Effect of Local Structure of NbSe2 on the Transport

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Effect of Local Structure of NbSe2 on the Transport Properties of [(SnSe)1.16]1[(NbSe2)]n Ferecrystals Matti B Alemayehu, Matthias Falmbigl, Kim Ta, and David C. Johnson Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00131 • Publication Date (Web): 24 Feb 2015 Downloaded from http://pubs.acs.org on March 5, 2015

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Chemistry of Materials

Effect of Local Structure of NbSe2 on the Transport Properties of [(SnSe)1.16]1[(NbSe2)]n Ferecrystals Matti B. Alemayehu a,*, Matthias Falmbigl a, Kim Ta a and David C. Johnson a,* a

Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403, United States Ferecrystals, polytypes, misfit layer compounds, thin films

ABSTRACT: ([SnSe]1.16)1(NbSe2)n ferecrystals were synthesized through modulated elemental reactants technique by increasing the number of Nb|Se layers in the precursor from 1 to 4. The c-lattice parameter of the intergrowth was observed to change as a function of n by 0.635(2) nm. The c-lattice parameter of SnSe was observed to be 0.588(8) nm and independent of n. The electrical resistivity does not decrease as n increases as expected from simple models, but instead the trend in the resistivity is (1,3) > (1,4) ≥ (1,1) > (1,2). The carrier concentration increases with n as expected, so the unusual trend in resistivity is a result of the carrier mobility decreasing with increasing n. In-plane X-ray diffraction line widths and STEM images of the (1,4) compound show that it has small in-plane grain sizes and a large diversity of stacking sequences respectfully, providing a potential explanation for the reduced carrier mobility.

INTRODUCTION A long standing question in misfit layer compounds, ([MX]1+δ)m(TX2)n, with M atoms from group 14, 15 (e.g. Pb, Sn and Bi) and rare earth metal, T atoms from group 4, 5 and 6 (e.g Ti, V, Nb or Ta) and X atoms from group 16 (e.g. S or Se),1 is what causes the unexpected electrical trends observed as a function of increasing the thickness of the conducting layers (TX2). While a simple picture would predict increasing the relative thickness of the conducting constituent would decrease resistivity, in ([PbSe]1.14)1(NbSe2)n and ([SnS]1.17)1(NbS2)n, the trend in electrical resistivity with (m,n) is (1,2) > (1,3) > (1,1) and (1,2) > (1,1) respectively.1,2 The cause of these unusual trends has not been explored, in part due to the limited ability to prepare misfit layered compounds with n larger than 1. Two structural factors could contribute to this unanticipated trend in transport properties: the distortion of the constituent layers to achieve a commensurate b axis,1,3 and/or different stacking sequences of the dichalcogenide layers.4,5 The distortion may reduce carrier concentration by localizing electrons and both factors will influence the mobility of the carriers. Stacking sequences, which result in polytypism,6 are the main structural influence on the properties of transition metal dichalcogenides (TMDs).4,5,7 The symmetries of the TMDs can be trigonal (T), hexagonal (H) or rhombohedral (R) depending on the stacking sequence (see Figure 1).8 Only one type of trigonal (T) and several hexagonal (2H and 4H)4,8 and rhombohedral (3R and 6R)7,8 stacking sequences of TMDs have previously been reported. TMDs containing transition metals V and Ti have been reported to have only one type of stacking sequence: 1T,9,10 while TMDs of Nb, Mo, W and Ta exhibit polytypism: 1T, 2H(Ha), 2Hb 3R, 4H, 4HdI and 4HdII (see

Figure 1).4 Different polytypes of the TX2 compounds result in different transport properties.11,12 For example, a 2H- NbSe2 superconducts below a transition temperature of 7.2 K, while the 3R- phase does not have a superconducting transition.7 Wiegers et al have attributed different stacking sequences of NbX2 in misfit layer compounds to different physical properties.1 In the ([PbS]1.14)1(NbS2)2 compound, NbS2 crystallizes in 2/3rd of a 3R- polytype and does not superconduct, whereas in the selenide analogue the NbSe2 bilayer crystalizes as a 2H- polytypes and has a superconducting critical temperature of 3.4 K.13,14 Stacking sequence was also reported to impact transport properties for the misfit layer compounds ([SnS]1.17)1(NbS2)n where n = 2 and 3.1,15 However, no clear relationship between the different stacking sequences of NbX2,4 in misfit layer compounds, and their resulting transport properties has been established, due to the limited number of examples.

Figure 1. Different polytypes of TMDs of formula NbSe2 with the symmetry of each orientation given. Highlighted in red are their orientations in a unit cell.

The modulated elemental reactants technique (MER), has been used to synthesize a number of misfit layer compounds with higher m and n.16,17 These compounds have been called ferecrystals due to the extensive turbostratic disorder between subsequent layers.16,17 This

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disorder inhibits the in-plane structural distortions found in conventional misfit layer compounds prepared at high temperature. In this paper, we present the structural and electrical properties of ([SnSe]1.16)1(NbSe2)n where n is increased from 1 to 4. By controlling the thickness of the NbSe2 layer (n), we can explore the evolution of properties without the structural distortion in the constituent layers. Assuming a single band model, the carrier mobility continuously drops with increasing number of NbSe2 layers. Different stacking sequences within one layer of NbSe2 as well as regions with new stacking sequences different than the well-established polytypes are observed in the ([SnSe]1.16)1(NbSe2)4 compound, and are proposed as the reason for the decrease in mobility of carriers. EXPERIMENTAL SECTION The compounds ([SnSe]1.16)1(NbSe2)n where n = 1-4 were synthesized using the modulated elemental reactants (MER) technique described in detail elsewhere.18 The technique utilizes a custom-built vacuum chamber with a base pressure of 1×10-8 Torr. The elemental sources, Sn (99.999% purity), Nb (99.999% purity) and Se (99.999% purity), were purchased from Alfa Aesar. The designed precursors were evaporated onto a (100) oriented silicon substrate and fused quartz (electrical measurements) using 3 kW electron beam gun. The elemental sources Sn, Nb and Se were evaporated at the rate of 0.04 nm/s, 0.02 nm/s and 0.05 nm/s respectively. A preprogrammed LabView program was used to position the substrates on top of each elemental source with a pneumatic shutter controlling the amount of material deposited. The same sequence as the target compound was deposited until a total film thickness of ~50 nm was achieved. The precursors were annealed in a N2 atmosphere glovebox with O2 content below 0.5 ppm. Atomic composition of the precursors and the final products was determined by a Cameca SX-100 electron probe microanalyzer (EPMA) using the method developed by Donovan et al.19 Atomic and thickness ratio calibration of this family of compounds were performed as described previously.20 X-ray reflectivity (XRR) and X-ray diffraction (XRD) were used to determine the total thickness and crystallinity of the films respectively. Both measurements were performed on a Bruker D8 AXS diffractometer equipped with a Cu Kα (0.154 nm) radiation source operated at 40 kV and 40 mA, a Göbel mirror, and Bragg–Brentano optics geometry. Locked coupled θ - 2θ scans were collected from (0–10°) and (6–65°) 2θ for XRR and XRD respectively. In-plane X-ray diffraction of the compounds was collected on a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm). Samples for high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging were prepared using FEI Helios dual beam. HAADFSTEM images were acquired on an FEI Titan 80-300. Temperature-dependent transport measurements were conducted through a standard van der Pauw method using films with a cross pattern on fused quartz. Indium was used to make contact to the four corners of the cross arms. A programmable current source was used to send current to two of the cross arms while measuring

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the potential between the remaining two cross arms with a nanovoltmeter. The sheet resistance is then extracted from the slope of the current-voltage (IV) curve. Using the total film thickness, the sheet resistance is converted to resistivity. Measurements were conducted between 20 K and 295 K. Hall measurements, also using the standard van der Pauw method, were conducted at a constant current value of 0.100 A. The Hall voltage was obtained from the slope of magnetic field (0-1.6 T) vs. voltage curve. RESULTS AND DISCUSSION The synthesis procedure to form designed precursors was described in detail elsewhere.20 Briefly, stoichiometric amount of the constituting elements with sequences that correspond to the desired products were evaporated onto a silicon substrate. Stoichiometric ratios of 1:1, 2:1 and 1.16:1 for Se:Sn , Se: Nb and Sn: Nb were targeted respectively. The amount of material in each Se:Sn and Nb:Se bilayer was calibrated to be a two planes of a rock salt structured SnSe and one Se-Nb-Se trilayer respectively. One bilayer of Sn and Se and n bilayers of Nb and Se, where n ranges from 1-4, were sequentially deposited until a total thickness of approximately 50 nm was achieved. Excess Se (4-5 at.%) was incorporated into each precursor to compensate for the loss of selenium during annealing. An annealing study was performed to determine the optimal formation condition for the compounds. Five pieces of the same (1,3) compound were annealed at temperatures between 100 °C and 600 °C for 20 minutes. Out-ofplane X-ray diffraction was collected after each annealing experiment. The diffraction pattern of the sample in the as-deposited state and after annealing at 200 °C contain sharp Bragg reflections at low angles due to the nanoarchitecture of the precursor and weak broad reflections at higher angles that reflect the mostly amorphous nature of the precursor. With increasing annealing temperature, additional (00l) reflections grow in intensity as the precursor self assembles while maintaining the initial nanoarchitecture of the precursor. The Bragg maxima shift slightly to higher angles as annealing temperature increases due to a decrease in the repeat thickness. This results from the loss of selenium during annealing and the crystallization of the individual layers. At temperatures above 400 °C, two different phenomena occur. The superlattice degrades, which is indicated by peak broadening and decreased intensities until a complete decomposition occurs at 600 °C when only peaks corresponding to NbSe2 are observed. At the same time impurity peaks of SnO2 appear and increase in intensity as previously reported.20 The annealing conditions that produced the narrowest full width at half maximum (FWHM) and the highest intensity in the 00l diffraction peaks, 400 °C for 20 minutes, was chosen to be the optimal annealing conditions. After the calibration conditions that produced stoichiometric constituents were determined, compounds of formula ([SnSe]1.16)1(NbSe2)n with increasing number of NbSe2 layers (n) were synthesized. Figure 3 shows the specular diffraction patterns of the four compounds synthesized. All Bragg maxima can be indexed as 00l reflections, yielding the c-lattice parameters listed in Table 1.


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a) b) Figure 2. a) Specular X-ray diffraction data for the ([SnSe]1.16)1(NbSe2)3 compound collected after annealing the sample at the indicated temperatures for 20 minutes. 400 °C was chosen as the optimal annealing condition based in intensity and line width of the (00l) reflections. b) The evolution of the (008) reflection after annealing the sample at the indicated temperatures

The respective thicknesses of the NbSe2 and SnSe layers were determined from the slope and intercept of the linear regression line for the c-lattice parameter vs n plot. The thickness of the SnSe layer, 0.588(8) nm, is within the previously reported range, 0.576-0.580 nm, for misfit layer compounds and ferecrystals.1,21 The thickness of the NbSe2 layer was marginally thinner, 0.635(2) nm versus 0.655 nm, than reported for 2H-NbSe2.22 To better understand the structural property of the four compounds, Rietveld refinement was performed using the FullProf package.23 The Rietveld refinement of the three compounds ([SnSe]1.16)1(NbSe2)n, with n = 1, 2, and 3, reveals several noteworthy findings (Figure 4). The puckering, the distortion in the SnSe layer, where the negatively charged Se in the neighboring NbSe2 layer attracts the Sn and the Se is slightly repulsed, remains constant in all compounds. Due to the mirror plane in the dichalcogenide compound along the Nb plane in the NbSe2 layer, the (1,1) compound is symmetric. In the (1,2) compound the mirror plane resides between the NbSe2 trilayers and the dichalcogenide layers are slightly asymmetric, with the Nb layer closer to the inner Se layer. This asymmetry is more pronounced than in the misfit layer compound: ([SnS]1.16)1(NbS2)3.15 The inner layer in the (1,3) compound contains a mirror plane similar to the NbSe2 layer in the (1,1) compound. The layers adjacent to the SnSe constituent in ([SnSe]1.16)1(NbSe2)3 are

symmetric within- error, but the Se-Nb-Se trilayer thickness along the caxis is smaller than found in the trilayer in the other two-

Figure 3. X-ray diffraction of ([SnSe]1.16)1(NbSe2)n , where n = 1-4. All reflections can be indexed as 00l reflections, and selected peaks for each compound are indexed. (*) designates the silicon substrate peaks.

Table 1: c-lattice, and in-plane lattice parameters of ([SnSe]1.16)1(NbSe2)n compounds with the FWHM of the 00l Bragg reflection at 14° 2θ and in-plane (110) NbSe2 reflection for each compound.

n [Number of NbSe2 layers] 1

c-lattice [nm]

FWHM

a-lattice of NbSe2 [nm] 0.346(1)

a-lattice of SnSe [nm] 0.599(1)

1.223(2)

[°] 0.300(2)

2

1.858(7)

0.260(2)

-

-

-

-

3

2.492(7)

0.307(4)

0.346(1)

0.598(1)

0.16(1)

6(1)

4

3.129(9)

0.335(4)

0.346(1)

0.598(1)

0.16(1)

5(1)


Misfit Parameter [δ] 0.16(1)

Crystallite Size [nm] 9(1)


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-two compounds and also smaller than found in bulk NbSe2. In-plane diffraction patterns of selected compounds (n = 1, 3 and 4) are displayed in Figure 5. Typical for ferecrystals, independent crystal structures for the constituents are observed.16 The SnSe basal unit remains square and the dimensions also remain constant as n is increased (See Table 1). Similarly, the a-lattice parameter of the hexagonal NbSe2 constituent remains constant as a function of n, with a basal plane lattice parameter more similar to that of the 2H- NbSe2 polytype (0.345(1) nm)24 rather than a 1T-NbSe2 polytype (0.353(1) nm).25 Interestingly, broadening of the NbSe2 peaks is observed as a function of n suggesting a reduction in in-plane coherency as a function of n. Based on the lack of a change of in-plane lattice parameters as n increases and prior reports that show that free standing flakes of ferecrystals remain flat, we assume that internal strains are small and do not contribute significantly to the line broadening. Using the FWHM of the (110) and (120) NbSe2 peaks and assuming all of the broadening is caused by crystallite size, crystallite sizes for n = 1, 3 and 4 were determined to be 9(1) nm, 6(1) nm and 5(1) nm respectively, based on the Scherrer equation. This crystallite size reduction with n indicates the presence of more grain boundaries in the higher n compounds.

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along the c-axis are displayed. Mirror planes of the space group P-3m1 are designated by dotted lines. The errors of all atomic plane distances are below 0.002 nm. The results 20 for the (1,1) compound were reported previously. A standard Pseudo-Voigt peak shape function was used for the refinements, which resulted in low residual values of RF = 0.08 and RI = 0.04 for the (1.2) compounds and of RF = 0.10 and RI = 0.09 for the (1,3) compound.

Figure 5. In-plane diffraction pattern of ([SnSe]1.16)1(NbSe2)n ferecrystals, where n = 1, 3 and 4. The inset shows the broadening of the (110) peak of the NbSe2 as a function of n.

Figure 4. Rietveld refinement results for ([SnSe]1.16)1(NbSe2)n , where n = 1, 2 and 3. Atomic positions of all the three compounds for half of the unit cell

ELECTRICAL PROPERTIES Figure 6a displays the temperature dependent resistivity of all four compounds and single crystalline NbSe226 within a temperature range of 20-295 K. All compounds exhibit metallic behavior. The resistivity minimum observed for compounds (1,3) and (1,4) at low temperatures might arise from electron-electron scattering effects in narrow anisotropic materials which results in carrier localization.27 -

Figure 6. a) Temperature dependent electrical resistivity of ([SnSe]1.16)1(NbSe2)n ferecrystals and that published previously for a single crystal NbSe2.(ref) b) The variation of the room temperature resistivity as a function of n compared with that calculated using the data for the (1,1) compound, using a parallel resistor equivalent circuit model and assuming that all of the transport occurs through NbSe2.


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Figure 7. a) Temperature dependent Hall coefficient of ([SnSe]1.16)1(NbSe2)n ferecrystals with single crystalline NbSe2 ted as a comparison b) Actual and predicted carrier concentration for all four compounds at room temperature.

The magnitude of the resistivity at room temperature (RT) follows an unexpected trend, (1,3) > (1,4) ≥ (1,1) > (1,2), as a simple equivalent circuit model of parallel resistors28 that assumes that the charge transport occurs in NbSe2 and that there is no interaction between the constituting layers (SnSe and NbSe2) predicts that the electrical resistivity should decrease systematically with increasing number of conducting layers. Figure 6b shows the deviation of the measured RT resistivity from the predicted based on this parallel resistor model. This unexpected trend in the resistivity may be a consequence of either variations in carrier concentration or the mobility of the dominant carriers. To gain more insight into the unexpected transport behavior, Hall measurements were conducted for all the compounds. The Hall coefficients of all the four compounds are positive, indicating that the dominant carriers are holes. The same is true for bulk NbSe2, which has been reported to have a positive Hall coefficient above the charge density wave transition temperature (35 K).26 Figure 7a shows the temperature dependent Hall coefficient of all the four compounds graphed with that for single crystalline NbSe2.26 Upon cooling, the Hall coefficient of all the compounds increase, suggesting localization of carriers similar to that observed and reported for the 3R-NbS2 bulk compound.27 The (1,1) compound has a larger increase, with a distinct change in slope around 150 K, similar to that observed in bulk VSe2 at the temperature of its charge density wave onset.29 The difference between the temperature dependence of the (1,1) compound compared to the other compounds might result from a change in dimensionality, with the single NbSe2 sheet resulting in increased localization of carriers as previously reported for 2D- NbSe2.30 The magnitude of the Hall coefficient at room temperature decreases systematically from the (1,1) to the (1,4) compound, with the (1,4) compound approaching the value of bulk NbSe2. The systematic decrease in the Hall coefficient of the compounds suggests that there is charge transfer from the SnSe to the NbSe2 layers, which is spread over all of the conducting NbSe2 layers as n is increased. The systematic trend in the carrier concentration calculated

26

plot-

from the Hall coefficient assuming a single band model (see Figure 7b) points towards the mobility as the main cause of the unexpected resistivity trend observed. Mobility values, for all four compounds, were calculated based on the single band model approximation using the Hall coefficient and resistivity values. As displayed in Figure 8, the temperature dependent mobility values decrease with increasing thickness of NbSe2. At room temperature, the mobility of the (1,1), (1,2) and (1,3) compounds are almost equivalent to the mobility of a single crystalline NbSe2 (3-4 cm2V-1s-1)26 while the mobility of the (1,4) compound is significantly lower than the single crystalline NbSe2.26 The lower mobility of the (1,4) compound might be a consequence of increased stacking disorder or the presence of more grain boundaries as suggested by the broadening of the in-plane reflections.

Figure 8. Log –log plot of the temperature dependent mobility of ([SnSe]1.16)1(NbSe2)n ferecrystals with single crystalline 26 NbSe2 plotted as a comparison.

To gain more insight into the cause of the lower mobility in the (1,4) compound, HAADF-STEM images were collected. Consecutive units of four NbSe2 layers and one double layer of SnSe are observed (Figure 9) with atomically sharp interfaces between constituting layers as expected for the (1,4) layering scheme. The HAADFSTEM image contains different zone axes for both con-



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stituents throughout the compound (e.g. (110)- SnSe and (100)- SnSe), revealing the presence of a rotational disorder. Several crystallographic orientations for the NbSe2 constituent can also be identified both within the same 4 layer repeat and in different layers. Figure 9 shows schematic representations of this turbostratic disorder between consecutive layers of the (1,4) compound, which is a key feature of ferecrystals.16

Figure 9. HAADF-STEM image of the ([SnSe]1.16)1(NbSe2)4 compound with the repeat unit and schematic representations of the structure highlighting the rotational disorder.

Two types of coordination environments can also be observed for the Nb atoms in the (1,4) compound, with chevron shapes corresponding to a (100) orientation of NbSe2 identifying trigonal prismatic coordination for the Nb atoms (see Figure 10a) and straight barbells identifying regions with octahedrally coordinated Nb atoms (see Figure 10b). In bulk NbSe2, octahedrally and trigonal prismatically coordinated Nb atoms are found in the 1Tand 2H, 3R and 4Ha- NbSe2 polytypes respectively.4,31 A mixture of the two coordinations is found in the 4Hb – NbSe2 polytype.8 The presence of different coordination environments of Nb, which result in different splitting of the d-band, can lead to differences in transport properties. In the previously reported ([SnSe]1.16)1(NbSe2)1 compound, only the trigonal prismatic coordination for the Nb atom was observed.20

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([PbS]1.14)1(NbS2)2 where the NbS2 forms 2/3rd of a 3R polytype.5 However, this was not observed in the selenide analogue, which crystallizes in a 2H-NbSe2 polytype.14 Figure 11b, c and d contain the HAADF-STEM images, for the (1,4) compound. Several regions with 2H- NbSe2 and 2/3rd- of 3R-NbSe2 stacking sequences are observed (see Figure 11b). In addition, a new stacking sequence, with four consecutive layers of NbSe2 oriented in the same direction, can be found (see Figure 11d). This stacking sequence has not been reported in misfit layer compounds or any of the bulk NbSe2polytypes.5 The variation of stacking sequences reflects the formation mechanism and the presence of turbostratic disorder. Figure 11b also shows high concentrations of grain boundaries within the same NbSe2 layer. Going from left to right in Figure 11b, two out of four consecutive layers of the NbSe2 change orientation within the same layer; consequently a change in the stacking sequence from 2/3rd of a 3R-NbSe2 to a 2H-NbSe2 is observed. A similar scenario is found in Figure 11c where the same NbSe2 block is part of a 2H- and 3R-NbSe2 stacking sequence. Interestingly, several areas where the direction of the chevrons abruptly switches, a cross-like geometry with Nb in the center is observed (see Figure 11d and e). It appears that at this specific grain boundary, the Nb atom is coordinated to eight Se atoms instead of six. NbSe4 and NbTe4 have previously been reported with the Nb atom located in the center of an antiprism made up of two square Se4 or Te4 units.32 However, in this case the two squares would form a cuboid around the Nb atom due to the symmetry restrictions from the neighboring NbSe2. Consistent with the broadening observed in the in-plane X-ray diffraction, the HAADF-STEM images show that increasing the thickness of NbSe2 leads to more grain boundaries and different stacking sequences. In general, the disorder within the NbSe2 constituent increases as a function of n, and thus results in decreasing carrier mobility.

Figure 10. a) Trigonal prismatically and (b) Octahedrally coordinated Nb in ([SnSe]1.16)1(NbSe2)4 compound

Increasing the thickness of NbSe2 allows for the formation of different stacking sequences (see Figure 1). Previously, for the ([SnS]1.16)1(NbS2)n with n = 2 and 3 misfit layer compounds, several different polytypes like 2/3rd of a 3R for n = 2 and 3/4th of a 4Ha for n = 3 were reported.15 Similar observations were made for


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Figure 11. a) An HAADF-STEM image of ([SnSe]1.16)1(NbSe2)4 ferecrystals with multiple repeating units. b) Different polytypes of NbSe2 within the same unit of NbSe2 c) The same layer of NbSe2 as part of two polytypes is highlighted. d) A new type of stacking sequence other than the well-established polytypes e) A possible NbSe4 type of coordination of Nb is highlighted.

CONCLUSION ([SnSe]1.16)1(NbSe2)n ferecrystals with increasing number of NbSe2 layers (1-4) were synthesized. A change in the c-lattice parameter by 0.635(2) nm per NbSe2 layer confirmed the synthesis of the intended superlattice. Increasing the thickness of the NbSe2 allowed the formation of different varieties of stacking sequences with distinctive orientations within the same layer of NbSe2. Several grain boundaries were observed in the HAADFSTEM image of the (1,4) compound. A corresponding decline in crystallite size as a function of n was also observed. Two Nb coordination environments, trigonal prismatic and octahedral, were identified in the STEM images, with trigonal prismatic predominating. Different stacking sequences of the NbSe2 layer including fractions of a 2H, 3R and a new stacking sequence were found. Transport property measurements revealed the carrier concentration systematically increasing as a function of n. The resistivity does not vary systematically with n because the carrier mobility decreases with increasing disorder as a function of n. AUTHOR INFORMATION

for assistance in preparing TEM samples and collecting STEM images. Grant MRI 0923577 provided funding for the dual beam FIB used to make TEM cross sections. The authors acknowledge support from the National Science Foundation under grant DMR-1266217. Coauthor MF acknowledges support from the National Science Foundation through CCI grant number CHE-1102637.

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Rietveld refinement analysis with fit parameters are provided as a supplementary information. This information is available free of charge via the Internet at http://pubs.acs.org/

ACKNOWLEDGMENT The authors thank R. Fischer and J. Razink from CAMCOR

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(13) Meerschaut, A.; Guemas, L.; Auriel, C.; Rouxel, J. Eur. J. Solid State Inorg. Chem. 1990, 27, 557–570. (14) Rouxel, J.; Meerschaut, A. Physics and Chemistry of Low-Dimensional Inorganic Conductors; Schlenker, C.; Dumas, J.; Greenblatt, M.; van Smaalen, S., Eds.; Plenum press: New York, 1996; pp. 59–70. (15) Hoistad, L. M.; Meerschaut, A.; Bonneau, P.; Rouxel, J. J. Solid State Chem. 1995, 114, 435–441. (16) Beekman, M.; Heideman, C. L.; Johnson, D. C. Semicond. Sci. Technol. 2014, 29, 064012. (17) Moore, D. B.; Beekman, M.; Disch, S.; Johnson, D. C. Angew. Chem. Int. Ed. Engl. 2014, 53, 5672–5. (18) Fister, L.; Johnson, D. C. J. Am. Chem. Soc. 1992, 114, 4639–4644. (19) 1–7.

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(20) Alemayehu, M. B.; Falmbigl, M.; Grosse, C.; Ta, K.; Fischer, S. F.; Johnson, D. C. J. Alloys Compd. 2015, 619, 861–868. (21) Atkins, R.; Disch, S.; Jones, Z.; Haeusler, I.; Grosse, C.; Fischer, S. F.; Neumann, W.; Zschack, P.; Johnson, D. C. J. Solid State Chem. 2013, 202, 128–133. (22) Li, L.; Xu, Z.; Shen, J.; Qiu, L.; Gan, Z. J. Phys. Condens. Matter 2005, 17, 493–498. (23) Roisnel, T.; Rodriguez-Carvajal, J. Anal. Mater. Sci. Forum 2001, 118, 378–381. (24) Brown, B. E.; Beerntsen, D. J. Acta Crystallogr. 1965, 18, 31–36. (25)

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(26) Lee, H. N. S.; McKinzie, H.; Tannhauser, D.S.; Wold, A. J. Appl. Phys. 1969, 40, 602. (27) Nazi, A.; Rastogi, A.K.; J. Phy. Condens. Matter. 2001, 13, 6787-6796. (28) Alemayehu, M. B.; Mitchson, G.; Hanken, B. E.; Asta, M.; Johnson, D. C. Chem. Mater. 2014, 26, 1859–1866. (29) Bayard, M.; Sienko, M.J. J. Solid State Chem. 1970, 19, 325-329. (30) Novoselov, K.; Jiang, D. Proc. Natl. Acad. Sci. 2005, 102, 10451-10453. (31) Kadijk, F. Recl. des Trav. Chim. des Pays-Bas 1964, 83, 768–775. (32) Selte, K.; Kjekshus, A. ACTA CHEM SCAND 1965, 1002–1024.


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Table of Content


9


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Figure 1. Different polytypes of TMDs of formula NbSe2 with the symmetry of each orientation given. Highlighted in red are their orientations in a unit cell. 254x88mm (150 x 150 DPI)


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Figure 2. a) Specular X-ray diffraction data for the ([SnSe]1.16)1(NbSe2)3 compound collected after annealing the sample at the indicated temperatures for 20 minutes. 400 °C was chosen as the optimal annealing condition based in intensity and line width of the (00l) reflections. b) The evolution of the (008) reflection after annealing the sample at the indicated tem-peratures.

201x75mm (150 x 150 DPI)



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Figure 3. X-ray diffraction of ([SnSe]1.16)1(NbSe2)n , where n = 1-4. All reflections can be indexed as 00l reflections, and selected peaks for each compound are indexed. (*) designates the silicon substrate peaks. 83x72mm (150 x 150 DPI)


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Figure 4. Rietveld refinement results for ([SnSe]1.16)1(NbSe2)n , where n = 1, 2 and 3. Atomic positions of all the three compounds for half of the unit cell along the c-axis are displayed. Mirror planes of the space group P-3m1 are designated by dotted lines. The errors of all atomic plane distances are below 0.002 nm. The results for the (1,1) compound were reported previously.20 84x43mm (150 x 150 DPI)



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Figure 5. In-plane diffraction pattern of ([SnSe]1.16)1(NbSe2)n ferecrystals, where n = 1, 3 and 4. The inset shows the broadening of the (110) peak of the NbSe2 as a function of n. 78x72mm (150 x 150 DPI)


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Figure 6. a) Temperature dependent electrical resistivity of ([SnSe]1.16)1(NbSe2)n ferecrystals and that published previously for a single crystal NbSe2.(ref) b) The variation of the room temperature resistivity as a function of n compared with that calculated using the data for the (1,1) compound, using a parallel resistor equivalent circuit model and assuming that all of the transport occurs through NbSe2. 260x97mm (150 x 150 DPI)



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Figure 7. a) Temperature dependent Hall coefficient of ([SnSe]1.16)1(NbSe2)n ferecrystals with single crystalline NbSe2 26 plotted as a comparison b) Actual and predicted carrier concentration for all four compounds at room temperature. 406x151mm (150 x 150 DPI)


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Figure 8. Log –log plot of the temperature dependent mo-bility of ([SnSe]1.16)1(NbSe2)n ferecrystals with single crystal-line NbSe2 26 plotted as a comparison. 141x103mm (150 x 150 DPI)



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Figure 9. HAADF-STEM image of the ([SnSe]1.16)1(NbSe2)4 compound with the repeat unit and schematic representations of the structure highlighting the rotational disorder. 84x58mm (150 x 150 DPI)


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Figure 10. a) Trigonal prismatically and (b) Octahedrally coordinated Nb in ([SnSe]1.16)1(NbSe2)4 compound

84x35mm (150 x 150 DPI)



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Figure 11. a) An HAADF-STEM image of ([SnSe]1.16)1(NbSe2)4 ferecrystals with multiple repeating units. b) Different poly-types of NbSe2 within the same unit of NbSe2 c) The same layer of NbSe2 as part of two polytypes is highlighted. d) A new type of stacking sequence other than the well-established polytypes e) A possible NbSe4 type of coordination of Nb is highlighted. 174x75mm (150 x 150 DPI)


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112x74mm (72 x 72 DPI)


Effect of Local Structure of NbSe2 on the Transport

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