Selective Fabrication of Mott-Insulating and Metallic Monolayer TaSe2

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Selective Fabrication of Mott-Insulating and Metallic Monolayer TaSe

Yuki Nakata, Takuya Yoshizawa, Katsuaki Sugawara, Yuki Umemoto, Takashi Takahashi, and Takafumi Sato ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00184 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Nano Materials

Selective Fabrication of Mott-Insulating and Metallic Monolayer TaSe2 Yuki Nakata1*, Takuya Yoshizawa1, Katsuaki Sugawara2,3, Yuki Umemoto1, Takashi Takahashi1,2,3, and Takafumi Sato1,3 1

Department of Physics, Tohoku University, Sendai 980-8578, Japan 2

WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

3

Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan *E-mail: [email protected]

ABSTRACT: We report the selective fabrication of monolayer TaSe2 with trigonal prismatic (1H)

or

octahedral

(1T)

crystal

molecular-beam-epitaxy method.

structure

on

bilayer

graphene

by

the

By angle-resolved photoemission spectroscopy, we

found that monolayer 1H-TaSe2 shows the metallic electronic structure with a dispersive band across the Fermi level (EF) similarly to the bulk crystal, while monolayer 1T-TaSe2 has a sizable energy gap at EF indicative of the insulating nature in contrast to the metallic nature of bulk 1T-TaSe2. We discuss the origin and implications of differences in the electronic structure among monolayer and bulk TaSe2 by comparing the experimental results with first-principles band-structure calculations as well as previous studies on bulk TaSe2.

KAYWORDS: transition-metal dichalcogenides, 2D materials, TaSe2, Mott insulator, angle-resolved photoemission spectroscopy, electronic states

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The discovery of novel exotic properties in graphene such as quantum Hall effect at room temperature1 has invoked intensive studies to search for new atomic-layer materials with novel functional properties which are absent and/or superior to those of bulk. Among many atomic-layer materials, transition-metal dichalcogenides (TMDs) have attracted significant attention because they provide a fertile platform to realize various exotic properties with a rich variety in the combination of constituent atoms and crystal structures2,3. Monolayer TMDs [here, we define “monolayer” a unit of three atomic layers (chalcogen-metal-chalcogen)] are categorized into two different groups, 1H (trigonal prismatic coordination) and 1T (octahedral coordination) (see Fig. 1), according to the difference in the coordination of six chalcogen atoms around the metal atom. It is well known from previous studies of bulk TMDs that such a difference in crystal structure leads to a sizable change in the electronic structure and physical/chemical properties4-7. In contrast to bulk TMDs, the electronic structure and its crystal-structure (1H and 1T) dependence in the monolayer counterparts are not well elucidated8-10. This is mainly due to the difficulty in obtaining a high-quality monolayer film with selecting the crystal structure of 1H or 1T. Bulk TaSe2 has been intensively studied because both 2H and 1T phases stably exist in the bulk form11 (It is noted here that the 2H phase is composed of the AB stacking of 1H layers). Bulk 2H-TaSe2 shows a two-step charge density wave (CDW) transition at T = 120 K and 90 K with incommensurate (~3×3) and commensurate (3×3) lattice deformations, respectively4,12, followed by a superconducting transition at low temperature (Tc ~ 0.2 K)6. Bulk 1T-TaSe2 also exhibits a two-step CDW transition, but at higher transition temperatures of T ~ 600 K and 473 K for incommensurate and commensurate phases, respectively4. The CDW periodicity of 1T-TaSe2 (√13 × √13) is 2 ACS Paragon Plus Environment

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different from that of the 2H phase (3×3). The mechanism of CDW transition in both 2H and 1T phases of bulk TaSe2 has been intensively studied by angle-resolved photoemission spectroscopy (ARPES) in relation to the Fermi-surface nesting, electron correlation, and electron-phonon coupling13-16. Although only a few studies have been performed on monolayer or thin multilayer films17-20, some previous studies17,20 reported the emergence of CDW even in monolayer 1H-TaSe2. On the other hand, there is no report on the fabrication nor the characterization of monolayer 1T-TaSe2. It is thus very important to fabricate a monolayer 1T-TaSe2 film and unveil its electronic structure to explore the possible exotic properties in monolayer TaSe2. In this paper, we report an ARPES study of monolayer TaSe2 grown epitaxially on bilayer graphene on SiC(0001). We have succeeded in selectively fabricating 1H- and 1T-TaSe2 monolayer by the precise control of substrate temperature (Ts) during the deposition of constituent atoms on the substrate. In monolayer 1H-TaSe2, we observed the metallic electronic states characterized by a large Fermi surface, similarly to the bulk 2H counterpart. In contrast, monolayer 1T-TaSe2 shows an energy gap at the Fermi level (EF) in sharp contrast to the bulk counterpart.

We discuss the origin and

implications of differences in the electronic structure among monolayer and bulk TaSe2 by comparing the present results with first-principles band-structure calculations and previous studies on bulk TaSe2. Monolayer

TaSe2

films

were

grown

on

bilayer

graphene

by

the

molecular-beam-epitaxy (MBE) method in a vacuum of 3×10-10 Torr9,19-21. Bilayer graphene was prepared by annealing an n-type Si-rich 6H-SiC(0001) single crystal by resistive heating at 1100ºC in a vacuum better than 1×10-10 Torr for 20 min. Then, monolayer TaSe2 was grown by evaporating Ta on the bilayer-graphene/SiC substrate in



Se atmosphere. The substrate temperature (Ts) was precisely controlled to keep at 450ºC or 560ºC during the evaporation in order to selectively fabricate the 1H and 1T phase, as detailed later. The deposition rate of Ta and Se atoms was monitored with a quartz oscillator. The as-grown film was annealed at 400ºC for 30 min to improve the crystallinity of film and avoid adsorption of extra Se atoms (vapor) on the TaSe2 film. Then the film was transferred to the ARPES-measurement chamber without breaking vacuum. The growth process was monitored by the reflection high-energy electron diffraction (RHEED). The characterization of sample surface was performed by the atomic-force microscopy (AFM) (Park NX10) under ambient condition. ARPES measurements were performed using a MBS A1 (MB Scientific) electron-energy analyzer with a high-flux helium discharge lamp and a toroidal grating monochromator at Tohoku University. The He-Iα resonance line (hv = 21.218 eV) was used to excite photoelectrons. The energy and angular resolutions were set to be 20-40 meV and 0.2°, respectively. The Fermi level (EF) of sample was referenced to that of a gold film deposited onto the sample substrate. First-principles band-structure calculations for free-standing monolayer 1H- and 1T-TaSe2 were carried out by using the Quantum Espresso code22 with generalized gradient approximation23. Spin-orbit interactions were included in the calculations. Ultrasoft pseudopotentials were used, and the wave functions and the charge density were expanded using 30 Ry and 150 Ry cutoff, respectively. The k-point mesh was set to be 12×12×1. The thickness of inserted vacuum layer in a model crystal was set to be more than 10 Å. Figure 1(c) shows the RHEED pattern of pristine bilayer graphene grown on SiC(0001). We clearly observe the 1×1 and 6√3×6√3R30° streak patterns which correspond to bilayer graphene and buffer layer beneath graphene, respectively9,21. After


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co-evaporation of Ta and Se atoms onto the substrate kept at 450ºC under ultrahigh vacuum (note that this condition is suitable to fabricate the 1H phase, as described later), the RHEED intensity from SiC and graphene is remarkably reduced, and at the same time a new 1×1 streak pattern appears [see Fig. 1(d)]. This drastic change in RHEED pattern indicates the formation of monolayer TMDs9,20,21 which was also confirmed by the AFM measurement. Figures 2(e) and 2(f) show ex-situ AFM images of our films grown at Ts = 450ºC (1H) and 560ºC (1T), respectively, where several TaSe2 islands are seen on the bilayer-graphene substrate. A side-by-side comparison of Figs. 2(e) and 2(f) reveals that the terrace size of 1H island is larger than that of 1T. Although the typical terrace size is ~10 nm as seen in Figs. 2(e) and 2(f), we could fabricate a film with a larger terrace size by increasing the deposition time and/or rate. On the other hand, we found that the line profiles across the 1H and 1T islands [Figs. 1(g) and 1(h)] show a typical height of ~1.2-1.4 nm. This is larger than the thickness of ML TaSe2 [~0.75 nm (ref. 17)], but is understandable since the adsorbates should have a finite thickness, as revealed in ML NbSe2 (ref. 24). Considering the experimental fact that the observed island height is still smaller than that of bilayer TaSe2 (~1.5 nm), it is likely that our film is composed of monolayer islands, but not multilayer ones. We have estimated the in-plane lattice constant a for the monolayer TaSe2 film to be ~3.5 Å by comparing the interval of adjacent 1×1 streak between graphene and TaSe2. This value is very close to those of bulk 1T-TaSe2 (3.48 Å) and 2H-TaSe2 (3.43 Å)25, suggesting a weak lattice-strain effect as expected from the weak van-der-Waals coupling between graphene and TaSe2. To clarify the electronic structure of monolayer TaSe2, we have performed ARPES measurements on several TaSe2 films prepared under various conditions, and revealed



that the temperature of substrate (Ts) during the film formation determines the electronic structure and there are essentially two groups in the electronic structure. Figure 2(a) shows the ARPES-intensity plot along the Γ-M cut for monolayer TaSe2 fabricated at Ts = 450ºC. One can clearly see several dispersive bands in the valence-band region, reflecting the high single-crystallinity of the film. In Fig. 2(b), we compare the ARPES-derived band structure with the calculation for monolayer 1H-TaSe2. One finds a reasonable agreement between the experiment and the calculation, as seen in a band which crosses EF at the midway between the Γ and M points and a few holelike bands centered at the Γ point at around 1-2 eV in binding energy (EB). These spectral features are similar to those in the previous ARPES studies for a few-layer 2H-TaSe2 (ref. 19) and monolayer 1H-TaSe2 (ref. 20). These results indicate that monolayer TaSe2 grown at Ts = 450ºC has the 1H crystal structure. Figure 2(c) shows the ARPES-intensity plot for a TaSe2 film fabricated at Ts = 490ºC. One can recognize that the band structure became much more complicated compared with that of a film prepared at Ts = 450ºC [1H-TaSe2, Fig. 1(a)]. For example, we observe that two additional bands appear at the Γ point; a flat band at EB ~ 0.3 eV and a highly dispersive holelike band at EB ~ 0.3-1 eV. The emergence of these additional features in the band structure suggests an inclusion of other crystal phases in the film prepared at higher temperature. To examine this possibility, we increased Ts up to 560ºC to see the change of the band structure. The result is shown in Fig. 2(d). Now one can see that the bands ascribable to the 1H phase almost disappear and as a result the obtained band structure represents a new electronic state of monolayer TaSe2. It is inferred that this new electronic state is likely due to the 1T phase of TaSe2, because the ARPES-derived band structure shows a good correspondence in the gross band structure


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to the calculated one for the 1T phase [see Fig. 2(e)]. This suggests that monolayer 1Hand 1T-TaSe2 films can be selectively fabricated by simply tuning the substrate temperature during the film growth, as schematically shown in Fig. 3(a). This is also corroborated by the experimental fact that the observed large difference in the band dispersion between the insulating and metallic states is similar to the case for 1T and 1H (2H) phases of ML NbSe2 (ref. 9) and bulk TaSe2 (refs. 13, 15, 16 and 26) (we also note that ex-situ measurements such as ex-situ Raman spectroscopy turned out to have a large difficulty in distinguishing the crystal phases because of the quick surface oxidization of film in the air). Here one may notice in Fig. 2(e) that the dispersive feature of bands near EF shows a sizeable difference between the experiment and the calculation. We discuss this point in details below. We have shown that there is a large difference in the electronic structure between 1H and 1T phases, for example, in the near-EF band structure and the Fermi surface. The 1H phase is metallic as characterized by a sharply dispersive band across EF [Fig. 3(b)] and a large hole pocket centered at the Γ point [Fig. 3(d)]. On the other hand, in the 1T phase, the band never touches EF [Fig. 3(c)] and no Fermi surface exists [Fig. 3(e)]. We observe an energy gap of ~ 0.2 eV at the Γ point in the 1T phase, which cannot be assigned to a conventional band gap because the band-structure calculation does not show a gap-like feature around the Γ point [Fig. 2(e)]. It is inferred that the Mott-Hubbard transition7,13,26 induces the gap opening and the observed flat band is assigned to a lower Hubbard band. In fact, a similar flat band with a finite energy gap is observed in bulk 1T-TaS2 (ref. 27) and monolayer 1T-NbSe2 (ref. 9), in which the strong electron correlation and the narrow d band due to the CDW transition cooperatively dominate the electronic structure.



The present result sheds light on unusual spectroscopic properties observed in bulk 1T-TaSe2 (refs. 13, 26). It was reported that the surface of bulk 1T-TaSe2 in the commensurate CDW phase shows an insulating behavior below ~ 260 K despite the metallic nature of bulk. This metal-insulator transition at the surface has been discussed in terms of the localization of Ta 5d band due to the modulation of atomic position near the surface caused by the CDW transition13,26. Since the topmost surface is substantially isolated from the bulk and may be regarded as almost a single monolayer, a similar Mott-Hubbard scenario is applicable to both monolayer and topmost surface of bulk. In fact, the overall band dispersion and the energy location of the lower Hubbard band are quite similar between monolayer 1T-TaSe2 and the surface of bulk. It is also worthwhile to note that we found a difference in the temperature dependence of the insulating gap between monolayer and bulk. The Mott insulating phase in monolayer 1T-TaSe2 persists even at room temperature (not shown), whereas that at the surface of bulk 1T-TaSe2 vanishes above 260 K13,26. We speculate that such a difference in the transition temperature could be related to the difference in the interlayer interaction, since the change in the interlayer interaction would trigger variation in the lattice constant and band-width (and also effective Coulomb energy), affecting the stability of the Mott phase. Further ARPES studies of monolayer TaSe2 on different substrates would help clarify this point. Finally, we briefly comment on possible device application of TaSe2 monolayer. The hetero-junction of metallic 1H- and insulating 1T-TaSe2 monolayers would be feasible for fabricating very thin field effect transistors or switching devices. One may also notice that TaSe2 films fabricated at intermediate substrate temperature between 450 and 560°C would consist of a mixed structure of 1H- and 1T-phases, providing an


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opportunity for realizing the in-plane hetero-junction of 1H- and 1T-TaSe2 monolayers. It is also noted that monolayer 1T-TaSe2 has several advantages over conventional band insulators; its Mott-insulating properties can be controlled by various external means such as carrier doping28,29, pressure30, and disorder31. Observed insulating behavior at room temperature would be very favorable to realize monolayer-TaSe2-based devices which stably work at room temperature. In conclusion, we reported the ARPES characterization of monolayer TaSe2 grown on bilayer graphene. We found that monolayer 1H- and 1T-TaSe2 films can be selectively fabricated by controlling the substrate temperature during the film growth. We observed that while monolayer 1H-TaSe2 holds the metallic character, the 1T counterpart exhibits an insulating nature caused by the Mott-Hubbard transition. The present study opens a pathway toward developing next-generation electronics and spintronics nano-devises based with monolayer TMDs.

Author Contributions Y.N., T.Y., K.S. and Y.U. carried out the fabrications of thin films, their characterization and ARPES measurements. Y.N. performed the band calculations. Y.N., K.S., T.T. and T.S. finalized the manuscript with input from all the authors.

Notes The authors declare no competing financial interest.

Acknowledgments We thank N. Shimamura and S. Souma for their help in the ARPES and AFM



experiments. We also thank T. Koretsune for his help to use the quantum ESPRESSO package. This work was supported by JSPS KAKENHI Grants (JP25107003, JP15H05853, JP15H02105, and JP17H01139), Grant for Basic Science Research Projects from the Sumitomo Foundation, Science Research Projects from Iketani Science and Technology Foundation, the Program for Key Interdisciplinary Research, and World Premier International Research Center, Advanced Institute for Materials Research. Y.N. acknowledges support from GP-Spin at Tohoku University.


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Fig. 1: (a,b) Crystal structures of monolayer (a) 1H- and (b) 1T-TaSe2. (c,d) RHEED patterns of (c) bilayer (BL) graphene and (d) monolayer TaSe2 on BL graphene. The RHEED patterns are obtained along the [1100] direction of the 6H-SiC(0001) substrate. The 1×1 pattern is observed in all the samples irrespective of the substrate temperature Ts (450ºC ≤ Ts ≤ 560ºC). (e,f) AFM images of 1H and 1T monolayer TaSe2 films on BL graphene fabricated at (e) Ts = 450ºC and (f) 560ºC. (g,h) Height profiles along a cut indicated by red line in (e) and (f), respectively. AFM measurements were performed on films with sub-monolayer thickness since we intended to estimate the film thickness by referring to the height of bilayer graphene in simultaneous imaging of TaSe2 islands and bilayer graphene.



(a) 1H (Ts=450℃) Γ EF Binding Energy (eV)

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(b) 1H (Calculation) (c) 1H+1T (Ts=490℃) (d) 1T (Ts=560℃)

M

Γ

M

M

Γ

M

Γ

(e) 1T (Calculation) Γ

M

1 2 High 3 Low 0.0

0.5 1.0 kx (Å-1)

0.0

0.5 1.0 kx (Å-1)

0.0

0.5 1.0 kx (Å-1)

0.0

0.5 1.0 kx (Å-1)

0.0

0.5 1.0 kx (Å-1)

Fig. 2: (a-e) ARPES intensity plotted as a function of wave vector and binding energy for monolayer TaSe2 along the Γ-M cut, for (a,b) the 1H-phase grown at Ts = 450ºC, (c) the mixed phase (Ts = 490ºC), and (d,e) the 1T-phase (Ts = 560ºC). ARPES spectra were measured with the He-Iα line (hv = 21.218 eV) at room temperature. (b) and (e) are the same ARPES intensity with gray scale compared with the calculated band structure (blue or red) obtained by the first-principles band calculations for monolayer 1H- and 1T-TaSe2, respectively.


Page 17 of 18

1H

(a)

Binding Energy (eV)

EF

K

1T

1H+1T

450℃

(b)

490℃

Γ

K

(c)

Ts

560℃

K

Γ

K

0.4 High 0.8 1.2 Low

(d) ky (Å-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 0.0 ky (Å-1)

0.4

Γ

M

-0.4 (e)

0.0 ky (Å-1)

0.4

0.3 0.0

M

Γ

High

-0.3 -0.6

0.0

K 0.5 1.0 kx (Å-1)

0.0

K 0.5 1.0 kx (Å-1)

Low

Fig. 3: (a) Schematic diagram of the relationship between the crystal structure and Ts. (b,c) ARPES intensity plotted as a function of wave vector and binding energy near EF for monolayer (b) 1H- and (c) 1T-TaSe2. (d,e) ARPES-intensity mapping at EF as a function of two-dimensional wave vector for monolayer (d) 1H- and (e) 1T-TaSe2. The intensity at EF was obtained by integrating the ARPES intensity within ±10 meV of EF.



E EF DOS

1T

EF 1

E

2

UHB EF ∆ Mott

3 Γ

LHB DOS

M

Binding Energy (eV)

1H Binding Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 18

EF 1 2 3 Γ


M

Selective Fabrication of Mott-Insulating and Metallic Monolayer TaSe2

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