Suppressing a Charge Density Wave by Changing Dimensionality in

Dec 29, 2014 - the Ferecrystalline Compounds ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, 4 ... The charge density wave transition observed in the resistivi...
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Suppressing a Charge Density Wave by Changing Dimensionality in the Ferecrystalline Compounds ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, 4 Matthias Falmbigl,† Andreas Fiedler,‡ Ryan E. Atkins,† Saskia F. Fischer,‡ and David C. Johnson*,† †

Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403, United States Novel Materials, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany

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S Supporting Information *

ABSTRACT: The compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4, were prepared using designed precursors in order to investigate the influence of the thickness of the VSe2 constituent on the charge density wave transition. The structure of each of the compounds was determined using Xray diffraction and scanning transmission electron microscopy. The charge density wave transition observed in the resistivity of ([SnSe]1.15)1(VSe2)1 was confirmed. The electrical properties of the n = 2 and 3 compounds are distinctly different. The magnitude of the resistivity change at the transition temperature is dramatically lowered and the temperature of the resistivity minimum systematically increases from 118 K (n = 1) to 172 K (n = 3). For n = 1, this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when n is greater than 1, and the temperature dependence of the Hall coefficient of the n = 2 and 3 compounds is very similar to the bulk, slowly decreasing as the temperature is decreased, while for the n = 1 compound the Hall coefficient increases dramatically starting at the onset of the charge density wave. The transport properties suggest an abrupt change in electronic properties on increasing the thickness of the VSe2 layer beyond a single layer. KEYWORDS: Charge density wave, vanadium diselenide, nanostructure, ferecrystal, quasi-2D material

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charge density wave state increases from single crystalline bulk VSe2 at 100 K13 to 135 K.14 However, recently the opposite trend was observed, with the transition temperature of VSe2 decreasing to 81.8 K at the lowest thickness of 11.6 nm for micromechanically exfoliated nanoflakes.15 Besides the mechanical and liquid exfoliation method used by Yang et al.15 and by Xu et al.,14 respectively, we believe the only other method used to synthesize vanadium diselenide ultrathin films reported in the literature is an atmospheric pressure chemical vapor deposition technique.16 All of these techniques lack the ability to control the film thickness by individual X−T−X trilayers or to prepare a film that is precisely a single trilayer thick. While a preferred orientation of the (001)14 and the (101) or (110)16 planes was observed, a control of the crystallographic orientation is not possible. The modulated elemental reactant (MER) method17 provides an elegant route to composite crystals, enabling absolute independent control of the layer thicknesses (m and n) of the two subunits of ([SnSe]1.15)m(VSe2)n. The highly preferred crystallographic orientation of the constituents permits the structure of the individual layers to be determined from diffraction patterns.18 Recently, the first charge density wave transition in an intergrowth compound was reported for

ince the discovery of the striking change of fundamental properties when the thickness of a compound is reduced to a single structural unit, such as three-dimensional (3D) graphite to a 2D sheet of graphene,1−3 much notice and effort has been focused on exploring additional quasi 2D materials that might be prepared as a single layer. Ultrathin 2D-nanosheets of layered transition metal dichalcogenides (TMD) with the general chemical formula TX2 (where T = transition metal, X = chalcogenide) have received special attention because their electrical properties range from insulating to semiconducting up to metallic depending on the transition metal.4 Nanosheets of these compounds have been explored for a wide range of applications, such as electrocatalysts for hydrogen evolution or hydrosulfurization, opto- and spin-electronics,5,6 electrodes in energy storage devices,7 and data storage devices.8 A challenge in these studies has been preparing high-quality, uniform, single layer thick TX2-nanosheets.7 VSe2 is among several TMDs, which exhibit charge density waves (CDW):4,9 a modulation of the conduction electron density accompanied by a modulation of the atomic positions within the crystal structure.10 Recently, it was demonstrated that the CDW transition temperature of a mechanically exfoliated TiSe2 film increases from ∼20011 to ∼240 K on lowering the overall thickness to a nanometer scale.12 A similar effect was reported for ultrathin nanosheets of VSe2 (4−8 trilayers of the TMD), which were prepared by a liquid exfoliation method. The phase transition temperature into the © 2014 American Chemical Society

Received: September 26, 2014 Revised: December 12, 2014 Published: December 29, 2014 943

DOI: 10.1021/nl503708j Nano Lett. 2015, 15, 943−948

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Nano Letters ([SnSe]1.15)1(VSe2)1 showing a large increase in resistivity accompanied by a decrease in carrier concentration below the onset temperature.19 Because no CDW was observed in the analogous misfit layer compound,20 the presence of the CDW was attributed to the turbostratic disorder observed in the ferecrystalline polymorph. 1 9 A series of samples ([SnSe]1.15)m(VSe2)1 with m = 1, 2, 3, and 4 showed that separating the single VSe2 layers by increasingly thick layers of SnSe increased the transition temperature of the CDW from 118 to 158 K.21 The temperature dependence of the electrical resistivity and Hall-coefficient were dramatically different than an ultrathin film of VSe2,14 VSe2 nanoflakes,15 or bulk VSe2.13 This observation prompted us to prepare a series of samples ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4 to investigate the effect of increasing the thickness of the VSe2 constituent. This is the first study demonstrating the influence of the layer thickness of the dichalcogenide constituent on the electrical properties of ferecrystals. X-ray diffraction and scanning electron microscopy studies confirmed the systematic addition of single layers of VSe2, creating compounds with 1 to 4 VSe2 layers sandwiched between a SnSe bilayer (see Figure 1). The

Figure 2. Specular X-ray diffraction patterns of ([SnSe]1.15)1(VSe2)n with n = 1−4. Two 00l diffraction peaks are indexed for each sample. Inset (a) displays rocking curves performed on the 00(2n + 2) reflection, and (b) displays the c-lattice parameter as a function of n.

half-maximum (fwhm) of rocking curves on the 00(2n+2) peak of each sample indicates the almost perfect orientation of the ab planes parallel to the substrates (See inset Figure 2 (a) and Table 1). The inset (b) of Figure 2 shows the c-lattice parameters of ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4 as a function of n, the number of consecutive VSe2-layers. The slope reveals a single layer thickness of 0.596(1) nm for the VSe2 subunit, which is slightly smaller than the c-lattice parameter range reported for bulk 1T-V1+xSe2 (0.6105−0.5970 nm), where the stoichiometric compound has a value of 0.6105 nm.23 The precisely linear trend of the c-lattice parameter vs n corroborates the ability of the MER-technique to precisely control the dimensionality of the VSe2 constituent by crystallizing a defined number of Se−V−Se trilayers of the 1T structure. STEM images of ([SnSe]1.15)1(VSe2)1 and ([SnSe]1.15)1(VSe2)3 confirm the formation of the layered superstructure with atomically abrupt interfaces between each layer. The precise alignment along the c-axis can be seen (Figure 3). The three consecutive units of VSe2 layers in ([SnSe]1.15)1(VSe2)3 display a similar crystallographic orientation as typical for the 1-T structure of binary VSe2 synthesized by the same method24 (Figure 3). However, the orientation changes between separate unit cells, which is due to the rotational disorder found in ferecrystalline compounds.18 An even more extensive amount of turbostratic disorder is observed for ([SnSe]1.15)1(VSe2)1, where the crystallographic orientation of each VSe 2 layer varies. Compositional information extracted from WDX-EPMA data (see Table 1) shows that the Sn/V-ratio for ([SnSe]1.15)1(VSe2)n is slightly below the misfit value reported from in-plane X-ray diffraction for the ([SnSe]1.15)1(VSe2)1 ferecrystal.19 The in-plane X-ray diffraction pattern for ([SnSe]1.15)1(VSe2)3 clearly exhibits two separate, independent crystallographic subunits with individual lattice parameters resulting in a misfit of 0.16(1), as plotted in Figure 4. The SnSe constituent can be indexed to a square basal plane with an alattice parameter of 0.595(1) nm, which is within the range reported for ([SnSe]1.15)m(VSe2)1 with m = 1−4 (0.59923(7) − 0.59976(2) nm).21 The thermodynamically stable bulkmodification of SnSe crystallizes in an orthorhombically distorted derivative of the rock salt structure (GeS-type),25 where the corresponding lattice parameters for the rock salt unit cell would be 0.628 and 0.588 nm, respectively. The peaks corresponding to the VSe2 constituent can be indexed as hk0

Figure 1. Scheme of the change in dimensionality by modifying the stacking sequence of ([SnSe]1.15)1(VSe2)n from n = 1 to n = 3 for (1,n).

large increase in electrical resistivity below 118 K observed in ([SnSe]1.15)1(VSe2)1 is not observed in ([SnSe]1.15)1(VSe2)2 or ([SnSe]1.15)1(VSe2)3, which contains two and three trilayers of VSe2 respectively. The electrical properties of the n = 2 and 3 compounds are similar to that observed for bulk VSe2 rather than ([SnSe]1.15)1(VSe2)1. Further increases in n, the thickness of the VSe2 constituent, result in only subtle changes in the electrical resistivity. The synthesis and calibration of the deposition system required to prepare the series of samples discussed in this report are described in detail elsewhere.19,21 Briefly, the synthesis technique utilizes a physical vapor deposition process22 to create precursors, which mimic the desired product already on an atomic scale.17 The sequential deposition of layers of elemental Se, Sn, and V with precisely defined thicknesses controls the formation of specific compounds.18 We used this approach to manipulate the thickness of the VSe2 layer from a quasi-2D scenario in ([SnSe]1.15)1(VSe2)1 to increasingly 3D blocks in ([SnSe]1.15)1(VSe2)n as n increases from 2 to 4. The products are highly oriented with the c-direction perpendicular to the substrate, and the specular X-ray diffraction patterns of all samples (Figure 2) reveal sharp superlattice peaks belonging to 00l reflections of the corresponding c-lattice parameter. The narrow full width at 944

DOI: 10.1021/nl503708j Nano Lett. 2015, 15, 943−948

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Table 1. c-Lattice Parameter, Full Width at Half-Maximum (fwhm) of a Rocking Curve on the 00(2n + 2) reflection, Sn/VRatio, Room-Temperature Electrical Resistivity (ρRT), Temperature of the ρ(T)-Minimum (Tmin), Ratio between Maximum (ρmax) and Minimum (ρmin) Electrical Resistivity, and Hall Coefficient (RH) at 230 K (m,n)

c-lattice parameter [nm]

fwhm [deg]

Sn/V ratio

ρRT [μΩ cm]

Tmin [K]a

ρmax/ρminb

RH at 230 K [*10‑4 cm3 C−1]

(1,1) (1,2) (1,3) (1,4)

12.03(1) 17.98(1) 23.92(1) 29.90(1)

1.211 0.750 0.671 0.479

1.06 0.54 0.36 0.24

506(1) 524(1) 387(1)

118(9) 166(7) 172(8)

2.24 1.06 1.05

6.5(4) 6.4(2) 6.6(2)

a Values determined from the minimum in a cubic spline fit. bRatio of electrical resistivity values at the maximum and the minimum of a cubic spline fit.

Figure 3. HAADF-STEM images of the samples ([SnSe]1.15)1(VSe2)n with n = 1 and 3. The different crystallographic orientations of the individual layers are highlighted.

Figure 4 . Grazing incidence in-plane diffra ction fo r ([SnSe]1.15)1(VSe2)3 displaying the two individual crystal structures in the a−b plane.

reflections of a hexagonal basal plane consistent with the bulk 1-T-structure. The resulting a-lattice parameter is 0.344(1) nm, which is slightly larger than reported for bulk 1-T V1+xSe2 (0.3358−0.3437 nm, where a increases with x).23 The value is within the range of 0.3414(3) − 0.34630(3) reported for ([SnSe]1.15)m(VSe2)1 with m = 1−4.21 Rietveld refinements were carried out for the out-of-plane diffraction data to obtain the positions or layers of atoms along the superlattice direction (see Figure 5). The distances between the V and Se layers (∼0.154 nm) within the dichalcogenides as well as the van der Waals gap between the VSe2 layers (0.289 nm) in the samples with n > 1 do not change within the measurement uncertainties and are close to the distances observed in 1-T VSe2 (0.153 and 0.306 nm, respectively).26 ([SnSe]1.15)1(VSe2)1 exhibits a significantly shorter distance between the VSe2 and the SnSe layer than the compounds with thicker VSe2 constituent layers. At the same time, the puckering, an out-of-plane distortion of the Sn in the rock

Figure 5. Rietveld refinement results of out-of-plane X-ray diffraction patterns for ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4. 00l reflections contain information on the distance between atomic planes along the c-direction of the crystal structure. *Data for the (1,1) compound are results from ref 28.

salt layer toward the Se in the dichalcogenides layer, is almost a factor of 2 higher in ([SnSe]1.15)1(VSe2)1 compared to the samples with higher n values. Both observations indicate a stronger interaction between the SnSe and VSe2 layers in ([SnSe]1.15)1(VSe2)1 than in the compounds with higher n values. The puckering of the rock salt layer is a well-known structural feature in common for misfit layer compounds20 and ferecrystals.27 For ([SnSe]1.15)m(VSe2)1 it was reported that the puckering for the rock salt layer adjacent to the VSe2 increases by 0.026 nm as a function of m reflecting a more pronounced interaction between the Sn in the rock salt layer and the Se in the dichalcogenide layer.21 945

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Nano Letters The normalized temperature dependence of the electrical resistivity, ρ, of ([SnSe]1.15)1(VSe2)n with n = 1, 2, and 3 differs significantly from bulk single crystalline VSe213 as well as from ultrathin nanosheets14 and mechanically exfoliated nanoflakes15 as displayed in Figure 6. The resistivity and the Hall-voltage

For the ferecrystalline compounds ([SnSe]1.15)m(VSe2)1 an increase of TCDW with increasing m was observed and attributed to a stronger interlayer interaction and/or approaching a truly 2D scenario as the separation between the VSe2-layers increases.21 Interestingly, in contrast to most other TMDs the charge density wave state of VSe2 is stabilized under pressure, which was explained by the pressure-induced broadening of the dconduction band in the presence of strong electron Coulomb repulsion.30 A similar behavior was observed for Nb substituted VSe2, where the stabilization of the CDW was also attributed to band broadening by decreasing the c/a-lattice parameter ratio and thereby lowering the Jahn−Teller distortion.31 In the present case, the c/a-lattice parameter ratio of 1.73 is significantly smaller than for bulk-VSe2 (1.82), which might stabilize the CDW state within the ferecrystalline compound. The smaller lattice parameter ratio in the ferecrystals can be attributed to the interlayer interaction between the two constituents. The temperature dependencies of the Hall-coefficients, RH, of ([SnSe]1.15)1(VSe2)n with n = 1, 2, and 3 are shown in Figure 7. Unlike bulk VSe2 and the nanoflakes,15 RH at room

Figure 6. Electrical resistivity normalized to the room temperature value (ρRT) for ([SnSe]1.15)1(VSe2)n with n = 1, 2, and 3. For comparison, data of single crystalline bulk VSe213 (ρRT = 220.7 μΩ cm) and an ultrathin film of VSe214 (ρRT = 1 × 105 μΩ cm) are also plotted. The inset shows the temperature dependence for selected samples on an expanded scale.

measurement for the single crystalline VSe2 were conducted perpendicular to the c-axis (corresponding to the in-plane geometry applied to the thin film samples within this investigation) and are in sound agreement with a similar study by van Bruggen and Haas.29 Whereas the electrical resistivity of the bulk sample shows a small anomaly around 100 K, ([SnSe]1.15)1(VSe2)n compounds exhibit a more pronounced discontinuity with a pronounced minimum and maximum in the electrical resistivity, shifting to higher temperatures as the number of consecutive dichalcogenide layers increases (Table 1). The increase of the electrical resistivity is also distinctively different from the behavior observed for the different nanosheets,14,15 which exhibit only a marginal increase of ρ at the transition temperature (see inset Figure 6). The electrical resistivity of the ferecrystalline compounds is in the same order of magnitude as bulk VSe2 (ρRT = 220.7 μΩ cm)13 and mechanically exfoliated nanoflakes (ρRT ∼ 230 μΩ cm),15 but by 2 orders of magnitude lower than the ultrathin nanosheets (ρRT = 1 × 105 μΩ cm).14 There is an abrupt decrease of the ρmax /ρmin ratio from 2.24 for ([SnSe]1.15)1(VSe2)1 to 1.05(1) for the samples with two and three consecutive VSe2-layers (see Table 1) indicating an abrupt change in the character of the transition as the thickness of the VSe2 is increased beyond a single layer. The distinct upturn in resistivity is most pronounced in ([SnSe]1.15)1(VSe2)1 and occurs for all three samples over a wider temperature range compared to bulk VSe2 and the different nanosheets (see Figure 6). The indications of an increase of the transition temperature (TCDW) with increasing thickness in the c-direction are consistent with the thickness dependence of the TCDW reported for VSe2 nanoflakes.15 However, for the 4−8 layers thick VSe2 sheet and the exfoliated TiSe2 samples, an increase of the transition temperature as the thickness decreases was reported.14,12 In the former case the increase might stem from residual organic molecules trapped between the layers.15

Figure 7. Hall coefficient for ([SnSe]1.15)1(VSe2)n with n = 1, 2, and 3. For comparison data of single crystalline bulk VSe213 are also plotted.

temperature is positive for all compounds (see Table 1) and remains positive throughout the temperature range investigated for ([SnSe]1.15)1(VSe2)1. This reflects charge transfer between the constituent layers of the ferecrystal, a phenomenon believed to be the main cause for stabilization of misfit layer compounds32 and ferecrystals.33 For ([SnSe]1.15)1(VSe2)1 RH increases by a factor of 10 in the same temperature interval where the resistivity changes, which is indicative of a CDW transition.4,34 The temperature-dependent Hall-coefficient in the samples with two and three trilayer thick VSe2 layers does not exhibit any anomalies in the temperature of the minimum in resistivity (see Figures 6 and 7) within the measurement uncertainties. The Hall data suggest that the charge density wave transition in the compound containing only a monolayer of VSe2 is distinctly different from that observed in samples with thicker VSe2 layers, which are much more similar to bulk. This suggests that the CDW is suppressed by the changing dimensionality in the ferecrystalline compounds ([SnSe]1.15)1(VSe2)n from n = 1 to n = 2 and 3 and/or by changes in the interlayer interactions as discussed above for the structural data. The sign of the Hall coefficient changes between 4 and 20 K in the samples ([SnSe]1.15)1(VSe2)n with n 946

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= 2 and 3 indicating that electrons become the majority charge carriers in this temperature regime. This observation also demonstrates that increasing the VSe2 thickness causes a more bulklike temperature dependence of RH. In conclusion, a series of ferecrystalline compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4 was synthesized in order to study the influence of the dimensionality on the charge density wave state in the VSe2 layer. The thickness along the c-axis of the superstructure was increased by adding single layers of VSe2 starting from one layer up to four consecutive layers. All samples exhibit a minimum in their electrical resistivity that is more pronounced than that observed in either bulk single crystals or nanosheets of VSe 2 . For ([SnSe]1.15)1(VSe2)1 there is a much larger change in resistivity accompanied by a steep increase of the Hall-coefficient in the same temperature range, pointing toward a charge density wave transition. This behavior is distinctly different in character compared to the compounds with higher values of n, which have a completely different temperature dependence of the Hall coefficient already closer to the bulk VSe2 rather than to ([SnSe]1.15)1(VSe2)1. The data presented demonstrate the importance of exact control of the layer sequence in nanostructured thin films. Experimental Methods. All thin film samples were synthesized using physical vapor deposition (PVD) in a custom-built vacuum deposition chamber.22 The PVD process was carried out on two different substrates, either (100) oriented Si-wafers for structural characterizations or silicon with thermally grown oxide (∼100 nm) for the measurement of electrical properties. Details on the operation of the deposition system and the calibration procedure for the initial precursor films are described elsewhere.19,21 All samples were annealed on a hot plate at 400 °C for 20 minutes inside a glovebox under nitrogen atmosphere (