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Resolving deep quantum well states in atomically thin 2H-MoTe flakes by Nanospot Angle-Resolved Photoemission Spectroscopy Hongyun Zhang, Changhua Bao, Zeyu Jiang, Kenan Zhang, Hao Li, Chaoyu Chen, José Avila, Yang Wu, Wenhui Duan, Maria Carmen Asensio, and Shuyun Zhou Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00589 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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Resolving deep quantum well states in atomically thin 2H-MoTe2 akes by Nanospot Angle-Resolved Photoemission Spectroscopy †

Hongyun Zhang,

Chen,

¶

†

Changhua Bao,

José Avila,

¶

§

Yang Wu,

†

Zeyu Jiang,

Wenhui Duan,

Zhou

†State

†

†

Kenan Zhang,

‡

Hao Li,

¶

Maria C. Asensio,

Chaoyu

and Shuyun

∗,†,k

Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P.R. China

‡School

of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China

¶Synchrotron

SOLEIL, L'Orme des Merisiers, Saint Aubin-BP 48, 91192 Gif sur Yvette Cedex, France

§Department

of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing, 100084, P.R. China

kCollaborative

Innovation Center of Quantum Matter, Beijing 100084, P.R. China

E-mail: [email protected] Phone: +86 010 62797928

1

ACS Paragon Plus Environment

Nano Letters 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

Transition metal dichalcogenides exhibit strong quantum connement eects and the electronic structure is strongly dependent on the number of layers. Resolving the thicknessdependent electronic structure is important. While the electronic structure of atomically thin 2H-MoSe2 or 2H-MoS2 have been explored, experimental electronic structure of 2H-MoTe2 is still missing. Here, by using nanospot ARPES (NanoARPES), we reveal the experimental electronic structure of exfoliated 2H-MoTe2 thin akes with dierent thickness (3, 5 and 7 monolayers). Well-separated quantum well states are clearly observed in thin 2H-MoTe2 akes at deep valence bands at energies between -3 to -5 eV, while those at the top of the valence band between -1 and -2 eV are much closely spaced compared to those from 2H-MoSe2 and 2H-MoS2 . First-principles calculation shows that the main dierence is attributed to the weaker hybridization and smaller energy dierence between Mo 4dz 2 and Te 5pz orbitals as compared to Se 4pz and S 3pz .

Our work demonstrates the power of NanoARPES in

resolving the electronic structure of atomically thin exfoliated akes.

KEYWORDS: NanoARPES, Transition Metal Dichalcogenides, MoTe2 , atomically thin akes, Quantum Well State (QWS)

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Transition metal dichalcogenides (TMDCs) exhibit interesting electronic properties with promising applications in nanoscale electronics, optoelectronics and valleytronics etc

13

.

These materials have weak van der Waals interaction between the layers and the electronic structure strongly depends on the sample thickness. For example, the band gap is widely tunable as a function of thickness

4,5

. For 2H-MoS2

6,7

, MoSe2

8

and WSe2

9,10

, there is an in-

direct to direct band gap transition due to the energy shifts of quantum well states (QWSs) at the

Γ

point and the maximum at the K point when the lm thickness decreases to one

monolayer (ML). While most of the 2H-phase materials formed by group VI transition metals have been widely explored, so far little has been known about the electronic structure of atomically thin 2H-MoTe2 . MoTe2 has attracted much attention due to its phase transiton

0 from 2H to 1T phase which makes it an important candidate for applications of 2D devices with excellent ohmic homojunction contact emitting diodes have been demonstrated

12,13

11

.

Recently, few-layer 2H-MoTe2 based light

, making it potentially useful for nanolaser and

nanodetector applications. Experimentally resolving the evolution of the electronic structure with lm thickness is important for understanding 2H-MoTe2 . Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to measure the electronic structure directly.

Molecular beam epitaxy (MBE) can be used to grow high

quality 2H-MoSe2 and 2H-WSe2 lms for ARPES measurements

8,9

, however, MBE growth

of pure phase, single crystalline 2H-MoTe2 lms is dicult due to the coexistence of 1T phase

1416

.

0

Mechanical exfoliation provides a convenient method for obtaining atomically

thin akes with dierent thickness, yet the size of such exfoliated akes (a few to tens of micrometers,

µm) is smaller than the typical ARPES beamsize (50 - 100 µm).

beam size down to hundred nanometer scale

17

By focusing the

, Nanospot ARPES (NanoARPES) provides

unique opportunities for probing the electronic structure of small samples with spatially resolved information

18,19

.

Here we report the electronic structure of exfoliated 2H-MoTe2

akes with dierent thickness from 3 ML to 7 ML. The high quality NanoARPES data allow to resolve the deep QWSs with much larger energy separation at high binding energies

3


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compared to those in 2H-MoSe2

8

9

and 2H-WSe2 .

Page 4 of 15

First-principles calculations reveal that

such dierence is attributed to the much weaker hybridization between Mo dz 2 and Te pz orbitals.

Our work demonstrates the unique advantages of NanoARPES in obtaining the

experimental electronic structure of atomically thin exfoliated akes.

MoTe2

Low

High

Figure 1: Characterization of atomically thin exfoliated 2H-MoTe2 akes with dierent sample thickness.

(a) Side view of the crystal structure and the Brillouin zone of 2H-MoTe2 .

(b) Optical image of MoTe2 akes on h-BN/Si substrate. (c) Zoom-in of the region marked by white dashed square in (b). (d) Optical contrast of the green channel extracted from the RGB along the white dashed line in (c). (e) AFM image of the white dashed square in (c) with measurements of step height appended. (f ) Raman spectra (measured under 633 nm) of 3 ML, 5 ML and 7 ML from dots indicated in (c). (g) Intensity ratio of B2g /E2g (green circles) and the peak position of E2g (red squares) extracted from (f ). The error bars are smaller than the symbol size.

2H-MoTe2 crystalizes in a hexagonal structure similar to 2H-MoS2 and 2H-MoSe2 . The building block is Te-Mo-Te sandwich (dened as 1 ML, 0.75 nm thick) as shown in Figure 1a. Figure 1b shows an optical image of exfoliated 2H-MoTe2 akes deposited on a thick h-BN ake on a silicon substrate. The zoom-in image in Figure 1c shows clear color contrast for akes with dierent thickness. Line prole analysis of the optical intensity contrast

20,21

in Figure 1d shows three steps with height ratio of 3:5:7, suggesting the ake thickness to be

4


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3 ML, 5 ML and 7 ML respectively. The ake thickness is further conrmed by atomic force microscope (AFM) measurements shown in Figure 1e. Raman spectra in Figures 1f,g show an increase in the number of

A1g

peaks which are sensitive to the out-of-plane vibrations

decrease of the relative peak intensity of

B2g /E2g

and a red shift of the Raman

3 ML to 7 ML, all in agreement with the increase of the sample thickness

23

E2g

22

, a

peak from

. By combining

optical, AFM and Raman measurements, the thickness of the thin akes is determined to be 3 ML, 5 ML and 7 ML.

Low

High

High

Low

M-K-Γ

Figure 2: Observation of QWSs in the layer dependent electronic structure of 2H-MoTe2 . (a) Spatially-resolved intensity map (interpolated by

2 × 2)

obtained by integrating the Te

4d core level peaks. The yellow broken lines are the extracted countour of the 2H-MoTe2 ake from the optical image (b), and the red broken line marks the edges between h-BN and the Si substrate. (b) Optical image of the same area as (a). (c) Zoom-in of the white dashed square marked in (b). The black arrow indicates the ARPES dispersions measured along the

Γ-K-M

Γ-K-M direction in the real space.

(d-f )

direction on MoTe2 akes with thickness of

3 ML, 5 ML and 7 ML respectively. (g-i) Calculated dispersions of 3 ML, 5 ML and 7 ML along the

Γ-K-M

direction.

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We utilize NanoARPES to spatially resolve the dierent akes and reveal the evolution of the electronic structure with ake thickness. Figure 2a shows a spatially-resolved intensity map obtained by integrating the Te 4d core level peaks at binding energy of 41.9 and 40.4 eV. A comparison with the the optical image in Figure 2b shows a good agreement in the sample contour (yellow broken lines). Such comparison also allows us to identify the regions of interests, namely, 3 ML, 5 ML and 7 ML thick akes marked in Figure 2c. Figures 2d-f show the measured dispersions along the

Γ-K-M

direction for all these akes, and a detailed

electronic structure of the 5 ML ake is shown in Figure S1 of the supplementary information. Figures 2g-i show the corresponding calculated dispersions for comparison. Overall, there is a good agreement between the experimental dispersions and calculated dispersions, except that the calculated band gap is underestimated which is typical for DFT calculations These akes all show semiconducting properties similar to the bulk crystal of the valence band at the

Γ

25,26

.

24

.

The top

point remains almost unchanged with ake thickness.

The

combined theoretical and experimental results suggest that the gap in MoTe2 does not change signicantly from 3 ML to 7 ML. This is dierent from results observed in MoS2 and MoSe2 , in which the top of the valence band at the

Γ

point shifts in energy with sample thickness,

resulting in a transition from indirect to direct band gap at 1 ML.

68

8

The critical thickness

at which the band gap of 2H-MoTe2 transforms from indirect to direct

2729

is still not clear.

To solve this problem, it requires determining the exact momentum location of the valence band maximum. In our data, the intensity for the top of the valence band at the K point is suppressed due to the dipole matrix element eect

30

which modulates the ARPES intensity.

Future ARPES measurements on thinner akes and measured at other photon energies are needed to resolve this question. Here we focus on the evolution of the electronic structure of the valence band with sample thickness, in particular the QWSs.

QWSs with obvious

separation are observed at deep valence bands at energies between -3 and -5 eV around

Γ

point, and the number of QWSs increases with sample thickness. This is strikingly dierent from previous ARPES work on 2H-MoS2 and 2H-MoSe2 thin lms where well-separated

6


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QWSs were observed on the top of the valence band near EF energies as our results in 2H-MoTe2 were not reported.

8,9

, while QWSs at such deep

The observed QWSs are also in

good agreement with calculated band structure shown in Fig. 2(g-i), which also shows a much larger splitting in the deep QWSs compared to the top valence bands.

We note

that although QWSs at deep energies have been observed in d-orbital derived quantum well states

3133

, most of those QWSs are generally observed at the top valence band closest to the

Fermi energy

810,20

, which is also the case for few layer graphene lms

34,35

. Our NanoARPES

data reveal clear QWSs in 2H-MoTe2 thin akes, which are located at deeper valence band as compared to the top valence band observed other 2D materials like 2H-MoS2 , 2H-MoSe2 and graphene. To further understand the origin for the dierence of electronic structure between 2HMoTe2 thin akes and other materials of 2H phase, a comparison of theoretical calculated electronic structure is needed. Here we apply rst-principles calculations to 7 ML thick 2HMoTe2 , 2H-MoSe2 and 2H-MoS2 along the high symmetry directions (Figures 3a-c) and the corresponding orbital analysis (Figures 3d-f ). The calculated electronic structure for 7 ML samples in Figures 3a-c show splittings at both the top valence band (indicated by green arrows) and the bands at much deeper energy (red arrows). Orbital analysis in Figure 3d indicates that the top valence bands from -1.3 to -1.6 eV are dominated by Mo 4dz 2 orbitals (indicated by blue color) while the deeper valence bands from -3 to -4.5 eV are mainly by Te 5pz (red color). Compared to the top valence band, the energy separation of MoTe2 at deep energies between -3 eV to -5 eV from the Te 5pz orbital is much larger, which is in agreement with our experimental observation of deep QWSs. The splitting for the top valence band QWSs increases from MoTe2 , MoSe2 to MoS2 , while the splitting for the deep QWSs shows an opposite trend. In addition, orbital analysis of top vanlence band QWSs in Figure 3d shows a gradual dependence of orbital contribution on energy from dominantly Mo 4dz 2 (blue color) to dominantly Te pz (red color) with a small energy separation of 0.29 eV. In 2H-MoSe2 and 2H-MoS2 , the energy separations increase to 0.57 eV and 0.72 eV respectively, with

7


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Figure 3: A comparison of the theoretical calculated electronic structure for 7 ML 2H-MoTe2 , 2H-MoSe2 and 2H-MoS2 . (a-c) Dispersions along the K-Γ-M directions for 2H-MoTe2 , 2HMoSe2 and 2H-MoS2 . The green and red arrows indicate the QWSs at the top valence band and deep energy respectively. (d-f ) Orbital analysis of the dispersions shown in (a-c). Blue represents Mo 4dz 2 and red represents the pz orbitals of Te, Se and S.

8


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Nano Letters

the orbital contribution more and more dicult to distinguish, which indicates a stronger hybridization between Mo dz 2 and the S pz (or Se pz ) orbital. Besides, the smaller splitting in the top valence band of MoTe2 is also related to the weaker hybridization and, more directly, smaller energy dierence between pz and dz 2 (indicated by two black solid lines). Therefore the deep QWSs observed are a combined eect of weaker hybridization and smaller energy separation between and Mo dz 2 and Te pz orbitals. To summarize, we have successfully resolved the electronic structure of few-layer 2HMoTe2 akes on h-BN/Si with

µm

size by NanoARPES measurements. Deep QWSs from

-3 eV to -5 eV are resolved and the number of bands increases with the thickness of the 2HMoTe2 ake. By comparing the experimental data and theoretical calculation, we conclude that the dierent electronic structure of 2H-MoTe2 is attributed by the weaker hybridization and smaller energy dierence between Mo 4dz 2 and Te 5pz orbitals. Our work demonstrates the power of NanoARPES in measuring small exfoliated akes with dierent sample thickness and can be extended to measure the band structure of other layered materials. Methods High quality 2H-MoTe2 single crystals were grown by chemical vapor transport

method using polycrystalline samples as precursors. By mechanically exfoliation, we have obtained high quality few-layer 2H-MoTe2 akes and transferred those akes onto h-BN ake which is also obtained by exfoliation method on p-type silicon.

The whole process

was prepared in the glove box and the sample was then sealed in a quartz tube which was pumped down to 10

−6

torr to protect the 2H-MoTe2 thin akes.

The sample was then

removed from the quartz tube inside the glove box and mounted into a vacuum suitcase, and then the sample was transferred into the UHV chamber after pumping down the suitcase. The akes were annealed in ultrahigh vacuum chamber at 150

◦

C for 1 hour to expose clean

2H-MoTe2 surface. The Te 4d core level for 2H-MoTe2 akes on h-BN is at lower binding energy compared to akes directly on Si substrate due to the change of chemical potential and the measured dispersion near EF is shifted by the same amount to compensate for that. NanoARPES measurements were performed at the ANTARES beamline of the syn-

9


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chrotron SOLEIL at France with the beamsize of

∼ 150 nm.

Page 10 of 15

All ARPES data were taken by

the photon energy of 100 eV. The energy and angular resolution were set to 25 meV and 0.1 degree, respectively. The sample was measured at T = 120 K in a working vacuum better than 2

× 10−10

mbar.

First-principles calculations are performed within the framework of density functional theory (DFT) implemented in the VASP Perdew-Burke-Ernzerhof (PBE) type

37

36

. A

code using the exchange-correlation functional of

18 × 18 × 1 Γ-centered

grid is adopted to sample

the Brillouin zone and the plane wave cut-o energy is set to 500 eV. The geometry of bulk phase is fully optimized with DFT-D2

38

van der Waals correction until residual forces are

less than 0.001 eV/Å, and the multilayers are simulated by supercell method with a vacuum of more than 15 Å to avoid the imaginary adjacent interactions.

In all the calculations

spin-orbit coupling (SOC) eect is taken into account.

Acknowledgement We thank Yichen Song and Yuanbo Zhang for the help during sample preparation.

We

acknowledge support from the National Natural Science Foundation of China (Grant No. 11334006 and 11725418), from Ministry of Science and Technology of China (Grant No. 2016YFA0301004, 2015CB921001), Science Challenge Project (No. 20164500122) and the Beijing Advanced Innovation Center for Future Chip (ICFC). The Synchrotron SOLEIL is supported by the Centre National de la Recherche Scientique (CNRS) and the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), France.

This work was also

supported by a public grant by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" (reference: ANR-17-CE09-0016-05)

Supporting Information Available Supporting Information. Constant energy contours and dispersions along

10


Γ-K

and

Γ-M

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of 5 ML 2H-MoTe2 are shown in the supporting information le. SI20180706v20.pdf: Energy contours and dispersions of 5 ML along shown in this le.

Γ-K

and

Γ-M

This material is available free of charge via the Internet at

are

http:

//pubs.acs.org/.

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Graphical TOC Entry TOC only.pdf

3 ML

5 ML

Energy

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

Nano Letters

5 μm

Momentum

15


7 ML

Resolving deep quantum well states in atomically ... - ACS Publications

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