Tunable Optical and Electrical Transport Properties of Size and

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C: Physical Processes in Nanomaterials and Nanostructures

Tunable Optical and Electrical Transport Properties of Size and Temperature Controlled Polymorph MoS Nanocrystals 2

Subhrajit Mukherjee, Souvik Biswas, Arup Ghorai, Anupam Midya, Soumen Das, and Samit K. Ray J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02585 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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The Journal of Physical Chemistry

1

Tunable Optical and Electrical Transport Properties of Size

2

and Temperature Controlled Polymorph MoS2 Nanocrystals

3

S. Mukherjee1, S. Biswas2, A. Ghorai3, A. Midya3, S. Das4 and S. K. Ray2,5,*

4 5 6 7 8 9

1

Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur- 721302 2 Department of Physics, Indian Institute of Technology, Kharagpur- 721302 3 School of Nano Science and Technology, Indian Institute of Technology, Kharagpur- 721302 4 School of Medical Science and Technology, Indian Institute of Technology, Kharagpur- 721302 5 Currently at S. N. Bose National Centre for Basic Sciences, Kolkata- 700106 * e-mail : [email protected]

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Abstract:

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The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the

12

metallic 1T to semiconducting 2H phase has been investigated in detail. Metallic 1T

13

phase NCs were prepared by Li+ intercalation-deintercalation exfoliation techniques

14

followed by prolong sonication. The effect of ex-situ thermal annealing on MoS2

15

polymorphs and their transformation from 1T to 2H phase has been extensively

16

monitored by X-ray photoelectron, Raman and optical absorption spectroscopy

17

techniques. Electrical conductivity measurements have also been carried out to probe the

18

phase transition of synthesized NCs. Temperature dependent (10-350 K) electrical charge

19

transport properties of variable sized NCs have been investigated to probe the scaling of

20

conductivity and activation energy with size, which are yet to be reported experimentally.

21

Charge transport mechanisms through the NCs assembly for different temperature

22

regions have been modeled and observed that the electron transport undergoes a

23

transition from the nearest neighbor hopping (NNH) to the variable range hopping (VRH)

24

upon decreasing temperature.

25 26

1. Introduction:

27

Two dimensional (2D) layered molybdenum disulfide (MoS2), an inorganic analogue of

28

graphene, has been studied extensively for its diverse structural and optical properties.

29

However, the electronic properties of zero-dimensional (0D) MoS2 nanocrystals (NCs)

30

with distinguishable different properties are still un-explored. Due to the manifold Page | 1 ACS Paragon Plus Environment

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increase in surface area and pronounced quantum mechanical effects, several fascinating

2

physical properties are exhibited by colloidal MoS2 NCs, which have made them

3

promising candidates as emerging materials for electronic and optoelectronic

4

applications1–3. Specifically, size-tunable optical and electronic properties and the

5

solution processability make them attractive for flexible and tunable LEDs,

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photodetectors, solar cells and transparent electronic devices4,5. A special feature of MoS2

7

is its existence in multiple crystal structures6,7. This polymorphism not only opens up a

8

new horizon in basic research but also provides the phase transition induced exciting

9

properties for device applications, unlikely graphene and hexagonal BN8,9. Intriguingly,

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one of these structural phases is trigonal prismatic, a semiconducting 2H phase, whereas

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the other is metallic 1T phase. This is in contrast to other group-V TMDs, where metal to

12

metal structural phase transitions have been demonstrated in multilayer TaS2 and

13

TaSe210,11. It is also possible to transform and restore their phases by some physical or

14

chemical means. For example, the intercalation of alkali metals into host MoS2 leads to

15

spontaneous transformation from the trigonal prismatic to the octahedral coordinated

16

phase, and most remarkably, subsequent deintercalation preserves the octahedral

17

coordination, yielding a metallic phase of MoS212,13. A few groups have successfully

18

synthesized metallic phase MoS2, by substitutional rhenium doping14, organic solvent

19

based chemical exfoliation from bulk13, potassium intercalation with subsequent

20

oxidation15, mechanical deformation16, electron or ion bombardment17 etc. Eda et al.

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reported a single layer coherent heterostructure of MoS2 consisting of both phases, with

22

matching lattices for novel molecular functionalities18. The focused electron

23

bombardment is a strategy to selectively fabricate monolithic devices consisting of both

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phases of MoS2 flakes18. Though, the structural transition and resultant properties of

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MoS2 polymorphs have been modeled theoretically12,19–21, experimental results during the

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progressive transformation are still lacking. Since the device functionality and efficiency

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rely on charge injection and transport in NC assemblies, the fundamentals of charge

28

transport through size controlled NCs need to be investigated in detail. A theoretical

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modeling of the transport is also challenging due to the coexistence of several transport

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mechanisms, like tunneling and hopping conduction. Therefore, developing an

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appropriate model of the size and temperature dependent charge transport in disordered

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MoS2 NCs requires the experimental conductivity data of NCs based devices.

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In this paper, we report a comprehensive study of the size- and temperature dependent

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electron transport in closely packed MoS2 NC assemblies together with the mechanism

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for phase transitions. The structural phase transformation from 1T to 2H phase has been

6

studied by optical and electrical probing to explain the possible mechanism in details.

7

The electron conduction at higher temperature regime follows an Arrhenius behavior,

8

implying a nearest neighbor hopping (NNH) transport mechanism. With decrease in

9

temperature, the transport mechanism undergoes a transition from NNH to the variable

10

range hopping (VRH), for smaller NCs. The activation energy for the NNH regime is also

11

found to be size-dependent. Collectively the size-, phase- and temperature-dependent

12

charge transport properties of MoS2 NCs assembly presented here provide a thorough

13

understanding of electrical conduction. The results suggest that due to much higher

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carrier mobility, 1T-MoS2 may have a greater potential over 2H-MoS2 as a promising

15

candidate for microelectronic devices, whereas the 2H phase has a clear advantage for

16

optical devices.

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2. Experimental:

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Alkali metal ions assisted technique was used to exfoliate bulk MoS2 by intercalating

19

lithium ions (Li+)22,23. The synthesis steps for 1T-MoS2 are presented in Figure 1(a-d),

20

resulting in variable size NCs. In short, 3 ml of 1.6 M n-butyl lithium and 300 mg of bulk

21

MoS2 powder were taken with hexane into an Ar-filled closed round bottom flux and the

22

whole mixture was stirred for 48 hrs in presence of argon atmosphere. This allowed the

23

diffusion of Li+ inside the inter-lamellar spaces to result in a lithiated metallic (1T) MoS2

24

phase (Figure 1b). Following, the successful intercalation, the black slurry was gently

25

washed with excess hexane to remove unreacted n-butyl lithium. Subsequently, the

26

remaining solution was sonicated for 1 hr in deionized (DI) water to exfoliate and release

27

very thin and free layers of 1T-MoS2 flakes (Figure 1c). Repeated DI water washing was

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done to make sure the MoS2 samples were free from residual lithium. Additionally

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prolonged sonication of the as-synthesized 1T MoS2 sheets in water is capable of Page | 3 ACS Paragon Plus Environment

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fragmenting the layers into small nano-sized MoS2 crystals (Figure 1d). The subsequent

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gradual centrifugation (5000, 10000, 15000 rpm) leads to the separation of variable sized

3

MoS2 NCs in large fraction. Thermal annealing was carried out in a high vacuum

4

chamber, with a base pressure of ~5 x 10-6 mbar, at different temperatures for 2 h for each

5

sample on a quartz substrate. Optical UV-Vis spectra were recorded in transmission

6

mode using a tungsten-halogen lamp as a broad band light source. Raman spectroscopy

7

was performed with a confocal micro-Raman setup using an Ar laser operating at 514

8

nm. The multiple dip coated films on Pt interdigitated electrodes with 100 μm spacing

9

were allowed to dry naturally at ambient conditions. The films were approximately ~55

10

nm thick, as determined by the scanning electron micrograph shown in Figure S1 (see

11

ESI copy). Electrical measurements were performed immediately after annealing by

12

employing two probe metal-semiconductor-metal (MSM) type geometry. All

13

measurements were performed in a close-cycle cryostat chamber under vacuum condition

14

using two source measure units (Keithley 2400).

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3. Results and Discussions

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The present study aims to a comprehensive experimental understanding of the formation

17

of 1T-phase induced by Li+ insertion-desertion and identify the progressive restoration to

18

2H-phase MoS2 NCs by ex-situ annealing by electrical and optical probing. The electrical

19

charge transport mechanisms through the MoS2 NCs polymorphs are also analyzed.

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3.1. Structural properties of MoS2 NCs

21

Chemically exfoliated MoS2 crystals consist of tri-layer S-Mo-S slabs held together by a

22

weak Van-der Waals (vdW) force, which are organized in either trigonal prismatic (D6h)

23

or octahedral (D3d) building blocks24. Initially, when Li+ are intercalated to D6h state of

24

host matrix, the structure transforms to the octahedral (D3d) coordination and intercalated

25

samples become more stable. This is expected since the D3d point group has the inversion

26

centers at Mo sites, whereas, the D6h point group is at the interstitial octahedral sites

27

within vdW gaps24. Therefore, only two kinds of vacant interstitial sites are available for

28

Li, specifically, one at octahedral sites within the gap and the other at tetrahedral sites.

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The occupation of a Li atom at a tetrahedral site raises the lattice stress along diagonal Page | 4 ACS Paragon Plus Environment

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direction of (110) plane, resulting in lattice distortion into a monoclinic shape and finally

2

Li atom is expelled out from the tetrahedral site leading to structural instability.

3

Therefore, for stable configuration, Li atoms occupy within the vdW gap by ruling out

4

the stress factor. X-ray diffraction spectra in Figure 1e reveal that the intercalated MoS2

5

undergoes a structural modulation. Samples collected after intercalation, exhibit extra

6

peaks for the (001) planes indicating that the material no longer maintains the 2H-type

7

symmetry25. Even, when all Li atoms are washed from the system, the structure still

8

remains stable in 1T-MoS2 phase. Atomic force (AFM) and transmission electron

9

microscopic (TEM) images have been shown in Figure 2 to investigate the surface

10

topography and microstructure of synthesized zero-dimensional NCs, respectively. A

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typical TEM micrograph of the MoS2 NCs (15000 rpm sample) is shown in Figure 2a,

12

exhibiting the formation of a roughly circular shape uniformly distributed nanocrystals.

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The lateral size distribution estimated from the image and the histogram is shown in the

14

inset of Figure 2a, reflecting an ensemble nature of synthesized MoS2 NCs having a peak

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size distribution of 2 nm in diameter. Typical 2D surface topographic image with

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corresponding histogram (inset) of the height distribution of spin coated MoS2 NCs of

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above sample are depicted in Figure 2b. The average height of the NCs distribution is

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found to be ∼7 nm, which indicates the local agglomerations of NCs on the substrate.

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The present synthesized approach is suitable for preparation of nanocrystals as compared

20

to complex template or stabilizer assisted approach, which always has a tendency to

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produce few layer MoS2, as reported previously26.

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The thermally driven phase transformations have been studied using X-ray photoelectron

23

spectroscopy (XPS). Figure 3 (a-d) exhibits a series of Mo 3d core-electron spectra of

24

~2.0 nm size MoS2 NCs with increasing annealing temperature. Characteristic doublet

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photoemission peaks of Mo 3d (Mo 3d5/2 and Mo 3d3/2) observed at 229.2 eV and 232.3

26

eV, respectively reveal the formation of 2H phase NCs. However, another Mo 3d

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photoemission doublet is clearly observed, shifted with respect to that for 2H MoS2 by

28

1.1 eV. The presence of additional Mo 3d peaks at 228.1 eV and 231.4 eV, confirm the

29

presence of 1T phase of MoS2 in NC assemblies. With Li intercalation, Mo 3d core level

30

energy is shifted towards the position of metallic Mo, which is consistent with the


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reported model27. XPS study reveals that both phases coexist initially. With increasing

2

temperature, as shown in Figure 3e, the 2H becomes more dominant and the 1T phase

3

diminishes, exhibiting a progressive transition from octahedral (1T) to a trigonal

4

prismatic (2H) phase. A similar trend of shift in XPS core level binding energy is also

5

observed for the S 2p electron in the emission spectra, as displayed in Figure S2 (see

6

ESI). The consistent results for both S 2p and Mo 3d core level spectra exclude the

7

decomposition or the formation of a modified structure on ultrasonication. XPS results

8

support the argument that the shift of Mo 3d and S 2p core lines to a lower BE is

9

attributed to the formation of 1T phase in un-annealed samples.

10

3.2. Optical properties of MoS2 NCs polymorphs:

11

We have investigated the optical properties of progressively annealed, mixed phase ~2.0

12

nm diameter MoS2 NCs samples by UV-Vis absorption measurements, as shown in

13

Figure 4a. The absorbance spectrum for the as-synthesized sample corroborates the

14

formation of 1T phase with a sharp peak around 300 nm and a small hump at 230 nm,

15

whereas the band edge transition feature of a 2H MoS2 phase is rather absent. However,

16

we could recover the semiconducting 2H phase on annealing with characteristic excitonic

17

features in the absorption spectra, as shown in Figure 4a. The photon energy agrees with

18

the two characteristics excitonic transition at K-point, commonly labeled as “A” and “B”,

19

originated from the spin-orbit coupled splitted valance bands to the degenerate

20

conduction band edge, in electronic structure for the 2H phase, as reported previously for

21

semiconducting MoS2 NCs28,29 and layered structure30,31. These observations suggest that

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the excitonic band gap transition is recovered in the 2H sample due to structural

23

restoration by thermal annealing. Two additional higher energy absorptions peaks in

24

spectrum, labeled as “C” and “D”, are likely to be attributed to band nesting transitions

25

arising in a localized region between K-point (finite momentum space) and - points

26

(K=0 momentum space) in the band structure of MoS2. The conduction and valence

27

bands are parallel to each other in a region between K and - points in the band structure.

28

This is known as the band nesting (BN) region, where the change of band gap is zero

29

with the change in momentum, giving rise to a local minimum in the optical band

30

structure as well as a singularity in the joint density of state (DOS) in the band structure Page | 6 ACS Paragon Plus Environment

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of these materials32,33. Band nesting implies that when the material absorbs a photon, the

2

produced electrons and holes propagate with exactly same, but opposite, velocity. The

3

“D”-peak is probably due to an excitonic state associated with either the higher energy

4

local minimum in the conduction band near C-point or an additional band nesting,

5

resulting in a minimum in the optical band structure. The experimental results exhibiting

6

additional peaks in the higher energy range are in good agreement with theoretical

7

prediction using density functional theory (DFT) calculation31.

8

To provide further evidence for the coexistence of both phases and the phase transition,

9

Raman spectra has been reported with a confocal micro-Raman setup using a continuous-

10

wave Ar+ laser operating at 514 nm. Characteristic Raman modes are used to extract

11

detailed information about the local content of these two phases and the phase transition

12

of MoS2 NCs. Figure 4b displays the spectra of deintercalated 1T MoS2 NCs with

13

increasing annealing temperature showing the gradual restoration of the 2H phase. The

14

as-synthesized sample exhibits the disordered characteristics signature; i.e., E2g and A1g

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modes at ~378 and ~403 cm-1, which are typical zone-center modes of 2H MoS27,34. The

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line width of the peak is broadened as compared to the original 2H phase. A small

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intensity E1g mode peak found at ~287 cm-1 also corroborates the existence of 2H phase.

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Three new intense features at lower frequency at ~150, ~225 and ~370 cm-1 are also

19

observed, which are characteristic features of 1T phase in agreement with the previous

20

report7,34. With increasing annealing temperature, the 1T vibrational modes become

21

significantly weaker, while the resonant features of the 2H phase are restored. The

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Raman frequency of 350 C annealed sample agrees well with the semiconducting 2H

23

phase26, indicating that the sample on restoration of phase possess a high crystalline

24

quality. The phase transformation can be easily followed by tracking the disappearance of

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the Raman modes associated with the 1T phase in annealed samples.

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The deintercalated stable allotrope of MoS2, the octahedral 1T phase exhibits the feature

27

of metallic phase, which could be explained with the help of the D3d symmetry and the

28

electronic structure of Mo-S building blocks. The intercalation of Li into host MoS2 is

29

governed by an ion–electron transfer topotactic reaction. Electrons from the s-orbital

30

valance band of Li ions are transferred to the lowest unoccupied energy levels of MoS2, Page | 7 ACS Paragon Plus Environment

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which have partially filled transition metal d-bands. They induce the changes in the

2

electronic band structure and properties of host materials. The Mo-S p-d interaction

3

enforces significant effect on electron injection due to Li intercalation and alters the

4

magnitude of the electrical transport. The 4d2 electronic configuration of Mo-4d in 2H

5

phase leads to the full occupation of the lowest sub-band, resulting in semiconducting

6

behaviour. In contrast, the incomplete occupation of Mo-4d orbitals under the 4d2

7

electronic configurations and weak bonding with S-p in 1T-MoS2 leads to the metallic

8

ground state. Therefore, the 1T-MoS2 lattice can be stabilized by the addition of

9

electrons, e.g. from Li atoms35. The octahedral 1T phase can be completely transformed

10

back to the stable tetrahedral 2H phase upon annealing. The valence electron

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configuration of Mo atoms in an octahedral coordination undergoes a transformation

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from d2 to d3 through heating, which eventually leads to loss of donor states. The 1T to

13

2H transformation is achieved following the theory of nucleation for solid−solid phase

14

transformations, where one S atomic layer translates with respect to the rest of the

15

crystal7. As the total energy of the system increases due to ex-situ thermal heating, the

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octahedral arrangement gets sufficient energy and a S-atom layer laterally translates by

17

half-unit cell with respect to the Mo layer to form the trigonal prismatic arrangement7.

18

With increasing thermal energy, more and more unit cells are transformed from 1T to 2H

19

phase by changing their atomic arrangements/co-ordinates. At an annealing temperature ~

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350 C, a completely phase transformation from 1T to 2H phase has been observed, as

21

confirmed by XPS and optical studies. Combining the experimental work with theoretical

22

discussions gives a better understanding about the process involved in the phase

23

transformation during liquid phase exfoliation and phase restoration upon annealing of

24

MoS2 NCs.

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3.3. Electrical properties of MoS2 NCs polymorphs:

26

To understand the nature of the electrical transport in two-dimensional arrays of MoS2

27

NCs, temperature dependent current-voltage (I−V) characteristics were carried out in

28

wide temperature range (10-350 K) for three different size (2 nm, 10 nm and 30 nm) as-

29

synthesized and annealed samples. It is presumed that a continuous 2D network is formed

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by MoS2 NCs clusters, which are separated by thin barriers acting as scattering centers Page | 8 ACS Paragon Plus Environment

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for the carriers. A systematic analysis of the electrical measurements allowed us to

2

estimate the conductivity, effective carrier mobility and activation energy of the transport

3

through NCs ensembles. Figure 5a and Figure 5b show the typical I-V characteristics of

4

the as-synthesized metallic phase and the restored semiconducting phase MoS2 NCs (~30

5

nm diameter) for several temperatures. The I-V characteristics for other two sizes

6

presented in Figure S3 and Figure S4, in ESI, also reveal similar behavior except at a low

7

current level. Figure 5b exhibits a charging behavior at low temperatures, which are

8

generally observed from a system where injected charge carriers are trapped and screen

9

the bias. The behavior indicates the presence of traps-limited conduction. Traps induced

10

within the energy band gap of MoS2 NCs due to defects at grain boundaries and

11

interfaces will capture the carriers, resulting in decrease of available carriers in thermal

12

equilibrium in MoS2 NC films. Unfortunately, the pronounced charge trapping resulting

13

in a voltage shift is observed as the temperature decreases, suggesting surface state

14

dominated carrier trapping rather than thermally activated one, indicating the presence of

15

an interfacial potential. The voltage shift values are plotted against temperature for each

16

of the samples in Figure S5 (see ESI). A smaller voltage is required to inject electrons

17

into films of larger sized NCs, which have lower quantized energy levels. This indicates

18

that the quantized energy level, determine the potential required for carrier injection in

19

the devices.

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The conductivity extracted from devices fabricated on interdigitised electrodes has been

21

calculated using the following equation36:

22

 T  

I T  d . (2n  1)lh V

………… (1)

23

where ‘d’ is the inter-electrode spacing, n is the number of electrodes, ‘l’ is their overlap

24

length, h is the film thickness, V is the applied voltage and I(T) is the measured current at

25

different temperatures. Figure 6 (a-c) shows the Arrhenius plot (ln vs 1000/T) of the

26

estimated conductivity using eqn. (1) for all the samples. Synthesized samples of 30 nm,

27

10 nm and 2 nm diameter MoS2 NCs exhibited metallic-like behavior. The conductivity

28

of the samples decreases upon annealing, indicating the restoration of semiconducting

29

phase progressively upon annealing. Figure 7 presents the estimated conductivity


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measured at 300 K, for all the samples. The conductivity decreases with increase of 2H-

2

to-1T phase ratio in the assembly and decreasing NCs size. The phase transition has been

3

observed with increasing annealing temperatures for all NCs, nearly above ~250 C.

4

Therefore, the band gap energy as well as the electrical conductivity of the MoS2 NCs

5

assembly can be tuned by varying the fractional 1T or 2H phase just by controlling the

6

anneal temperature. The ion–electron topotactic reactions leading to the electron transfer

7

from the valance s-orbital of Li+ to the lowest unoccupied energy levels of the MoS2

8

results in an important change in the electronic band structure and hence the property of

9

the host materials. To the best of our knowledge, this is the first study on the probing of

10

progressive phase transition through electrical conductivity in polymorph MoS2 NC

11

films.

12

3.4. Transport properties of semiconducting MoS2 NCs:

13

The following discussion is focused on charge transport properties of closely packed

14

semiconducting MoS2 NCs for 3 different sizes. Physical descriptions of charge transport

15

through semiconductor NCs assemblies should be different from their bulk counterpart

16

due to the carrier confinement in NCs and the presence of a multiple finite barrier at their

17

surroundings. Therefore, the charge transport through NCs assemblies can be viewed as

18

sequences of interparticle transport between localized states of individual NCs. Low

19

temperature electrical conductivity measurements exhibiting a clear deviation from

20

Arrhenius type temperature dependence of the conductivity as shown in Figure 8a,

21

implies that there are additional conduction mechanism rather than a single charge

22

transport mechanism over the entire temperature span. Figure 8a exhibits three distinct

23

conduction mechanisms, in which, initially a slow increase of conductivity is observed in

24

the low temperature range of 10–80K (III), and subsequently at a faster rate in the

25

temperature range of 80–200K (II), followed by a rapid increase in the range of 200K to

26

300K (I). The variation of conductivity for the entire temperature span is generally

27

expressed as37:     2  .exp       K BT 

   0 exp   28

……. (2)


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Where σ0 is the temperature independent pre-exponential factor,  is the spatial distance

2

between states that are involved in hopping, ‘’ is the localization length of a carrier, ΔE

3

is the energy difference between the states. In a randomly distributed system, the closest

4

range corresponds only with the spatially nearest neighbor if α> 1. The hopping

5

distance ‘’ is limited to the spatial nearest neighboring site at an average distance 0

6

and the conduction mechanism over localized states distributed in a periodic array, called

7

nearest neighbor hopping (NNH). However, if α  1, the variable range hopping (VRH)

8

process dominates. At low temperatures regime, hopping does not take place between the

9

nearest neighbor states due to requirement of larger energy (ΔE). Instead, sequential

10

hopping occurs between randomly distributed closely packed disordered states over an

11

optimum distance that maximizes eqn. (2) and net conductivity depends on the average

12

probability of sequential hops. Eqn. (2) also indicates an increasing hopping distance with

13

decreasing temperature. The experimental data imply the requirements of multiple

14

physical models to explain the charge transport through disordered NCs assembly over

15

wide temperatures. The conductivity can be written for combined VRH and NNH regime

16

as37, 





 T    0 exp    T0 T    

17

 

…… (3)

18

Where 0 and T0, the parameters associated with disorderness in the system can be

19

written as38:

  ( EF )   0  3e  ph    8 i T 

1

2

T0 

2

20

….. (4)

and

 i3

   ( EF )

…… (5)

21

where ph is phonon frequency (~1013 s-1) at Debye temperature, i is the inverse

22

localization length of wave function related to localized states, N(EF) is the density of

23

localized states for electrons at Fermi level, and β (~16) is a dimensionless parameter.

24

Distinguishing transport models have been confusing for some situation because of the

25

similar values of ‘Z’. Optimizing the hopping probability can be estimated from the ‘Z’

26

dependence by assuming a slowly varying Density of State (DOS) in the vicinity of the

27

Fermi level. Mott VRH, Efros-Shklovskii (ES) VRH and NNH regimes have been widely

28

observed in many types of disordered materials. We have used the method of Zabrodskii Page | 11 ACS Paragon Plus Environment

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Page 12 of 25

1

et al.39 to quantitatively determine the dominated transport for different temperature

2

regime, by estimating the exponent ‘Z’ (as described in eqn. 3), using the following

3

function f(T):

4

 d  ln    f (T )  ln    ln    ln T0    ln(T ) d ln T    

5

The best linear fit of the slopes for f(T) versus ln(T) plot as shown in Figure 8b, directly

6

gives the exponent, ‘Z’ (the exponent in eqn. 3). A clear transition in the slope is

7

observed for all NCs size. For the 30 nm diameter NC, at a temperature lower than a

8

transition temperature (Ttr) ~160 K, the slope is close to ~ 0.25 indicating the existence of

9

M-VRH conduction mechanism, whereas for T > Ttr, the slope is closer to ~1, suggesting

10

a crossover from M-VRH to the NNH regime. For other two NCs size, similar transition

11

phenomena are observed. In Table 1, the experimental values of Ttr are presented

12

providing a quantitative estimation of the crossover temperature with NCs size. Although

13

pin-pointing the exact transition temperature may not be trivial from the plot, the result

14

signifies the dependence, as the same increases with decreasing NCs size.

15

A. Transport in the temperature range 80-300 K:

16

A simple pathway for charges to cross a potential barrier between two localized states is

17

by jumping over the barrier on gaining sufficient kinetic energy from their thermal

18

environment. Since the hopping distance is constant regardless of temperature in the

19

NNH regime, i.e., the first exponential term in eqn. (2) is constant, the temperature

20

dependence of the overall conductivity follows a basic Arrhenius type relation. The

21

Arrhenius type conductivity plot over the temperature range of 80–300 K, shown in

22

Figure 8c, has two distinct slopes for the temperature range of 80–200 K and 200–300 K.

23

The conductivity following Arrhenius-like temperature dependence, can be described

24

as40,

…. (6)

25

   1 exp   ETh K T    2 exp   ENNH K T  B  B   

26

Where 1 and 2 are constants, ETh and ENNH are the activation energies related to two

27

different types of conduction processes. The solid line in Figure 8c represents the

………….. (7)


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Page 12 of 25

1

experimental data well fitted by eqn. (7). The extracted thermal activation energies, ETh

2

and ENNH for different NCs size tabulated in Table 1, are in the range of ETh ~ 95 to 110

3

meV and ENNH ~27 to 30 meV. The energy for thermally activated conduction process is

4

controlled by the impurity energy levels and the donor carrier concentration. The

5

activation energy ETh is associated with a group of deep donors, which are excited to the

6

conduction band at a temperature above 200 K, giving rise to charge transport. In the NC

7

ensembles, the edge defect due to S-ion termination and S-vacancies act as donors

8

forming deep energy levels. As the donor carrier concentration increases, the Fermi level

9

rises up in energy resulting in a decrease of the activation energy. At a temperature below

10

200 K, the deep donors bind most of the electrons, and thus carriers do not have sufficient

11

energy to be excited to the conduction band. On contrary, electrons may hop to the

12

shallow impurity band from an occupied level residing below the Fermi energy, thus

13

contributing to major contribution to the conduction mechanism at temperature range 80-

14

200 K. The localized states associated with such conduction process are formed due to

15

the imperfections present in NC films. The energy required for nearest neighbor hopping

16

process is much less compared to that required for thermally activated conduction

17

process.

18

The calculated activation energy of electron transport for the NNH charge transport

19

displays a monotonic increase with decreasing NCs size. The size-dependent activation

20

energy can be explained as the electron conduction through nearest-neighbor sequential

21

hopping between neighboring localized states depends on the carrier mobility, which is

22

affected by the multiple potential barriers in different sized NCs. It is reasonable that

23

electrons traveling via NNH through NC assemblies will require fewer hops to cross a

24

channel of a fixed length. Thus, one would expect that NC films made of larger size will

25

exhibit smaller Ea. A second explanation for the size dependence of ENNH involves

26

inhomogeneities in the position of the electron energy levels due to size distribution of

27

NCs. Film made of variable NCs size leads to the finite distribution in energy sates.

28

Therefore, the film that has a narrower distribution of energy will have stronger coupling

29

between NCs than those with a broader distribution. Therefore, a strong coupling leads to

30

lower activation energy. From simple effective mass approximation, the estimated width

31

of lowest electron state is ΔE1S = h2/(4m*R2).ΔR/R, where h, m*, R, and ΔR are the Page | 13 ACS Paragon Plus Environment

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Page 14 of 25

1

Planck’s constant, the effective mass of the electron, the radius of NC, and the variance in

2

size of NCs, respectively41. Therefore, for a given NCs size distribution, the smaller one

3

will have a broader ΔE1S leading to weaker coupling and thus higher ENNH values.

4

B. Transport in temperature range 10-80 K:

5

The variation of conductivity with temperature in the low temperature range of 10–80 K

6

agrees well with the Mott’s variable range hopping (MVRH) model. According to Mott,

7

the variation of conductivity with temperature in disordered materials is given by42,

8

 T     0 exp   0   T

1

4

………. (8)

9

Below 80 K, the conductivity varies slowly with temperature as the transport is

10

determined by carrier hopping via phonons, from very closely matched occupied energy

11

states to unoccupied ones irrespective of the spatial distribution, which are adjacent to the

12

Fermi level within the energy band gap of NCs. Such characteristic temperature

13

dependence has been observed in many low-dimensional systems and is a signature of

14

hopping transport via localized states. So, the hopping distance is not fixed in this type of

15

conduction. At low temperatures, most of the electrons remain in the vicinity of the Fermi

16

level (EF). Combining eqns. (4), (5) and (8), we obtain ln(T1/2) α T-1/4, which are plotted

17

for different sized NCs assembly in the temperature range of 10–80 K in Figure 8d. A

18

linear variation is observed for all the NCs indicating that M-VRH conduction

19

mechanism is dominant in the studied temperature range. Extracted M-VRH parameters,

20

calculated from the slope and y-axis intercept of the linear fitting, using eqns. (4) and (5),

21

are listed in Table 1. The value of T0 gives a measure of the disorder of a material, which

22

is found to be the lowest for 30 nm and highest for ~2 nm diameter NCs. The value of

23

inverse localization length (i) increases with size of NCs from 2 nm to 30 nm. This

24

means that the fall of carrier wave function becomes sharper with size. Assuming a

25

constant density of states [N=N(E)] near EF and the number of available states [N(E)]

26

near EF per unit energy separated by the distance *, the optimal hopping distance (*)

27

can be determined from a value that maximizes eqn. (4) or that gives d(())/d=0, such

28

that, Page | 14 ACS Paragon Plus Environment

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1

Page 14 of 25

1

  9  8 K BTN ( EF ) 

2

The above relation implies that the hopping occurs to higher distant sites at a lower

3

temperature. This is reasonable because at lower temperature with less available thermal

4

energy, states between which non-resonant tunneling can occur would be found at a

5

larger distance. From the average hopping distance, we have estimated the average

6

hopping energy (W), which can be defined as41,

*

W 3

4

…... (9)

7

43 .N ( EF )

8

The conditions to be satisfied for variable range hopping conduction process are αi*  1

9

and W > kBT. It is noted that all MoS2 NCs studied here satisfy the above conditions and

10

thus justify that Mott’s variable range hopping as the dominant conduction mechanism in

11

the low temperature region (10–80 K). As T decreases, hopping between closer energy

12

spacing are more favorable and the transport changes to Mott VRH. In the context of NC

13

films, the exponent (Z) has been found to be 0.5, which was also observed for other

14

semiconductor systems41,42 as well as for monodispersed metal43 and magnetite

15

nanocrystal arrays44. The correlation between the localization length and NC size is

16

expected because the localization length is essentially the decay length of an electron

17

wave-function in NC assemblies, which is inversely proportional to the number of hops

18

necessary to travel a fixed distance.

…….. (10)

19 20

4. Conclusions:

21

In conclusion, we have successfully synthesized metallic phase MoS2 NCs of three

22

different sizes and studied their transition from 1T metallic to 2H semiconducting phase

23

through optical absorption and XPS spectroscopy by thermal annealing. The variation of

24

conductivity and activation energy of the investigated NCs ensembles has been estimated

25

from electrical measurements to investigate the role of NC size, annealing temperature

26

and charge transport mechanism in NC films. Low temperature electrical conductivity

27

measurements reveal the presence of multiple dominant conduction mechanisms such as

28

M-VRH, NNH and thermally activated type of conduction in the temperature range of


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Page 16 of 25

1

10–80 K, 80–200 K, and 200–300 K, respectively. Upon increasing temperature, the

2

electron transport undergoes a transition in mechanism from M-VRH to NNH. In this

3

context, our experiments with MoS2 devices of different nanocrystal sizes reveal that

4

electrons are strongly localized, likely at the randomly occupied interfacial traps. The

5

ubiquity of surface trap states probably constitutes a generic source of disorder in such

6

ultrathin devices. The transport parameters also vary strongly and systematically with

7

NCs size. The transition temperature between these charge transport mechanisms is size-

8

dependent with the transition taking place at a higher temperature for films of smaller

9

size NCs. The activation energy for charge transport in the NNH model is also found to

10

be size-dependent. Overall, the size- and temperature-dependent charge transport

11

properties of NC films described here provide a thorough understanding of electrical

12

conduction in NC films. The study reveals the important role of short-range disorders on

13

the MoS2 NCs based device performance. We believe that this study provides

14

fundamental understanding of charge transport in NC films and thus would utilize their

15

characteristics for further applications in NCs based optoelectronic devices.

16

Associated content

17

Supporting Information: SEM image of MoS2 NCs thin film, XPS spectra of S 2s states

18

to show the 1T to 2H phase transformation, Temperature dependent I-V characteristics

19

for the ~10 nm and ~2 nm diameter MoS2 samples for 1T and 2H phase, Charge trapping

20

effect at different temperatures for variable size NCs.

21

Acknowledgments:

22

This work was partially supported by the Department of Science and Technology (DST),

23

Govt of India, under the ‘ICF’ Project at IIT Kharagpur. The XPS facility of the Dept. of

24

Physics under FIST project is gratefully acknowledged.

25

Contributions:

26

SM synthesized the NCs and performed the characterizations and worked on the device

27

fabrications. SM and SB have done all the electrical measurements. AG helped in the Page | 16 ACS Paragon Plus Environment

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Page 16 of 25

1

chemical synthesis. SM and SKR analyzed the data and wrote the manuscript. SB and SD

2

contributed to the preparation of the manuscript. All authors have given approval to the

3

final version of the manuscript.

4

Competing interests:

5

The authors declare no conflict of interest.

6

Data availability:

7

The data sets generated during and/or analyzed during the current study are available

8

from the corresponding author on reasonable request. References: (1)

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699–712.

(2)

Kim, S.; Konar, A.; Hwang, W. S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J. B.; Choi, J. Y.; et al. High-Mobility and Low-Power Thin-Film Transistors Based on Multilayer MoS2 Crystals. Nat. Commun. 2012, 3, 1011–1017.

(3)

Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable P-N Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262–267.

(4)

Mukherjee, S.; Maiti, R.; Katiyar, A. K.; Das, S.; Ray, S. K. Novel Colloidal MoS 2 Quantum Dot Heterojunctions on Silicon Platforms for Multifunctional Optoelectronic Devices. Sci. Rep. 2016, 6.

(5)

Sundaram, R. S.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A. C.; Avouris, P.; Steiner, M. Electroluminescence in Single Layer MoS2. Nano Lett. 2013, 13, 1416–1421.

(6)

Yang, H.; Kim, S. W.; Chhowalla, M.; Lee, Y. H. Structural and Quantum-State Phase Transition in van Der Waals Layered Materials. Nat. Phys. 2017, 13, 931–937.

(7)

Guo, Y.; Sun, D.; Ouyang, B.; Raja, A.; Song, J.; Heinz, T. F.; Brus, L. E. Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS2 Crystals. Nano Lett. 2015, 15, 5081–5088.

(8)

Wilson, J. A.; Yoffe, A. D. The Transition Metal Dichalcogenides Discussion and Interpretation of the Observed Optical, Electrical and Structural Properties. Adv. Phys. 1969, 18, 193-335.

(9)

Duerloo, K. A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in Two-Dimensional Mo-and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5, 1–9.

(10)

Kim, J.; Park, C. Observation of a Phase Transition from the T Phase to the H Phase Induced by a STM Tip in 1T-TaS2. Phys. Rev. B 1997, 56, 573–576.

(11)

Zhang, J.; Liu, J.; Huang, J. L.; Kim, P.; Lieber, C. M. Creation of Nanocrystals through a


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Solid-Solid Phase Transition Induced by an STM Tip. Science 1996, 274, 757-760. (12)

Enyashin, A. N.; Seifert, G. Density-Functional Study of LixMoS2 Intercalates (0

Tunable Optical and Electrical Transport Properties of Size and

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