that in case of hexagonal bulk geometry these values refer to an orthohexagonal setting), where both lattice parameters decrease with increasing y. 23 The smaller misfit parameters of the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures relative to the

Figure 4. Atomic resolution STEM micrographs of the n = 3, [(LaSe)1.17]1Vn(1+y)+1Se2n+2, heterostructure. The layers are c-axis oriented and show incommensurate and rotational disorder. 8294

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

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Chemistry of Materials resembling V3Se4 (zone axis [010]).19 Due to the alternating filled and half-filled V layers, the atomic position of the Se and V atoms along the c-direction are shifted above and below the single plane that would be expected for an ideal VSe2 crystal (marked in orange in Figure 5 for Se atoms). The difference in

between the experimental and the simulated ratios of the Se and V peak intensities (Figure 5) further supports the formation of stoichiometric V3Se4-type building blocks or at least a full occupancy of the respective atomic sites in this heterostructure. The space between the outermost Se layer in Vn(1+y)+1Se2n+2 and the LaSe bilayer does not contain any V atoms as depicted in the schematic structure in Figure 1b. Consequently, the stoichiometry is dependent on the number of building blocks n. Assuming that the Se atoms form a perfect anionic sublattice, the nominal stoichiometry of the vanadium block is V1.5n+1Se2n+2. Hence, for n = 3, the expected stoichiometry is V5.5Se8 or V2.75Se4, which is slightly vanadium deficient compared to V3Se4. EDX measurements of single V− Se layers yielded 42.2 at % and 57.8 at % for V and Se, respectively (see Figure S2 and Table S1 for details), which is slightly higher than expected for y = 0.5. While the majority of the inspected images contained V3Se4structured layers, regions with VSe-type structures could be located as well, emphasizing the flexibility of the V1+ySe2 layers sandwiched between LaSe units to accommodate any y-value between 0 and 1. Even within one layer, the structure was observed to change, for example, from V3Se4 to VSe (Figure 6).

Figure 6. Atomic resolution dark field (DF) STEM images of regions containing both V3Se4 and VSe building blocks. The red circles highlight the filling of the octahedral voids with V atoms characteristic for either VSe (fully filled) or V3Se4 (alternatingly filled).

The varying vanadium content reflects local fluctuations in the stoichiometry of the precursor and the stability of particular compositions within the heterostructure. Even in the bulk synthesis of V1+ySe2, locally significant differences in y were observed. According to a detailed study by Rost and Gjertsen, the local variation causes an expansion of the in-plane lattice parameters with increasing V content.23 Therefore, the in-plane lattice parameters of the vanadium constituent from the XRD studies presented above should be considered an average of the various local structural arrangements. Rietveld refinements of the specular XRD patterns of the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures with n = 2 and 3 were based on the positions of atomic planes along the c-axis extracted from the vertical intensity profiles in the HAADFSTEM images of the n = 3 compound. We used Gaussian peak fitting to determine peak locations with subpixel accuracy and averaged the results from different images to obtain initial estimates for the z-coordinate of each atomic plane in the heterostructures.25 The STEM produced initial model has two plane positions for the atomic layers of Se and the fully occupied V layer and one for the nominally half occupied V layer. The space group P3̅m1 containing mirror planes at 0 and 1/2 along the c-direction was selected for the structural model as the layers in the heterostructure are exposed to a similar bonding environment as showcased in the insets in Figure 7. A refinement of the occupancy of the half-filled V-positions yielded a composition for the V−Se constituent of V3.46Se6 for n = 2, which is slightly V-deficient compared to the expected

Figure 5. Enlargement of the area in the red square in Figure 4. Alternating filled and half-filled vanadium planes between Se sheets are observed, which is typical for V3Se4.19 V atoms are absent at the interface to LaSe. The atomic positions of the selenium layers alternate up and down in response to the half-filled vanadium planes (highlighted in orange). Simulations based on V3Se4 (zone axis [010]) show good agreement (yellow encircled). Line profiles (blue and red lines correspond to the simulated and experimentally obtained data, respectively) are depicted below.

the position of these planes in c-direction is 0.0427 ± 0.0050 nm. Interestingly, the shift of the Se and V atoms along the cdirection is also noticeable at the boundary to the LaSe-type layer, thus indicating weak interaction between the adjacent LaSe and Vn(1+y)+1Se2n+2 layers. Slight distortions compared to the ideal V3Se4 structure were observed. Similar to the results obtained by XRD measurements, the a-axis lattice parameter is slightly decreased to 0.611 nm. A distortion of the lattice is also observed, which when constructing the unit cell in a monoclinic I12/m1 symmetry alters the β*-angle from 91.73(5)° toward a range of 90−91° (see Figure S1 for comparison). As a result of this distortion, the unit cell gains an increasingly orthorhombic character. In addition, the atomic resolution STEM micrographs agree well with simulated images assuming stoichiometric V3Se4-type building blocks (Figure 5) (for simulation details, see the Supporting Information). The agreement 8295

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

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Chemistry of Materials

Figure 7. Rietveld refinement result for the compounds with n = 2 (a) and 3 (b) with refined compositions of [(LaSe)1.16]1V3.46Se6 and [(LaSe)1.18]1V5.63Se8, respectively. Structural schematics displaying the individual layers along the c-axis including the mirror planes (m) at 0 and 1/2 of the unit cell along the c-axis are provided as insets. # denotes a stage peak.

Figure 8. Refined structural models for the heterostructure with n = 2 based on V1+ySe2 (top)24 and for the heterostructure with n = 3 based on V3Se4 (bottom)19 compared to structural parameters of the corresponding bulk compounds. The resulting residuals (RI) for each model and the interlayer distances and occupancies for each atomic layer are provided (Note that the V-position at 0 is a one-fold position, whereas all others are 2-fold positions).

V4Se6, and V5.63Se8 for n = 3, which is slightly higher than the V5.5Se8 expected on the basis of an exactly half occupied layer. This result affirms the findings of the TEM investigations demonstrating that the energy difference between different yvalues for V1+ySe2 within these heterostructures must be relatively small and depends on the composition of the starting precursors. In order to gain more detailed information on the dominating structural arrangement of the V−Se constituent in each of these heterostructures, we tested two different models: one based on the V−Se layer having a V1+ySe224 structure and the second based on the V3Se4 structure.19 The n = 2 heterostruture fit best to a model based on the V−Se layer having the CdI2 structure type. The occupancy of the V atoms residing in the van der Waals gap and the La and Se atoms of the rock salt bilayer were refined to account for the misfit. The refined position of the atomic planes along the c-axis results in a rather asymmetric location of the interstitial V layer between the adjacent Se layers (Figure 8). This might arise from the presence of different local structural units (see Figure 6), which are not accounted for in this structural model. Using the model based on the V3Se4 structure resulted in unreasonably short distances between several atomic planes and an unstable refinement. The refinement for the n = 3 structure gave lower residual values using the model based on the V3Se4 structure,19

which included the distortion present in the Se layers and the fully occupied V layers as indicated by the STEM data. The highly stable refinement of the layer splitting in conjunction with the close resemblance of the interplanar distances compared to bulk V3Se419 and low RI of 2.1% provide strong evidence that structural distortions similar to V3Se4 are present in the V−Se layer of the n = 3 heterostructure. The difference between the two refinements is probably a consequence of the initial composition of the precursors. For the ideal n = 2 compound, [(LaSe)1.17]1V3.46Se6, the refined occupancy of the interstitial V layer is located between VSe2 and V3Se4. However, for the n = 3 compound, [(LaSe)1.17]1V5.64Se8, the refined occupancy is between V3Se4 and VSe. This higher occupancy results in the V−Se layer having the structural distortions observed for bulk V3Se4. The LaSe constituent was found to have a distortion relative to an ideal rock salt structured layer, where the La cation protrudes toward the V−Se constituent. This puckering distortion is similar to that observed in the misfit layered compound (La0.95Se)1.21VSe2.18 The magnitude of the distortion is larger than found in rock salt structured layers of divalent cations such as Sn and Pb in both misfit layer compounds and ferecrystals, indicating a stronger interaction between the two constituents.26−29 In contrast to the misfit 8296

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

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Chemistry of Materials layered compound, where ∼5% La vacancies were found in order to maintain charge balance and a 3+ oxidation state for both cations,18 no indications for the presence of vacancies were found refining the La and Se positions in the rocksalt layer independently in the compounds studied here. This difference is most likely related to the mixed oxidation state of the V atoms in the Vn(1+y)+1Se2n+2, layers, which results in an increased ability to accept transferred charge from the LaSe layer. To begin to probe the interplay between structure and properties, including the influence of charge transfer between the two constituents, the temperature dependence of the electrical resistivity was measured for all three compounds and is displayed in Figure 9. The [(LaSe)1.17]1Vn(1+y)+1Se2n+2

layer to donate electrons into the VSe2 layers. Because the local distortions to the vanadium atoms in the V(1+y)Se2 layer change as a function of the concentration of interstitial V and the identity of adjacent layers, a rigid band approximation is not appropriate. Understanding the interplay of properties with both composition (y) and nanoarchitecture (n) will be the focus of upcoming investigations, made possible by the ability to synthesize these metastable compounds. We have demonstrated the first successful bottom-up synthesis of chalcogenide based van der Waals heterostructures composed of individual constituents lacking a van der Waals gap in the 3D bulk structures. The [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures can be considered as the prototype for a tremendously large family of new compounds. STEM images show locally stabilized layers with structures resembling VSe2, V3Se4, and VSe. This supports the idea that a plethora of 3D structural units can be incorporated as constituents of heterostructures. The high quality Rietveld refinement of the [(LaSe)1.17]1[V5.64Se8] demonstrates the ability to control the structure through the use of designed precursors and that the global structure is very close to a superlattice containing alternating regions of LaSe bilayers with a distorted rock salt structure and blocks of V1.5n+1Se2n+2 adopting the V3Se4 structure. The resistivity systematically changed as n was increased; however, a more systematic investigation of properties, independently varying both n and y, is required to quantitatively understand the observed behavior.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. Electrical resistivity as a function of temperature for [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures with n = 1, 2, and 3. The inset shows the temperature dependence of all three compounds normalized to the value at 285 K.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02605. Details of synthesis and characterization of precursors; details of STEM-EDX measurements and data analysis; additional STEM images; details of Rietveld refinement procedure and tables of Rietveld refinement parameters (PDF)

heterostructures have systematically lower resistivity values and decreasing resistivity ratios (see Table 1) as n increases. The temperature dependence of the resistivity increase for the n = 1 compound is much weaker than expected for activated conduction in a semiconductor. The lack of a systematic trend for the Hall coefficient measured at 285 K (see Table 1) most likely arises from differences in V content, y, in [(LaSe)1.17]1Vn(1+y)+1Se2n+2. This behavior is not unexpected considering the complex electronic properties of bulk vanadium selenides, which differ considerably as structure and composition are varied. While VSe2 is thoroughly investigated owing to the presence of a charge density wave below 100 K,30 there is much less information on the physical properties of V3Se4 and VSe. V3Se4 exhibits metallic behavior and weak antiferromagnetism below 16 K.31,32 The NiAs-structured VSe shows weak semiconducting behavior and exhibits Pauli-paramagnetism.33 For heterostructures of vanadium selenides, it was demonstrated that the charge transfer and layer thickness have a distinct influence on the properties. The charge balanced misfit layer compound (La0.95Se)1.21VSe2 exhibits semiconducting behavior implying localization of the carriers in the VSe2 layer.18 Heterostructures of VSe2 interleaved with SnSe and GeSe2 are metallic with an even more pronounced charge density wave transition compared to the bulk compound. In both cases, a strong dependence of the properties on the constituent layer thicknesses is observed.16,34,35 In the [(LaSe)1.17]1Vn(1+y)+1Se2n+2 heterostructures prepared in this work, we expect the interstitial V atoms as well as the LaSe

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gavin Mitchson: 0000-0002-0840-2376 Danielle M. Hamann: 0000-0002-9262-1060 Andriy Lotnyk: 0000-0002-0000-9334 David C. Johnson: 0000-0002-1118-0997 Present Address ¶

M.F.: Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA.

Author Contributions

N.S.G. synthesized the films and collected structural and electrical data. G.M. performed parts of the X-ray diffraction, and U.R., T.D., and G.M. performed the electron microscopy experiments. T.D., M.F., N.S.G., U.R., G.M., D.M.H., A.L., L.K., and D.C.J. analyzed the experimental results. M.F. conducted the structural refinement. N.S.G., T.D., M.F., L.K., and D.C.J. cowrote the paper. Notes

The authors declare no competing financial interest. 8297

DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298

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Chemistry of Materials

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation under Grants DMR-1266217 and DMR-1729613.

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DOI: 10.1021/acs.chemmater.7b02605 Chem. Mater. 2017, 29, 8292−8298