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Supplementary Online Material

Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy Elizabeth J. McCormick,1 Michael J. Newburger,1 Yunqiu (Kelly) Luo,1 Kathleen M. McCreary,2 Simranjeet Singh,1 Iwan B. Martin1, Edward J. Cichewicz Jr.1, Berend T. Jonker,2 Roland K. Kawakami1* 1Department of Physics, The Ohio State University, Columbus OH 43210, USA 2Naval Research Laboratory, Washington DC 20375, USA *e-mail: kawakami.15@osu.edu

I. Flake 5 PL peak assignment

The characteristics of the PL spectra are quite different in flake 5 than in

flakes 1-4 because the neutral exciton peak is much smaller. We therefore

discuss the peak assignment for flake 5, as it differs from the other flakes.

Supplementary Figure S1 shows the PL spectrum for flake 5 over a broader

range than what is shown in the main text Figure 1c. In addition to the main peak

Figure S1. Photoluminescence with an extended wavelength range for Flake 5.

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(632 nm), there is a small peak at a shorter wavelength (615 nm) and very broad

emission (640-750 nm). We attribute the broad emission to defect emission, but

do not analyze it by peak fitting as it is not well described by a single Lorentzian,

and is likely a combination of multiple defect states.

The other two shorter wavelength peaks can be fit well by Lorentzians.

The 615 nm peak has a broadening parameter, which is half width at half max

(HWHM), of 2.2 nm, consistent with the exciton broadening parameter in flakes

1-4. The 632 nm peak has a broadening parameter of 4.4 nm, consistent with

the broadening parameter of the trion peak in flakes 1-3 (flake 4 does not have a

resolvable trion peak). The fitting parameters for the exciton, trion, and defect

peaks for all 5 flakes are shown in Supplementary Table 1. The defect peaks in

flakes 1-4 have broadening parameters from 10.0 nm to 13.5 nm, which is much

wider than what is observed for the 632 nm peak in flake 5, supporting the

assignment of the 632 nm peak in flake 5 as the trion peak. Although the

wavelength of the 632 nm peak in flake 5 is red shifted slightly compared to the

trion peaks in flakes 1-3, the exciton peak in flake 5 is also slightly red shifted,

and the wavelength of the peaks in WS2 can be heavily dependent on the

amount of strain in the sample1.

We understand the differing characteristics in the PL between flake 5 and

the other flakes through the mechanism discussed by Currie, et al.2. Here, it was

shown that a monolayer TMD sample that was exposed to laser light under

vacuum for relatively large periods of time (several minutes) and subsequently

excited optically could generate a very strong trion peak that dominates over the

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neutral exciton peak. As flake 5 was the first sample we investigated, it had

considerably more laser exposure than the other samples (all measurements

were done under vacuum). The other samples were measured more efficiently,

as we had the benefit of experience at that point, and therefore had much less

exposure to the laser under vacuum than flake 5.

II. Additional characterization of the CVD-grown monolayers of WS2

II.A. N-type doping

The CVD grown material used in this study was determined to be n-type

based on transport measurements. Two-probe field effect transistor (FET)

devices were fabricated from WS2 monolayers on SiO2/Si(100) substrates. The

n-doped Si substrate served as a back gate for electrical measurements.

Electrical contacts were defined using electron beam lithography and Au contacts

with Ti adhesion layers were deposited via electron-beam evaporation. A

constant bias of +12 V was applied between the two metal electrodes while the

current was measured as a function of back gate voltage. Measurements were

obtained under vacuum (< 10-5 Torr) and at 10 K. Figure S2 shows the current

Flake1 Flake2 Flake3 Flake4 Flake5

exciton l = 610.5nm l = 611.3nm l = 607nm l = 606.5nm l = 615nmb= 3.8nm b= 3.6nm b= 3.4nm b= 3.2nm b= 2.2nm

trion l = 624nm l = 627nm l = 620.3nm l = * l = 632nmb= 7.0nm b= 9.0nm b= 6.0nm b= * b= 4.4nm

defect l = 637.8nm l = 638nm l = 632.5 l = 632nm l = **b= 12.0nm b= 13.5nm b= 10.0nm b= 10.0nm b= **

Table S1. Wavelength, l, and broadening parameter, b, for the fits of the three peaks in the PL spectra for each of the 5 flakes. (* there was no resolvable trion peak), (** does not fit well with a single Lorentzian)

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as a function of gate voltage, and indicates that the material is n-type. We note

the intrinsic conductance (for gate voltage = 0 V) is low, suggesting a low

residual n-type doping. The low doping level is also suggested by the small

Stokes shift (<2 nm) when comparing the reflectivity spectrum and PL spectrum

(see Figure S2), as Stokes shifts are known to increase monotonically with the

doping level3. The low doping level is also consistent with the PL spectrum,

which is dominated by neutral exciton emission (X0), with only minor contributions

from charged trion emission (X-).

II.B. PL and reflectivity spectroscopy

The PL emission energy in our CVD grown samples is considerably different

than previously reported on mechanically exfoliated samples. While mechanical

exfoliation is typically carried out at room temperature, we perform CVD

synthesis at 825°C. As the sample cools to room temperature from the growth

Figure S2. Measured current versus gate voltage on a two-probe FET device. Measurements were done at 10 K with a source-drain voltage of +12 V.

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temperature, monolayer WS2 and the supporting substrate contract at different

rates, imparting strain to the WS2 monolayer, subsequently reducing the band

gap and red-shifting the emission energy of the exciton. While we observe a

small amount of sample-to-sample variation, room temperature PL emission is

near 633 nm (Figure S3), consistent with previously reported room temperature

PL energies of 625 nm – 636 nm4-6 for monolayer WS2 synthesized under

conditions similar to our growth parameters (i.e. ambient pressure, WO3 and

sulfur precursor, Tgrowth> 700°C, SiO2/Si growth substrate).

Figure S3 shows the PL and reflectivity spectra measured at room

temperature. To minimize unwanted contributions from the substrate in the

reflectivity measurement, the difference between the intensity measured from the

bare substrate (Ioff), and from the WS2 sample (Ion) is obtained then normalized to

the substrate intensity, I*= (Ioff – Ion)/Ioff. The peak position measured for PL and

I* = (Ioff – Ion )/Ioff

Figure S3. PL and reflectivity spectra of the WS2 flakes. These measurements were performed at room temperature.

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reflectivity are nearly coincident, confirming the PL arises from excitonic emission

rather than low-energy defect related recombination. The small Stokes shift (<2

nm) is comparable to7 or better than8 reported values on mechanically exfoliated

WS2.

III. Wavelength Dependence of TRKR signal

We investigated the wavelength dependence of the TRKR signal and

compared it with the wavelength dependence of the PL and reflectance.

Supplementary figure S4 displays the PL, differential reflectance, and TRKR

signals as a function of wavelength for flake 1 in the main text. The differential

reflectance data was taken at 8 K, as was the PL and TRKR (see methods in the

main text). The white light was focused down to ~5 µm spot, and the difference

between the intensity measured from the bare substrate and the intensity from

the sample was normalized to the substrate intensity to minimize substrate

contributions, as done for the room temperature data in Supplementary section

I.B. The reflectance peak is observed at the same energy as the exciton

emission and the TRKR signal. This is expected, as the exciton should exhibit

absorption, and the presence of a Kerr rotation in a semiconductor relies on the

absorption of circularly polarized light. Neither absorption nor Kerr rotation is

observed at the trion (~630 nm) for flakes 1-4 or defect peak (~640 nm) for flakes

1-5. Flake 5, however, does exhibit a Kerr rotation at the trion energy, ~625 nm.

Existence of a Kerr rotation signal indicates that the trion absorbs light, which

seems to occur when the trion luminescence is large relative to that of the

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exciton2. Photoluminescence from flake 5, main text Figure 1c, shows a very

large trion peak, ~20x larger than the neutral exciton (this varies with position).

Figure S4. Wavelength dependence of the photoluminescence, reflectance, and Kerr rotation, for Flake 1 from top to bottom. The Kerr rotation data shows the amount of Kerr rotation present at 100 ps. As can be seen from the three panels, the Kerr rotation only occurs at the energy of the exciton, where there is also a peak in the reflectance, however no Kerr rotation exists at the defect peak, which does not have a corresponding peak in the reflectance spectrum.

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IV. Laser Induced Spot on Flake 1

Throughout this study, we observed differing levels of laser induced

changes to WS2 flakes. Other studies have also noted that WS2 samples can

change over time, both with and without laser exposure9,10. Over time, all flakes

show some level of aging, but some flakes were affected much faster than

others. Flake 1 was especially sensitive to laser exposure. The anticorrelated

spot, circled in white in the main text Figure 3a, developed after a relatively short

period of exposure to the ultrafast laser. We investigated this specific spot for

~45 min to perform time delay scans for characterizing the spin/valley lifetime.

Afterwards, the spot showed a stronger TRKR signal, with suppressed PL. We

do not fully understand the mechanism in which the laser exposure was able to

induce an anticorrelation, however we suspect that cleaning the surface due to

laser annealing or photo-doping may be contributing.

V. Linecuts of TRKR and PL on Flake 5

Supplementary Figure 5 shows a series of TRKR delay scans and PL

spectra taken along a linecut within the triangular island (flake 5) shown in the

main text Figure 2, as indicated in Supplementary Figure 5a. The linecuts

elucidate a very important trend, showing that in regions with the strongest PL,

the TRKR has a fast decay (<10 ps) and shows very little amplitude in the long-

lived spin state. On the other hand, regions with high spin density in the long-

lived state have much weaker PL.

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Figure S5. Anticorrelation of photoluminescence and time resolved Kerr rotation for flake 5. a, The dashed line indicates the line-cut where TRKR and PL are compared. b-c, TRKR delay scans and PL spectra measured at 6 K at representative points along the line-cut. The position with the brightest photoluminescence has lowest spin density. d-e, Detailed spatial dependence of TRKR and PL along the line-cut.

600 640 680 720 Wavelength (nm)

0 1 2 3 Time Delay (ns)

Pho

tolu

min

esce

nce

Inte

nsity

(a.u

.)

Ker

r Rot

atio

n (a

.u.)

0

5

10

15

20 y

posi

tion

(µm

)

y po

sitio

n (µ

m)

y = 3 µm

y = 7 µm

y = 15 µm

y = 11 µm

y = 3 µm

y = 7 µm

y = 15 µm

y = 11 µm

0 1 2 3

0

5

10

15

20 600 640 680 720

Kerr rotation (a.u.) PL intensity (a.u.)

(b) (c)

(d) (e)

3.02.52.01.51.00.50.0

x10-3

Δt = 80 ps

5µm

y position (µm)

(a)

0

10

5

15

20

0 1 0 1

10

VI. Temperature dependence of TRKR signal in WS2

The temperature dependence of the TRKR signal in WS2 is shown in

Supplementary Figure S6. The TRKR signal was measured as described in the

main text, and the long spin/valley lifetime, as given by an exponential fit, was

plotted as a function of temperature. The TRKR signal is robust until ~130 K, at

which point it drops to below 200 ps. The inset of Supplementary Figure S6

shows representative scans at different temperatures.

Spi

n Li

fetim

e (n

s)

Temperature (K) 0 200 50 100 150

3

4

2

1

0

30 K 60 K 110 K 180 K K

err

Rot

atio

n (a

.u.)

0 4 Time Delay (ns)

2

(f)

Figure S6. Spin lifetime as a function of temperature for a flake not shown in the mapping data. The inset shows representative delay scans at different temperatures.

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