SUEMENTARY INRMATIN - Nature · .10/.2108 SUEMENTARY INRMATIN 11 When compared with iodine treated samples, the C1s region of XPS spectra for functionalized TMDs showed peak at 288.4

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.2108

1

Covalent functionalization of monolayered transition metal dichalcogenides by phase

engineering

Damien Voiry1, Anandarup Goswami2,3, Rajesh Kappera1, Cecilia de Carvalho Castro e Silva1,

Daniel Kaplan4, Takeshi Fujita5,6, Mingwei Chen5, Tewodros Asefa2,3, and Manish Chhowalla1,*

1 Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New

Jersey 08854, USA.

2 Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road,

Piscataway, New Jersey 08854, USA.

3 Department of Chemical and Biochemical Engineering, Rutgers University, 98 Brett Road,

Piscataway, New Jersey 08854, USA.

4 U.S. Army RDECOM-ARDEC, Acoustics and Networked Sensors Division, Picatinny Arsenal,

New Jersey 07806, USA.

5 WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 6 JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.

Supplementary Information

© 2014 Macmillan Publishers Limited. All rights reserved.



2

Table of Contents

Section Title Pages

Materials and Methods 3-6

Zeta Potential Measurement 6

Photoluminescence of Chemically Exfoliated TMDs 7

High Angle Annular Dark Field Scanning Transmission Electron Microscope (HAADF STEM)

8

XPS Spectroscopy 9-12

Thermogravimetric Analysis (TGA) 13-14

Solubilization of functionalized TMDs in organic solvent 15-16

Raman and FTIR Spectroscopy 16-18

Solid-state 13C Nuclear Magnetic Resonance Spectroscopy 19-21

Importance of iodine as leaving group 21-22

ATR-FTIR spectroscopy of 300°C-annealed TMDs 22-23

Restoration of the trigonal prismatic structure (2H phase) 23-29

Raman spectroscopy of functionalized 2H MoS2 29

X-ray Diffraction 30-31

Functionalization with 4-Bromobenzenediazonium tetrafluoroborate 31-33




3

Structures of the functionalized 1T TMDs 33-34

Field Effect Transistors 34-35

References 35-37

Materials and Methods:

All functionalized TMDs presented in the Supplementary Information are functionalized with

acetamide unless otherwise specified.

Chemicals and Reagents:

Bulk powders of TMDs were purchased from Alfa Aesar. Iodomethane, 2-iodoacetamide,

acetamide, n-butyllithium (1.6 M in hexane), iodine (>99.8%) were obtained from Sigma

Aldrich. Organic solvents were purchased from either Sigma Aldrich or Alfa Aesar and used as

received. Hydrochloric acid (36.5%) was received from Fischer Scientific.

Reaction of exfoliated nanosheets with iodomethane:

The intercalated powder was exfoliated in water at 1mg/mL, sonicated for 1 h to facilitate

the exfoliation and centrifuged to remove lithium compounds as well as the non-exfoliated

materials. The solution of chemically exfoliated TMDs was then diluted with 2-propanol to get a

final ratio water:2-propanol equal to 2:1. Iodomethane was then added to the TMDs solution

with a 10-fold excess. We noticed that the reaction goes faster than in the case of 2-




4

iodoacetamide. After 12 hours, the functionalized nanosheets were washed with 2-propanol (3 x

50 mL), ethanol (3 x 50 mL) and water (3 x 50 mL).

Reaction of 2H nanosheets with 2-Iodoacetamide:

As-exfoliated nanosheets were dried and annealed at 300°C for 15 min to restore the 2H

phase following the protocol reported in Ref 10. The 2H nanosheets were dispersed in water and

sonicated for 10 minutes. The nanosheets suspended in water were then treated with a 10-fold

excess of 2-iodoacetamide for 5 days and washed with isopropanol (3 x 50 mL), ethanol (3 x 50

mL) and water (3 x 50 mL).

Annealing of Functionalized TMDs:

Functionalized TMDs were placed in a quartz tube and vacuum was applied for 20 min to

remove any trace of oxygen. The tube was then refilled with Ar and annealing was performed

under a flow of Argon at 80 sccm for 15 min. Non-functionalized TMDs were annealed

following the same protocol.

Quantification of the functionalization:

The extents of functionalization reported in the manuscript correspond to the ratio the of

the atomic percentage of nitrogen N1s peak at ~ 400 eV attributed to the nitrogen from the amide

groups and the atomic percentage of the transition metal measured by X-ray Photoelectron

Spectroscopy (XPS)1,2.




5

Chemical Vapor Deposition of electronic grade single layer MoS2 nanosheets:

Single layer MoS2 nanosheets were grown by chemical vapor deposition (CVD) using

MoO3 and sulfur powder as precursors. MoO3 powder (50 mg) was placed in an alumina boat in

the center of the heating zone of the furnace. Pieces of silicon wafer capped with 300 nm oxide

layer wafers and 0.75 g of sulfur were placed next the boat containing the MoO3 powder and at

the entrance of the furnace, respectively. Vacuum was performed for 15 minutes and the furnace

tube was then refilled with Ar. The furnace was heated under a constant Ar flow of 50 sccm.

After 15 minutes at 800°C, the furnace was cooled down to room temperature and samples were

removed from the furnace. Triangular MoS2 nanosheets grown on SiO2 were further observed by

optical microscope with dimensions ranging from 20 m to 100 m.

Functionalization of single –layer MoS2 crystals:

Single-layer MoS2 flakes grown by CVD on Si/SiO2 were first converted to the 1T phase.

In a glove box, butyllithium (1.6 M in hexane) was dropped on the wafer. After 24 hours of

reaction, the wafer was washed with hexane (4x 2 mL) and dried. 0.1 M of 2-iodoacetamide in

dimethylsulfoxide was then added on the MoS2 flakes and the reaction was carried out for 48

hours. Finally the excess of the 2-iodoacetamide solution was removed and the wafer was

washed with DMSO (3 x 5 mL), THF (3 x 5 mL), isopropanol (3 x 5 mL), ethanol (3 x 5 mL)

and water (3 x 5 mL).




6

Fabrication of MoS2 field-effect transistors (FET):

Field effect transistors were fabricated using poly(methyl metacrylate) (PMMA)

patterning and e-beam evaporation to deposit gold sources/drain electrodes. The PMMA was

removed by washing with large amount of HPLC grade acetone. Gold (50 nm) was deposited on

the MoS2 flake at 0.5 Å/s and the source-drain length and width were 15 µm and 10µm

respectively.

Zeta Potential Measurement:

Zeta-potentials of exfoliated MoS2, iodine-treated MoS2 and functionalized MoS2 were

measured using Malvern Instrument Zetasizer Nano-ZS90 at pH ≈ 7.4 (using phosphate buffer

saline buffer). The samples were prepared as followed: The representative solid sample (1mg)

was first dispersed in DI water (1 mL), 1 mL of PBS buffer was added to it and sonicated for 30

mins. 1 mL of that solution was taken out and put into the zeta-potential cell for measurement of

zeta-potential. The values are reported as an average of three measurements.

Entry Zeta-potential (, mV)

Exfoliated MoS2 -40.8

Iodine-treated-MoS2 -27.1

Functionalized-MoS2 -20.2




7

Supplementary Table 1. Zeta-potential values of exfoliated MoS2, iodine-treated MoS2 and

functionalized MoS2 at pH ≈ 7.4.

Photoluminescence of Chemically Exfoliated TMDs:

MoS2, WS2 and MoSe2 are known to undergo an indirect-to-direct band gap transition

when in the form of single layers. Thin film of TMDs has been prepared by vacuum filtration

and transferred on SiO2 following the same methods as reported earlier (Ref 10). After annealing

at 300°C under inert atmosphere to restore the semiconducting structure to the nanosheets, we

detected photoluminescence signals from each material confirming the single-layer nature of the

exfoliated materials prepared for this work (Supplementary Fig. 1).

Supplementary Figure 1. Photoluminescence spectra of chemically exfoliated TMDs for an

excitation wavelength of 514 nm (2.41 eV).

600 700 800

Inte

nsity

(a.u

.)

Wavelength (nm)

MoS2

WS2

MoSe2

2 1.8 1.6

Energy (eV)




8

High Angle Annular Dark Field Scanning Transmission Electron Microscope (HAADF

STEM):

HAADF STEM images were obtained using JOEL JEM-2100F TEM/STEM with double

spherical aberration (Cs) correctors (CEOS GmbH, Heidelberg, Germany) and a collecting angle

between 100 and 267 mrad. The acceleration voltage was set to 120 kV.

Our HAADF-STEM study of functionalized single-layer MoS2 nanosheets revealed no

noticeable changes in the 1T structure between the bare and the functionalized MoS2 nanosheets.

This indicates that the functionalization do not lead to a profound change in the atomic structure

of the TMDs nanosheets. In 2H phase, the S atoms from the upper and lower layer of MoS2 are

in top of each of other. In the case of the 1T phase, due to the octahedral coordination of the

metal atoms, the Sulfur atom S atoms in the upper and lower planes are offset from each other

and their intensity is significantly decreases (Supplementary Fig. 2)3.

(b)(a)




9

Supplementary Figure 2. High annular angle dark field STEM images of the (a) chemically

exfoliated and (b) functionalized single-layer MoS2 nanosheets. The positions of the atoms are

shown in blue and yellow for Mo and S respectively. Scale bar: 0.5 nm.

XPS Spectroscopy:

X-ray photoelectron spectroscopy (XPS) measurements were performed with a Thermo

Scientific K-Alpha spectrometer with a detection limit of 0.1 at.%. All spectra were taken using a

Al-Kα microfocused monochromatized source (1486.7 eV) with a resolution of 0.6 eV and a spot

size of 400 µm.

Supplementary Fig. 3 shows the survey spectra for iodine treated and acetamide-

functionalized MoS2, WS2 and MoSe2. No signals from I3d5/2 expected at ~ 620 eV can be

indentified confirming the absence of pristine reagents.




10

Supplementary Figure 3. XPS survey spectra of acetamide-functionalized TMDs compared to

their iodine-treated counterpart.

1200 1000 800 600 400 200 0

WS2

Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-WS2

1200 1000 800 600 400 200 0

MoSe2

Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-MoSe2

1200 1000 800 600 400 200 0

Fct-MoS2

Inte

nsity

(a. u

.)

Binding energy (eV)

MoS2

(a)

(b)

(c)




11

When compared with iodine treated samples, the C1s region of XPS spectra for

functionalized TMDs showed peak at 288.4 eV associated with carbonyl peak coming from the

amide group on the surface of the nanosheets (Supplementary Fig. 4).

Supplementary Figure 4. C1s high-resolution spectra for (a) Fct-MoS2, (b) Fct-WS2 and (c)

Fct-MoSe2 compared to their iodine-treated equivalent. The band at ~ 288.4 eV is attributed to

the amide groups grafted on the TMDs nanosheets.

At the respective metal regions, the spectra from the functionalized transition metals

didn’t show significant difference compared to the iodine treated TMDs indicating that the

environment of the metal atoms is not significantly affected by the grafting of functional groups

and thus the metal atoms are not attached to the amide group (Supplementary Fig. 5).

292 288 284 280

Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-MoS2

MoS2

292 288 284 280

Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-MoSe2

MoSe2

292 288 284 280

Inte

nsity

(a. u

.)

Binding energy (eV)

WS2

Fct-WS2

(a) (b) (c)




12

Supplementary Figure 5. High-resolution spectra from Mo3d (a and c) and W4f (b) regions of

Fct-TMDs and iodine-treated samples.

40 35 30

WS2Inte

nsity

(a. u

.)

Binding energy (eV)

Fct.-WS2

236 232 228 224

MoSe2

Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-MoSe2

236 232 228 224

MoS2Inte

nsity

(a. u

.)

Binding energy (eV)

Fct-MoS2

(a)

(b)

(c)




13

Thermogravimetric Analysis (TGA):

TGA was performed using Perkin Elmer TGA 7 Thermogravimetric Analyzer equipped

with Thermal Analysis Controller. Measurements were done under nitrogen from 50 °C up to

600 °C with a ramp of 10 °C/min after purging the system for 15 min at 50 °C (Supplementary

Fig. 6). The traces of the first derivatives of the TGA traces show that the decompositions of the

functional groups occurs at 257°C, 242°C and 262°C for respectively MoS2, WS2 and MoSe2

with an onset decomposition temperature of ~ 200°C. The degree of functionalization has been

calculated by measuring the weight loss at 400 °C and assuming that the difference of weight

loss between functionalized TMDs and their iodine treated equivalents is solely due to the

degradation of the functional groups.




14

Supplementary Figure 6. Thermogravimetric traces of bulk, iodine-treated and acetamide-

functionalized TMDs from 50 °C to 500 °C with ramp of 10 °C /min under nitrogen. First

derivative of the TGA traces are displayed in dashed lines.

100 200 300 400 50070

80

90

100

Wei

gth

(%)

Temperature (oC)

Bulk WS2

As-exfol. WS2

Fct-WS2

0.00

-0.05

-0.10

Derivative w

eight (%. oC

-1)

100 200 300 400 50070

80

90

100

Wei

ght (

%)

Temperature (oC)

Bulk MoSe2

MoSe2 _ iodine

Fct-MoSe2

0.0

-0.1

-0.2

-0.3

Derivative w

eight (%. oC

-1)

100 200 300 400 50070

80

90

100

Wei

ght (

%)

Temperature (oC)

Bulk MoS2

As-exfol. MoS2

Fct-MoS2

0.0

-0.1

-0.2

-0.3

Derivative w

eight (%. oC

-1)

(a)

(b)

(c)




15

Solubilization of functionalized TMDs in organic solvent:

Functionalized MoS2 and WS2 were dissolved in isopropanol at 1-2mg/mL and were

gently sonicated for 30 min using regular bath sonicator. The colored solution was then

centrifuged at 1500 rpm (~ 500 g) for 10 minutes to remove the aggregates. The solution thus

obtained from 1T and 2H functionalized XS2 (X= Mo or W) are found highly stable and no

sedimentation is observed after several days. Interestingly the different phase can be identified

easily by the color. The solutions from 1T TMDs have a grey-reddish color, whereas the

solutions of 2H phase are yellow-greenish (Supplementary Fig. 7).

Supplementary Figure 7. Digital pictures of the functionalized 1T and 2H XS2 solutions in

isopropanol. From left to right: Fct-1T MoS2, Fct-2H MoS2, Fct-1T WS2 and Fct-2H WS2.

The concentration of the solution has been estimated by using the Beer-Lambert law

(Supplementary Table 2). We have measured the extinction coefficient for the 1T phase of MoS2




16

and WS2 by combining UV-vis spectroscopy and dried-extract of the solutions. We found at 500

nm, 1T MoS2 = 27.4 L.g-1.cm-1 and 1T WS2 = 35.6 L.g-1.cm-1. For the 2H phase, we have used

previous values from the literature4.

Fct-MoS2 Fct-2H MoS2 Fct-WS2 Fct-2H WS2

Concentration (mg/mL) 0.09 0.04 0.06 0.03

Supplementary Table 2. Concentration of functionalized TMDs solution in IPA.

ATR-FTIR Spectroscopy of acetamide functionalized TMDs:

We performed ATR-FTIR spectroscopy on the acetamide-functionalized TMDs. 2-

iodoacetamide has intense signals at 1610 and 1655 cm-1 attributed to N-H bond and –C=O

respectively. After functionalization, the similar peaks are detected from Fct-TMDs confirming

the presence of the functional group (Supplementary Fig. 8). The presence of the band at 1223

cm-1 attributed to -CH2 wagging from S-CH2 in Fct-MoS2 and Fct-WS2 indicates the formation

of the S-CH2 bond consecutive to the covalent functionalization on the sulfur atoms of MoS2 5,6.




17

Supplementary Figure 8. ATR-FTIR spectra of 2-iodoacetamide (blue), iodine-treated WS2

(black), iodine-treated MoS2 (grey) and MoS2 (green) and WS2 (red) functionalized with

acetamide.

Raman Spectroscopy:

Raman spectra were obtained using a Renishaw 1000 system operating at 514 nm (2.41

eV). TMDs have two main Raman modes, the in-plane mode: 12GE and the out-of-plane mode:

A1G. After the chemical intercalation, additional Raman signals appear in the spectra of the

different TMDs, attributed to the 1T structure presumably because of the conversion of 2H phase

to 1T 7. The Raman signals of the 1T MoX2 and WX2 (X= S or Se) are still largely unexplored.

In the case of MoS2, new signals named J1, J2 and J3 have been reported in the early studies of

Li-intercalated MoS2. Recently Calandra has proposed different vibration modes for the J1, J2

600 1200 3000 4000

WS2-iodine

Fct-MoS2

Fct-WS2

2-iodoacetamide

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

MoS2-iodine

600 1200 3000 4000




18

and J3 modes as well as for the other peaks at 147 cm-1, 223 cm-1 and 286 cm-1 8. The Raman

signals from the 1T phase are not affected by the iodine treatment indicating that the 1T phase is

preserved. On the other hand, strong bands rise up after the functionalization, which are

attributed to the covalent grafting of molecule on the nanosheets (Supplementary Fig. 9).

Notably, the A1G bands of the different TMDs split into 2 signals after functionalization most

likely due the presence of functional groups bonded to the S or Se atoms, which perturb the out-

of-plane vibrations (Supplementary Fig. 9). The J1 signals of MoS2 slits into 2 bands with a new

peak at 167 cm-1. The J2 and the J3 peaks at ~ 226 cm-1 and 328 cm-1 get significantly stronger

and downshifted to 208 cm-1 and 323 cm-1 respectively. According to the work of Calandra, the

J1, J2 and J3 peaks involve mainly the vibrations from the sulfur atoms. Thus these changes

suggest the perturbation of the 1T vibration modes and in particular the vibration from the sulfur

atoms caused by the anchorage of the functional groups.

Supplementary Figure 9. Raman spectra of functionalized TMDs compared to their iodine-

treated counterparts.

200 400

WS2

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

Fct-WS2

E12G

A1G

200 400

MoSe2

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

Fct-MoSe2

A1GE1

2G

200 300 400

A1GE12G

MoS2

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

Fct-MoS2

J2

J1

J3

(a) (b) (c)




19

Solid-state 13C Nuclear Magnetic Resonance Spectroscopy:

The solid-state 13C (100.64 MHz) NMR spectra were acquired on a Bruker 400 MHz

NMR spectrometer (Probe: Bruker 3.2 mm bore HXY probe operating in HX mode) at 298K.

The samples were prepared mixing cal-SBA-15 with the samples followed by careful packing of

the mixture into a 3.2 mm zirconia rotor. For 13C CP-MAS NMR experiments, 10.0 kHz spin

rate, 5 s recycle delay, 2 ms contact time, π/2 pulse width of 5 μs (at 66 W), and at least 64 K

scans using “Spinal 64” 1H decoupling method were employed. The spectra were referenced

based on external adamatane standard in which the peak at higher chemical shift was set at 38.43

ppm. The spectra were processed in Bruker Topspin (v 3.2) using conventional techniques and

50Hz of line broadening window function was applied to all the spectra.

210 190 170 150 130 110 90 70 50 30 10 -10 -30

Chemical Shift (ppm)

* = spinning s ide-bands

178.1 22.8

*

XNH2

O

H HC

C

X = H(Acetamide)




20

Supplementary Figure 10. 13C CP-MAS spectrum of acetamide. The green peak designates the

carbon signal corresponding to carbonyl group and blue one shows aliphatic carbon signal.

Proton solution nuclear magnetic resonance (1H-NMR) spectroscopy

Proton solution nuclear magnetic resonance spectra (1H-NMR) were recorded using

Varian VNMRS-500 MHz instrument using solvent as an internal standard (DMSO-d6 at 2.50

ppm). For “PRESAT” mode, the water signal at 3.32 was saturated in order to suppress the peak

as best as can be done. The spectra were processed using MNOVA (version 8) using standard

processing parameters and reported in ppm. The sample was prepared by adding freshly opened

DMSO-d6 to the respective sample and sonicating them for 5 mins at room temperature.

Supplementary Fig. 11 shows the extended region of 1H liquid NMR spectra of (a)

iodoacetamide, (b) MoS2-bulk, (c) Fct-MoS2-1T and (d) Fct-MoS2-1T (under water-suppressed

PRESAT mode) using DMSO-d6 as solvent. In case of pure iodoacetamide (Supplementary Fig.

11a), dissolved in DMSO-d6, the 1H signal corresponding to methylene protons showed up at

3.57 ppm. This peak was expectedly absent in case of MoS2-bulk due to absence of such protons

(Supplementary Fig. 11b). In case of Fct-MoS2-1T the peak at 3.51 ppm (both in normal and

PRESAT mode, Supplementary Fig. 11c-d) clearly established the presence of methylene groups

that could only stem from functionalization. The trend in 1H NMR chemical shift values from

iodoacetamide to Fct-MoS2-1T can be explained by 1) smaller electronegativity of the sulfur

than iodine, and 2) the introduction of relatively more negative charge to the methylene protons

upon functionalization, both of which allow better shielding and presumably make methylene

peak to be more upfield than iodoacetamide.




21

Supplementary Figure 11. Extended region of 1H liquid NMR spectra of (a) iodoacetamide, (b)

MoS2-bulk, (c) Fct-MoS2-1T and (d) Fct-MoS2-1T (under water-suppressed PRESAT mode)

using DMSO-d6 as solvent.

Importance of iodine as leaving group:

In order to get better understanding of the process, we replaced 2-iodoacetamide with

acetamide in the functionalization reaction. No evidence of functionalization was observed by

XPS spectroscopy as shown in Supplementary Fig. 12. This result confirms the key role of

iodide presumably as leaving group in the mechanism of the reaction.

Chemical Shift (ppm)

3.57

3.51

(under water-suppressed PRESAT mode)

(a)

(c)

(d)

(b)

3.6 3.5 3.4 3.3




22

Supplementary Figure 12. XPS high-resolution spectra of N1s region for 1T MoS2 treated with

2-iodoacetamide (red) and acetamide (black).

ATR-FTIR spectroscopy of 300°C-annealed TMDs:

ATR-FTIR spectroscopy was performed on methyl-functionalized TMDs after annealing

at 300°C for 15 min (see methods above). The ATR-FTIR spectra confirm the presence of

methyl groups covalently attached to the surface of MoS2 and WS2 (Supplementary Fig. 13). The

S-CH3 peaks can be clearly identified at 1288 cm-1 and 943 cm-1 in the case of 2H MoS2 and

1289 cm-1 and 950 cm-1 in the case of 2H WS2. These peaks appear to be slightly shifted

compare to their position for functionalized 1T TMDs possibly due to the change of the crystal

structure. The ATR-FTIR spectra of 2H MoS2 and WS2 confirm the presence of covalently

attached functional groups on the 2H TMD nanosheets after annealing.

404 400 396 392

Mo3p

Inte

nsity

(a. u

.)

Binding energy (eV)

N1s

acetamide

2-iodoacetamide




23

Supplementary Figure 13: FTIR spectra of methyl-functionalized 1T MoS2 (black), 2H MoS2

(red), 1T WS2 (blue) and 2H WS2 (green).

Restoration of the trigonal prismatic structure (2H phase) upon thermal annealing:

The 2H phase was restored in the TMDs nanosheets by thermal annealing (see Methods

above). We used Raman and XPS spectroscopy to follow both the evolution of the extent of

functionalization upon annealing and the restoration of the trigonal prismatic 2H phase

(Supplementary Fig. 14).

3000 1500 1250 1000 750

2H WS2-CH

3

1T WS2-CH

3

2H MoS2-CH

3

1T MoS2-CH

3

Tran

smitt

ance

(%)

Wavenumber (cm-1)

r(CH3)d(CH3) n(S-C)




24

Supplementary Figure 14. Restoration of the 2H phase in the acetamide-functionalized MoS2

nanosheets. (a) Evolution of the Raman spectra of Fct-MoS2 with increasing temperature

compare to the spectrum of chemically exfoliated 2H MoS2. Spectra are normalized to the A1G

404 402 400 398 396 394 392

250oC

200oC

300oC

Inte

nsity

(a. u

.)

Binding energy (eV)

As-Fct.

100 200 300 400 500

2H MoS2

300oC

250oC

200oC

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

As-Fct.

(b)

(a)

0 50 100 150 200 250 3000

10

20

30

40

Fct-MoS2

Fct-WS2Nitr

ogen

per

met

al (%

)

Temperature (oC)

(c)




25

peak at ~ 405 cm-1. (b) XPS spectra of the N1s region for Fct-MoS2 as-prepared and annealed at

200 °C, 250 °C and 300 °C. Signals from the Mo3p peaks and its satellite peaks are shown in

blue and red for the 1T and 2H phase respectively. Spectra are normalized to the Mo3p peak. (c)

Evolution of functionalization degree as function of the annealing temperature.

The change in the 2H ratio was tracked by XPS spectroscopy. It appears that the

restoration kinetics is significantly slower for Fct-MoS2 and Fct-WS2 that their bare counterparts

(Supplementary Fig. 15) are in good agreement with the Raman spectra (Supplementary Fig. 16).

Supplementary Figure 15. Evolution of the 2H phase as function of the annealing temperature

for non-functionalized exfoliated and functionalized MoS2 (a) and WS2 (b).

The restoration of the 2H phase of MoS2 is accompanied with a rise of the Raman modes:

12GE and A1G. At 200 °C in the case of as-exfoliated MoS2, the two modes have high intensities

indicating that the trigonal prismatic structure is virtually completely restored. At the opposite,

0 50 200 250 3000

20

40

60

80

100

Fct-MoS2

MoS2

2H p

hase

(%)

Temperature (oC)

0 50 200 250 300

0

25

50

75

Fct-WS2

WS2

2H p

hase

(%)

Temperature (oC)

(a) (b)




26

Fct-MoS2 exhibits still strong 1T signatures with the J1, J2 and J2 modes at ~ 160 cm-1, ~ 230 cm-

1 and ~ 330 cm-1 respectively (Supplementary Fig. 16) 7. The evolution of the 1T phase

monitored by XPS and Raman spectroscopy suggests that the 1T phase could be stabilized by the

covalent attachment of the functional groups, which would also agree the slight increase of the

1T concentration in the as-functionalized samples (Supplementary Fig. 5).

Supplementary Figure 16. Raman spectra of as-exfoliated, iodine-treated and Fct-MoS2 after

annealing at 200°C for 15 min. Spectra are normalized to the A1G peak.

Up to 200 °C virtually no change of the functionalization extent can be detected

confirming the TGA results which show that the loss of the grafted group starts at temperatures

higher than 200 °C (Supplementary Figs 14 and 17a). When increasing the annealing

temperature, the density of functional groups decreases and the extent reaches ~30, ~ 15 and ~ 12

at.% at respectively 200 °C, 250 °C, and 300 °C in the case of MoS2 (Supplementary Fig. 14c).

100 200 300 400 500

Inte

nsity

(a. u

.)

Wavenumber (cm-1)

Fct-MoS2

Iodine-treated

As-exfoliated




27

Similar trend can be observed in the case of WS2 although the restoration of the 2H is not

complete at 300°C (Supplementary Figs 14, 15 and 17b). Supplementary Fig. 17c shows the

evolution of the C1s region when increasing the annealing temperature. The peak for the amide

groups at ~ 288.4 eV decreases relative to the α-C peak at ~ 285 eV suggesting that the amide

function is more susceptible to decompose during annealing than the α-C directly bonded to

MoS2.




28

Supplementary Figure 17. Combined plots of the N1s regions from Fct-MoS2 (a) and Fct-WS2

(b) at increasing temperature up to 300°C. Peaks of the Supplementary Figs 15a and 15b are

normalized to the Mo3p peak and the total amount of tungsten detected by XPS respectively.

406 404 402 400 398 396 394

Inte

nsity

(a. u

.)

Binding energy (eV)

250oC

200oC

300oC

As-Fct.

294 291 288 285 282

Inte

nsity

(a. u

.)

Bindin energy (a. u.)

As-Fct. 200oC 300oC

404 402 400 398 396 394 392

Inte

nsity

(a. u

.)

Binding energy (eV)

As-Exfol. As-Fct.

200oC 250oC 300oC

(a)

(b)

(c)




29

Evolution of the C1s regions for annealed Fct-MoS2 (c). The carbon peaks are normalized to the

total percentage of carbon detected in the samples.

Raman spectroscopy of functionalized 2H MoS2:

12GE and A1G are the two main Raman signatures of the trigonal prismatic (2H phase)

structure of MoS2. The strong signals: 12GE at ~ 384 cm-1 and A1G at ~ 405 cm-1 from Fct-2H

MoS2 indicate that the crystallinity of the phase is largely preserved after annealing. Minimal

increase in the FWHM and the absence of significant shift of the 12GE and A1G bands suggests

that disorder induced by the covalent grafting is moderate (Supplementary Table 3). A slight

blue shift of the 12GE peak coupled with an increase of the FWHM of the 2 modes can be noticed

in the case of Fct-2H MoS2 (Supplementary Table 3).

Peak position (cm-1) FWHM (cm-1)

12GE A1G 1

2GE A1G

Fct-2H MoS2 382.5 ± 0.2 405.4 ± 0.5 10.1 ± 0.2 10.4 ± 0.5

2H MoS2 383.6 ± 0.2 405.5 ± 0.5 9.23 ± 0.6 9.4 ± 0.5

Supplementary Table 3. Summary the peak positions and the width of the 12GE and A1G modes

of 2H MoS2 and functionalized 2H MoS2.




30

X-ray Diffraction:

The wide-angle x-ray diffraction (XRD) patterns of MoS2 containing samples were

obtained by use of a Bruker HiStar area detector and an Enraf-Nonius FR571 rotating anode x-

ray generator equipped with a graphite monochromator (Cu K; = 1.5418 Å) operating at 40

kV and 63 mA. The beam was monochromatized by use of a Rigaku osmic mirror in parallel

mode. All of the data were collected at room temperature (ca. 21 ºC) over a period of about 5

minutes. The sample to detector distance was (9.00 cm and the standard spatial calibration was

performed at that distance. Scans were 2 degree wide for the sample stage axis () while rotating

the spindle axis at 2 deg min-1 and having fixed detector, or Bragg, angle (2) of 35 deg, and

fixed horizontal tilt angle of 45 deg. In all cases, the count rate for the area detector did not

exceed 100,000 cps. The samples were prepared by coating the tip of a 0.5 mm glass capillary

with paraffin oil and placing grains of the powder on it. The center of the capillary was centered

on the instrument.

The XRD data were analyzed by use of programs Bruker GADDS (data collection and

chi integration and MDI Jade7 (background fitting and IUCR Powder Diffraction File, PDF,

matching). The 2H phase of MoS2 has been indexed following PDF numbered as 77-1716. The

X-ray diffraction peaks from Fct-MoS2 nanosheets annealed at 300 °C match very well to the 2H

MoS2 structure, confirming that the crystallinity of the Fct-2H MoS2 is largely preserved

(Supplementary Fig. 18).




31

Supplementary Figure 18. X-ray diffraction patterns of exfoliated 2H MoS2 (black) and

functionalized 2H MoS2 (red).

Functionalization with 4-Bromobenzenediazonium tetrafluoroborate:

The electron-rich 1T phase of TMDs can react with other types of electrophyle molecules

such as aryl diazonium salts.

To functionalize TMDs (MoS2 and WS2) with aryl groups using aryl diazonium salts, we

followed the method developed by Bekyarova et al.2. Solutions of chemically exfoliated MoS2

and WS2 were obtained following the method reported above. Thin films of each of MoS2 and

WS2 were then prepared by vacuum filtration and transferred on SiO2 wafer using the same

method as in Ref 15. Under inert atmosphere, a solution of 10 mM of 4-bromobenzenediazonium

tetrafluoroborate in degassed acetonitrile was dropcasted on the TMDs thin films. The samples

10 20 30 40 50 60

Fct-2H MoS2

Exfol.-2H MoS2

110

105104

103

101

100

Inte

nsity

(a. u

.)

2degree

002




32

were protected from light and left 24 hours to react. Finally the films were washed with

acetonotrile (2 x 10 mL), isoproanol (2 x 10 mL), ethanol (2 x 10 mL) and water (2 x 10 mL).

The functionalized MoS2 and WS2 films have been characterized by XPS (Supplementary

Fig. 19). Bromine was found in both cases whereas no fluorine from the tetrafluorobarate was

detected confirming the successful attachment of the bromophenyl functions. No changes in the

Mo3d or W4f regions were visible indicating that they are not affected by the reaction similarly

to the results obtained with 2-iodoacetamide. At the opposite the S2p spectra are found

significantly changed compare to the iodine-treated samples (Fig. 2) suggesting the covalent

attachment of the aryl functions. The ratios Br/Mo and Br/W reach 0.9 and 0.75 respectively.

Such high ratio can be attributed to the formation of aryl oligomers as observed previously in the

case of graphene9. Traces of nitrogen also suggest that the attachment could be achieved without

the release of nitrogen molecules10,11.

165 162 74 72 70 68

Br3d

Inte

nsity

(a. u

.)

Binding energy (eV)

1T 2H C-S

Br3d




33

Supplementary Figure 19. X-ray photoelectron spectra of the S2p (160-167 eV) and Br3d (68-

75 eV) regions of as-exfoliated MoS2 (bottom) and WS2 (top) functionalized with 4-

bromophenyl.

Structures of the functionalized 1T TMDs:

We have performed first-principles calculations using the SIESTA density-functional

theory software package 12. For this study we have utilized the Perdew-Burke-Ernzerhof (PBE)

exchange correlation functional along with Norm conserving Troullier-Martin pseudopotentials

and split-valence double-zeta basis sets with polarization functions and a plane wave cutoff of

300 Ry 13,14. A uniform k-grid consisting of 225 points (a 15 x 15 x 1 Monkhorst-Pack grid) for

Brillouin zone integration was sufficient to converge the total energy to less than 1 meV. We

have used 90 Å of vacuum spacing in order to eliminate all spurious interactions between

repeating images of the system. All pseudopotentials were tested and resulted in calculated

parameters which were in good agreement with previously reported results. All structures were

allowed to relax until forces were less than 0.04 eV/Å. Electronic convergence was accelerated

by Pulay mixing of the density matrix without loss of accuracy in calculated energies.

The structure of the functionalized 1T TMDs have been calculated with a 2x2x1 supercell

considering methyl (-CH3) as functional groups and a degree of functionalization of 25%

(C/Metal = 25%). The results of the DFT calculations show that the structure of functionalized

1T TMDs corresponds to a structure analogue to the distorted 1T phase with a zig-zag pattern

formed by the metal atoms (Supplementary Fig. 20).




34

Supplementary Figure 20: Top (a) and side (b) view of the calculated structure of single layer

methyl-functionalized 1T MoS2. The 2x2x1 supercell used for the calculations can be identified

with the black lines.

Field Effect Transistors:

The output (left plot) and transfer characteristics (right plot) of the back gated FETs are shown

below in Supplementary Fig. 21. It can be seen that the non-functionalized 1T phase cannot be

gate modulated due to its metallic character. After functionalization, however, gate modulation is

clearly visible. The FET results reveal that the functionalized 1T phase is less conducting than

the pure 1T phase, which is expected since covalent bonding of functional groups decreases the

concentration of free electrons. This localization of electrons due to their participation in

covalent bonds with functional groups is also responsible for the semiconducting properties of

the functionalized 1T phase. These results clearly demonstrate that it is possible to dramatically

alter the optoelectronic properties of TMDs by functionalization.

(a) (b)




35

Supplementary Figure 21: (a) Output and (b) transfer characteristics of back gated FETs

consisting of 2H MoS2, 1T MoS2 and functionalized 1T MoS2 channel materials. Contrary to 1T

MoS2, which does not show any gate modulation due to its metallic nature, gate modulation of

the drain-source current can be obtained in functionalized 1T MoS2. (a) Ids-Vds curves obtained

for a back-gate voltage of 20 V. The current for the pure 1T phase material is 10 times higher

than in the Fct-1T material. (b) Transfer characteristics of 2H MoS2, 1T MoS2, and Fct-1T-MoS2

measured for a bias voltage Vds of 1 V.

References:

1. Lomeda, J. R., Doyle, C. D., Kosynkin, D. V., Hwang, W.-F. & Tour, J. M. Diazonium

Functionalization of Surfactant-Wrapped Chemically Converted Graphene Sheets. J. Am.

Chem. Soc. 130, 16201–16206 (2008).

2. Bekyarova, E. et al. Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of

Aryl Groups. J. Am. Chem. Soc. 131, 1336–1337 (2009).

3. Eda, G. et al. Coherent Atomic and Electronic Heterostructures of Single-Layer MoS2. ACS

Nano 6, 7311–7317 (2012).

0.0 0.5 1.0 1.5 2.0

0

200

400

600

800

1000I ds

(nA

)

Vds (V)

2H MoS2

1T MoS2

Fct-1T MoS2

x0.1

-50 0 500.01

0.10

1.00

10.00

100.00

10000.00

2H MoS2

1T MoS2

Fct-1T MoS2

I ds (n

A)

Vg (V)

(a) (b)




36

4. Coleman, J. N. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of

Layered Materials. Science 331, 568–571 (2011).

5. Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts.

(John Wiley & Sons, 2001).

6. Vorontsov, A. V., Lion, C., Savinov, E. N. & Smirniotis, P. G. Pathways of photocatalytic gas

phase destruction of HD simulant 2-chloroethyl ethyl sulfide. J. Catal. 220, 414–423 (2003).

7. Jiménez Sandoval, S., Yang, D., Frindt, R. & Irwin, J. Raman study and lattice dynamics of

single molecular layers of MoS2. Phys. Rev. B 44, 3955–3962 (1991).

8. Calandra, M. Chemically exfoliated single-layer MoS2: Stability, lattice dynamics, and

catalytic adsorption from first principles. Phys. Rev. B 88, 245428 (2013).

9. Hossain, M. Z., Walsh, M. A. & Hersam, M. C. Scanning Tunneling Microscopy,

Spectroscopy, and Nanolithography of Epitaxial Graphene Chemically Modified with Aryl

Moieties. J. Am. Chem. Soc. 132, 15399–15403 (2010).

10. Laforgue, A., Addou, T. & Bélanger, D. Characterization of the Deposition of Organic

Molecules at the Surface of Gold by the Electrochemical Reduction of Aryldiazonium

Cations. Langmuir 21, 6855–6865 (2005).

11. Agnès, C. et al. XPS study of ruthenium tris-bipyridine electrografted from diazonium

salt derivative on microcrystalline boron doped diamond. Phys. Chem. Chem. Phys. 11,

11647–11654 (2009).

12. Soler, J. M. et al. The SIESTA method for ab initio order-N materials simulation. J. Phys.

Condens. Matter 14, 2745 (2002).

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Rev. Lett. 77, 3865–3868 (1996).




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14. Troullier, N. & Martins, J. L. Efficient pseudopotentials for plane-wave calculations.

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SUEMENTARY INRMATIN - Nature · .10/.2108 SUEMENTARY INRMATIN 11 When compared with iodine treated samples, the C1s region of XPS spectra for functionalized TMDs showed peak at 288.4