Recombination kinetics and effects of superacid treatment in sulfur and selenium based transition metal dichalcogenides

Transcription

1 Supporting Information For Recombination kinetics and effects of superacid treatment in sulfur and selenium based transition metal dichalcogenides Matin Amani 1,2, Peyman Taheri 1, Rafik Addou 3, Geun Ho Ahn 1,2, Daisuke Kiriya 1,2, Der-Hsien Lien 1,2, Joel W. Ager III 2, Robert M. Wallace 3, and Ali Javey 1,2,* 1 Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, United States 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States 3 Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, TX 75080, USA. * ajavey@eecs.berkeley.edu Methods Sample Preparation and Chemical Treatment. Monolayer flakes were prepared from naturally occurring MoS 2 crystals (SPI Supplies) or crystals grown by chemical vapor transport (CVT) in the case of WS 2, MoSe 2, and WSe 2 (HQGraphene and 2D Materials). Samples were directly exfoliated either on Si/SiO 2 substrates for Raman measurements or on quartz substrates for absorption, QY, or TRPL measurements. TFSI treatment was performed using an identical procedure as our previous work. 1 In brief, 20 mg of TFSI (Sigma-Aldrich) was dissolved in 10 ml of 1,2-dichloroethane (DCE) (Sigma-Aldrich) to make a 2 mg/ml solution; this is then further diluted with 1,2-dichlorobenzene (DCB) (Sigma-Aldrich) or DCE to make a 0.2 mg/ml TFSI solution. The sample was then immersed in the 0.2 mg/ml solution in a tightly closed vial for 10 min on a hotplate (100 C). The sample was removed and blow dried with nitrogen without rinsing and subsequently annealed at 100 C for 5 min in ambient. For treatment of WSe 2 and MoSe 2, in addition to the previously described condition, treatments were attempted using TFSI

2 concentrations of 0.02 mg/ml and 2 mg/ml. This was performed both in DCE at room temperature and 100ºC as well as 9:1 vol% DCB/DCE at 100ºC. Additionally, we found that annealing, even at relatively moderate temperatures ( ºC) in forming gas tends to reduce the PL QY of selenium based TMDCs; for this reason this step was omitted for these materials. Optical Spectroscopy: Methods for the various optical measurements as well as the details of the calibration used to establish quantum yield are presented in our previous work. 1 The same home-build optical setup and procedures were utilized here, with the only exception being that an 80 MD Plan (Olympus) objective (NA = 0.9) was used for photoluminescence (PL) excitation/collection due to its improved transmission at longer wavelengths. In brief, the nm line of an Ar + laser was used for excitation in Raman and steady-state PL experiments. The microscope is configured such that the beam is sampled (sampled beam intensity is ~130 times greater than intensity on the sample) throughout the duration of the measurement either with a photodiode power meter (ThorLabs S120C) or calibrated photodiode using lock-in techniques. Raman spectra were measured using a Si CCD camera (Andor idus BVF) and a triple spectrometer in subtractive mode with a 2400 g/mm grating in the final stage. PL spectra were collected using a Si CCD camera (Andor idus BEX2-DD) and a single spectrometer with a 150 g/mm grating. A 550 nm long-pass filter was used to block the excitation signal. Calibration of the wavelength, instrument function, and collection efficiency were performed prior to each measurement using previously described methods 1. Absorption spectra measurements were performed using illumination from a supercontinuum source (Fianium WhiteLase SC-400) while the absorption coefficient at the excitation wavelength was measured using lock-in detection. The system calibrations were performed using quartz and silver as reference transmission and reflectance standards respectively. External quantum efficiency (EQE) is converted to quantum

3 yield (QY) by determining the fraction of light which falls within the escape cone, which is given as 1/4n 2, where n is the refractive index of the medium. 2 Time resolved measurements were performed using the monochromated beam (λ = 514 nm, 2 nm measured bandwidth, ps pulse duration) of a supercontinuum source (Fianium WhiteLase SC-400). The repetition rate was set to either 20, 10, or 5 MHz depending on the sample being measured. The signal was detected by an avalanche photodiode operating in single photon counting mode (IDQuantique) and analyzed using a time correlated single photon counting module (TCSPC) (Becker-Hickl GmbH). Scanning Tunneling Microscopy (STM): An Omicron scanning probe microscope (SPM) system integrated with a preparation chamber, and an X-ray/UV photoelectron spectroscopy spectrometer was utilized for STM measurements. 3,4 The base pressure in the STM chamber was in the low mbar. The SPM incorporates a spring suspension and magnetic damping system to reduce vibration-induced noise. Electrochemically etched W tips were used and STM images were processed using WSxM 5. The various crystals were loaded within 1 min of exfoliation into the SPM chamber load lock.

4 Figure S1. Normalized Raman spectra of both as-exfoliated (blue) and chemically treated (red) WS 2 (a), WSe 2 (b), and MoSe 2 (c) monolayers measured using the nm line of an Ar + laser.

5 Figure S2. Pump-power dependence of the integrated PL for as-exfoliated and treated WS 2 and MoS 2 (a). Dashed lines show power law fits for the dominant recombination regimes. Pump-power dependence of the integrated PL for as-exfoliated and treated WSe 2 and MoSe 2 (b). Dashed lines show power law fits for the dominant recombination regimes.

6 Figure S3. Pump-power dependence of the QY for multiple as-exfoliated and chemically treated MoS 2 samples; includes data from 1 (open upper triangle and solid diamond).

7 Figure S4. Pump-power dependence of the QY for multiple as-exfoliated and chemically treated WS 2 samples.

8 Figure S5. Radiative decay of as-exfoliated (a) and chemically treated (b) WSe 2 measured at several pump fluences. Radiative decay of as-exfoliated (c) and chemically treated (d) MoSe 2 measured at several pump fluences. Initial carrier concentrations (n 0 ) and the instrument response function (IRF) are labeled in each graph.

9 References 1. Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; Kc, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A. Near-unity photoluminescence quantum yield in MoS 2. Science 2015, 350, Yablonovitch, E.; Cody, G.D. Intensity enhancement in textured optical sheets for solar cells. IEEE T. Electron Dev. 2002, 29(2), Wallace, R. M. In-Situ Studies on 2D Materials. ECS Trans. 2014, 64, Addou, R.; Colombo, L.; Wallace, R. M. Surface Defects on Natural MoS 2. ACS Appl. Mater. Interfaces 2015, 7, Horcas, I; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78,