Supplementary Materials for

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1 advances.sciencemag.org/cgi/content/full/4/2/e /dc1 Supplementary Materials for Photocarrier generation from interlayer charge-transfer transitions in WS2-graphene heterostructures Long Yuan, Ting-Fung Chung, Agnieszka Kuc, Yan Wan, Yang Xu, Yong P. Chen, Thomas Heine, Libai Huang This PDF file includes: Published 2 February 2018, Sci. Adv. 4, e (2018) DOI: /sciadv section S1. Spatial resolution and sensitivity of TAM section S2. Raman spectroscopy section S3. Steady-state PL measurements of the 1L-WS2/G heterostructure fig. S1. AFM measurement of the 2L-WS2/G heterostructure. fig. S2. Schematics of homebuilt TAM setup. fig. S3. Spatial resolution of TAM. fig. S4. TAM images of a control 2L-WS2. fig. S5. The amplitude of the TA signal at a 0-ps time delay versus pump fluence for the 2L-WS2/G heterostructure at area 1 (pump, 1.57 ev; probe, 1.99 ev). fig. S6. Band structure calculation along the Γ-M path. fig. S7. Raman spectra of the 1L-WS2/G heterostructure with an excitation energy of 1.96 ev and the Fermi level shift of graphene. fig. S8. Enhancement of Raman intensity in the WS2/G heterostructures with an excitation energy of 2.32 ev. fig. S9. Correlated transmission and reflection images of the same 1L-WS2/G heterostructure on a transparent sapphire substrate shown in Fig. 4 (C and D). fig. S10. TA spectrum of the A-exciton resonance for strong- and weak-coupling locations of the 1L-WS2/G heterostructure on a sapphire substrate. fig. S11. Pump intensity dependent TA spectra of a control 1L-WS2 flake. fig. S12. PL spectra of the 1L-WS2/G and a control 1L-WS2 excited at 2.78 ev. References (70 73)

2 section S1. Spatial resolution and sensitivity of TAM The spatial resolution of TAM is determined by fitting a very small feature in the image with Gaussian function as shown in fig. S3, which is around 400 nm. There are two main sources of noise contributing to TAM imaging: laser fluctuation noise and electronic noise from the detection system (for example, detector and lock-in amplifier). Noise due to laser intensity fluctuations can be effectively eliminated by using heterodyne lock-in detection with MHz modulation (70) where the intensity of the excitation beam (or additional local oscillator) is modulated by an acoustic-optical modulator. Subsequently, a lock-in amplifier referenced to this modulation frequency can sensitively extract the induced signal. The fluctuation of laser intensity ( 1 noise) usually occurs at low frequency (< 10 khz). When f is in the MHz range, the f laser intensity noise becomes near the quantum shot noise limit, which is always present because of the Poissonian distribution of the photon counts at the detector. The pixel dwell time should be significantly longer than the modulation period to allow for reliable demodulation for each pixel. Such a modulation scheme has been successfully applied to transient absorption microscopy to achieve single-molecule sensitivity. In our experiments, we use a modulation frequency of 1 MHz. The TAM instrumentation described here is capable of detecting a differential transmission T T of 10 7, three orders of magnitude higher sensitivity than conventional TA spectroscopy (71). section S2. Raman spectroscopy All Raman spectra were collected by a confocal Raman system (HORBIA Jobin Yvon, XploRA). A CW laser with excitation wavelengths of 532 nm (2.32 ev) and 633 nm (1.96 ev) were used to excite

3 the sample, which was focused on the sample with a 100 /NA = 0.90 objective. A grating of 1800 lines per mm used in the measurements could provide a spectral resolution of 1.9 cm -1. To minimize any thermal damage in the air, all measurements were taken with excitation power of 1 mw and an integration time of 1 s. The spectrometer was calibrated with a Si substrate at 520 cm -1 before the measurements. section S3. Steady-state PL measurements of the 1L-WS2/G heterostructure A picosecond pulsed diode laser (PicoQuant, LDH-P-C-450B) with an excitation photon energy of 2.78 ev and a repetition rate of 40 MHz was used to excite the sample, which was focused by a 100 NA = 0.95 objective. The PL emission was collected with the same objective, dispersed with a monochromator (Shamrock 303i-H, Andor Technology) and detected by a thermoelectric cooled charge-coupled device (CCD) (Newton 920, Andor Technology).

4 Supplementary Figures fig. S1. AFM measurement of the 2L-WS2/G heterostructure. (a) AFM image of the 2L- WS2/G heterostructure. (b) Line profiles taken along the black and red dashed lines indicated in (a). Scale bar represents 1 µm. fig. S2. Schematics of homebuilt TAM setup. OPO: optical parametric oscillator; AOM: acoustic optical modulator; BBO: beta barium borate.

5 fig. S3. Spatial resolution of TAM. The spatial resolution of TAM is determined to be ~ 400 nm by fitting a very small feature (red dash line in a) in the TAM imaging with a Gaussian function (b). The scale bar represents 1 μm. fig. S4. TAM images of a control 2L-WS2. TAM images of a control 2L-WS2 flake at time delays of 0 ps (a) and 5 ps (b) with 3.14 ev pump and 1.99 ev probe. (c) Normalized line profiles at time delays of 0 and 5 ps taken along the dash line indicated in (a). (d) Normalized transient dynamics of positions 9 and 10 in 2L-WS2 with a pump fluence of 8.4 µj cm -2. All scale bars represent 1 µm.

6 fig. S5. The amplitude of the TA signal at a 0-ps time delay versus pump fluence for the 2L- WS2/G heterostructure at area 1 (pump, 1.57 ev; probe, 1.99 ev). fig. S6. Band structure calculation along the Γ-M path. Band structure of the incommensurate model of the perfect unit cells calculated along the -M directions (at the HSE06/SOC level). No additional transitions between graphene and WS2 are expected in comparison with the -K directions shown in Fig. 3.

7 fig. S7. Raman spectra of the 1L-WS2/G heterostructure with an excitation energy of 1.96 ev and the Fermi level shift of graphene. No obvious change in the position of G peak ( 1586 cm -1 ) in the two regions, whereas there is a blue shift of the 2D peak ( 15 cm -1 ) for the graphene layer on WS2. By analyzing the correlation of the two peaks (72), we estimate a p-doping density of cm -2 in graphene on 1L-WS2 corresponding to a Fermi level shift of 0.17 ev (73). Spectra are shifted vertically for clarity. fig. S8. Enhancement of Raman intensity in the WS2/G heterostructures with an excitation energy of 2.32 ev. Raman spectra of (a) a strong coupling location in a 1L-WS2/G heterostructure and a control 1L-WS2 layer and (b) a strong coupling location in a 2L-WS2/G heterostructure and a control 2L-WS2 layer. Spectra are individually normalized to the intensity of their respective Si 520 cm -1 peak.

8 fig. S9. Correlated transmission and reflection images of the same 1L-WS2/G heterostructure on a transparent sapphire substrate shown in Fig. 4 (C and D). Scale bars: 200 nm. fig. S10. TA spectrum of the A-exciton resonance for strong- and weak-coupling locations of the 1L-WS2/G heterostructure on a sapphire substrate. Pumped at 3.14 ev.

9 J/cm R/R 0 (Norm) Energy (ev) fig. S11. Pump intensity dependent TA spectra of a control 1L-WS2 flake. Pump photon energy = 3.14 ev. fig. S12. PL spectra of the 1L-WS2/G and a control 1L-WS2 excited at 2.78 ev. Normalized PL spectra of a control 1L-WS2 (a) and the 1L-WS2/G heterostructure (b). Lorentzian functions are employed to decompose the PL peak to neutral exciton emission (blue) and trion emission (green). Red line is the fitting.