advances.sciencemag.org/cgi/content/full/3/10/e1701661/dc1 Supplementary Materials for Defect passivation of transition metal dichalcogenides via a charge transfer van der Waals interface Jun Hong Park, Atresh Sanne, Yuzheng Guo, Matin Amani, Kehao Zhang, Hema C. P. Movva, Joshua A. Robinson, Ali Javey, John Robertson, Sanjay K. Banerjee, Andrew C. Kummel This PDF file includes: Published 20 October 2017, Sci. Adv. 3, e1701661 (2017) DOI: 10.1126/sciadv.1701661 Supplementary Materials and Methods fig. S1. The SEM of as-grown MoS2 on HOPG via CVD showing two different areas. fig. S2. STS of MoS2 ML taken far away from defects with fitting in a linear scale. fig. S3. The large-area STM image of bare MoS2 grown by CVD on HOPG. fig. S4. Raman spectra of a MoS2 ML before and after deposition of TiOPc under a 488-nm laser excitation. fig. S5. The deposition of TiOPc molecules on MoS2 ML via molecular beam epitaxy at 300 K. fig. S6. Reproduced subset of di/dv/i/v near the TiOPc molecule on MoS2 ML. fig. S7. Tip-induced diffusion of TiOPc molecule on MoS2. fig. S8. STM image and STS recorded in bulk MoS2 deposited TiOPc molecules. fig. S9. Full ML of TiOPc on bulk MoS2 and corresponding STS of a TiOPc ML. fig. S10. Thermal stability of a TiOPc ML on an MoS2 ML. fig. S11. DFT calculations of net charge in a TiOPc/MoS2 ML; three different locations in MoS2 ML are selected, as shown in the circles. fig. S12. Back-gated leakage current of a single-layer MoS2 FET, with VD = 1 V before and after deposition of a TiOPc ML. fig. S13. Back-gated transfer characteristics of a single-layer MoS2 FET, with VD = 0.1 V before and after deposition of a TiOPc ML. table S1. Summary of relative net charge of TiOPc and MoS2 (neutral) from three different locations. table S2. Net charge of TiOPc and MoS2 ( 2e).
References (33, 42 60)
Supplementary Materials and Methods 1. Preparation of MoS2 In this report, two different MoS2 monolayers are employed for scanning tunneling microscopy (STM) and electronic/photoluminescence (PL) characterization; for electric and optical characterization, mechanically exfoliated MoS2 MLs are prepared using the standard scotch-tape method onto thermally grown SiO2/Si substrates. After transferring MoS2 MLs on substrates, the samples are annealed in ultra-high vacuum (2x10-10 torr) at 523 K for 15 30 min to remove the any tape residue or adsorbates introduced from ambient air. All exfoliated MoS2 flakes are characterized before deposition of TiOPc. Frequently, exfoliated MoS2 from bulk MoS2 has variable electronic properties area to area or grain to grain, because of variable defects types, as reported in previous results (42). Therefore, for consistent analyses using scanning tunneling microscopy, MoS2 MLs were deposited on Highly Oriented Pyrolytic Graphite (HOPG) using chemical vapor deposition (CVD) as previously described (43); MoO3 powder (Sigma-Aldrich) is placed at the center of furnace and vaporized by annealing at 750 C, while the substrates are placed above the MoO3 powder. At same time, S powder is located upstream in the furnace and vaporized by heating to 180 C. 100 sccm of Ar gas transfers S vapors to the substrates to deposit MoS2. The CVD grown MoS2 is characterized using scanning electron microscopy (SEM) and Raman spectroscopy. As shown in SEM images of fig. S1, two different areas are imaged; in both areas, CVD grown MoS2 on HOPG surface has < 0.5 ML coverage. In area 2, the HOPG step edge serves as a favorable nucleation site for MoS2, consistent with step flow growth (44, 45), while in area, 1the growth of MoS2 ML on the terrace.
fig. S1. The SEM of as-grown MoS2 on HOPG via CVD showing two different areas. 2. STM and STS and determination of band edge in STS After recording the STS, the raw STS curves are converted to di/dv/i/v, then STS modeling is employed on a linear scale to determine the conduction and valence band edges, as shown the STS of MoS2 ML in fig. S2. The detailed fitting method is performed using techniques in previous STM/STS reports (46, 47). It is noted that the standard error is obtained during fitting process. The corresponding uncertainties are statistical, less than thermal broadening, and do not include the band edge states (48).
fig. S2. STS of MoS2 ML taken far away from defects with fitting in a linear scale. 3. Density Functional Theory calculation The atomic and electronic structure simulation of MoS2 and TiOPc interaction was performed with the plane wave pseudopotential density functional theory code CASTEP (49). The standard ultra-soft pseudopotentials distributed with the code were used with the recommended cut off energy of 400 ev. A slab of 25 Å vacuum was inserted to simulate the monolayer with periodical boundary conditions. A supercell of 20Å was used for size convergence test. BFGS method was used to relax the total energy until the residual force was less than 0.03eV/Å. Only one k-point was used for the integration in reciprocal space due to the large size of the supercell. It is well known that local functionals underestimate the band gap of semiconductors. It has been confirmed by many calculations that the band structure of 2D materials can be reproduced by local density approximation (LDA) and PBEstyle general gradient approximation (GGA). Considering the large size of our simulation and the surface configuration, PBE-style GGA functional was used for all the calculations. The van der Waals interaction was included in TS scheme as implemented in CASTEP code with the defaults parameters (50).
Additional data 4. Large area STM image of bare CVD grown MoS2 deposited on HOPG As shown in fig. S3, the large area CVD MoS2 flakes are imaged in the empty state by STM. The triangular island growth of MoS2 can be observed on HOPG surfaces, consisting of monolayer (ML), bilayer (BL) and tri-layer (TL) MoS2. Each layer is about 0.7 nm height consistent with three atomic layer (S-Mo-S), as shown the line trace of fig. S3(b) (51, 52). Although the CVD grown MoS2 samples were exposed in ambient air before loading an UHV chamber, air induced adsorbates are not observed at the terraces. Conversely some adsorbates can be detected along the step edges, even after annealing at 573 K for 6 hr. Previously, it has been observed that the edge of MoS2 is more chemically reactive than terrace (53 55), therefore, it can be hypothesized that the edges of MoS2 is reactive to ambient air. fig. S3. The large-area STM image of bare MoS2 grown by CVD on HOPG. (a) large area empty state STM image of bare MoS2/HOPG. (Vs = 2 V, It = 20 pa) (b) Line trace taken along the white dash line in (a). 5. Analysis of Raman spectra Non-destructive Raman spectroscopy of exfoliated MoS2 ML is carried out in ambient conditions using the excitation layer a 300 micro-raman confocal microscope, with the laser
operating at a wavelength of 488 nm. Parameters in the present mapping were (i) grating (Raman) = 1800 g/mm, (PL) = 600 g/mm; (ii) integration time/pixel = 1 s; (iii) resolution = 3 pixels/µm. As shown in fig. S4, two prominent peaks are observed at 386.9 cm -1 and 405.8 cm -1, referred as the in plane vibrational E 1 2g mode and out of plane vibrational A1g mode respectively, and the difference of these two peaks is about 18.9 cm -1, consistent with single layer of MoS2 (56). The peak-to-peak ratio (E 1 2g/ A1g) is obtained as 0.421. After deposition of a TiOPc ML on the same MoS2 ML, similar peaks also can be observed at nearly identical positions as shown in the red curve. The difference between the two peaks is still about 18.9 cm -1, and the peak-to-peak ratio of TiOPc/MoS2 is about 0.415, similar to bare MoS2. This nearly identical behavior of MoS2 ML in Raman before and after deposition of TiOPc ML indicates that during deposition of TiOPc, there is no change in the crystal structure of MoS2 or the introduction of defects in MoS2. fig. S4. Raman spectra of a MoS2 ML before and after deposition of TiOPc under a 488- nm laser excitation. 6. Control of the coverage of TiOPc molecules on MoS2 ML via deposition duration The coverage of TiOPc molecules can be controlled by the deposition duration, while the sample temperature is held at 300 K. As shown in fig. S5(a), after deposition of TiOPc for
10 s on MoS2 ML, a few TiOPc molecules are adsorbed on the MoS2 terrace. With an increase of the deposition duration to 30 s, the number of TiOPc molecules increases in the STM images by about 4x as shown in fig. S5(b). It is noted that during STM imaging at 100 K, there is interaction between STM tip and single molecular adsorbed TiOPc molecules, thereby inducing noise into the STM images. After deposition TiOPc for over 60 s, a full monolayer of TiOPc is obtained as shown in Fig. 2(e) fig. S5. The deposition of TiOPc molecules on MoS2 ML via molecular beam epitaxy at 300 K. (a) After deposition for 10 s, a few TiOPc molecules are observed on MoS2 ML. (Vs = 2 V, It = 20 pa) (b) After deposition for 30 s, a larger coverage of TiOPc molecules is adsorbed on MoS2 ML. (Vs = 2 V, It = 20 pa) 7. Reproducible spatial STS of single TiOPc molecules on CVD grown MoS2 ML Another set of spatial STS is recorded near a single TiOPc adsorbate on CVD grown MoS2 ML to verify the charge transfer from the MoS2 ML to the TiOPc molecule; note this is a different TiOPc molecule from the one shown in Fig. 2. In fig. S6(a), a single TiOPc molecule can be observed in empty state imaging, and the STM tip is positioned at a blue dot near TiOPc molecule. As a result, the blue STS curve of MoS2 ML is observed with the Fermi level close to valence band in fig. S6(b). As STM tip is moved away from TiOPc and
positioned at the red dot, the Fermi level approaches the conduction band, consistent with the negative charge transfer from MoS2 to the TiOPc molecule. fig. S6. Reproduced subset of di/dv/i/v near the TiOPc molecule on MoS2 ML. (a) STM image showing the adsorption of single TiOPc molecule on MoS2 ML. (Vs = 2 V, It = 40 pa) (b) di/dv/i/v recorded at a distance from TiOPc. 8. Tip induced diffusion of TiOPc molecule on MoS2 An isolated TiOPc molecule adsorbed on MoS2 can be induced to diffuse by applying bias to the W STM tip, as shown in fig. S7. After the STM tip is approached near an isolated TiOPc molecule on ML MoS2 (d < 1 nm to TiOPc), the STS bias on the W tip is ramped from 2 to -2 V. As a result, the TiOPc molecule diffuses along the opposite direction to the W tip. This diffusion of TiOPc molecules indicates that the TiOPc/MoS2 interface relies on van-der- Waals interaction, rather than chemical bonds or ionic bonds which are the much stronger interactions than van-der-waals interactions.
fig. S7. Tip-induced diffusion of TiOPc molecule on MoS2. STS bias (sweeping from 2 V to - 2 V) is applied at the X position near the TiOPc molecule (Vs = 2 V, It = 20 pa). 9. Inverse charge transfer direction at TiOPc bulk MoS2 interface In contrast to TiOPc/MoS2 ML, inverse charge transfer is observed in the interface of TiOPc/bulk MoS2, as shown in fig. S7. In fig. S7(a), the two TiOPc molecules are deposited on bulk MoS2 surface with about 1.8 nm diameter, similar with TiOPc molecules on MoS2 ML. However, as shown in the spatial STS of fig. S7(b), the direction of charge transfer at TiOPc/bulk MoS2 is inverse to the charge transfer for TiOPc/MoS2 ML. Near the TiOPc molecule, the bulk MoS2 has a Fermi level close to the conduction band. Conversely, as the STM tip moves away from the TiOPc molecule, the Fermi level is shifts to valence band, To elucidate the charge transfer between a TiOPc molecule and bulk MoS2, a TiOPc monolayer is deposited on bulk MoS2 using MBE, as shown in fig. S8. It is noted that on bulk MoS2, the deposition method was different from deposition of TiOPc on MoS2 ML. On MoS2 ML, deposition was performed at 300 K without post deposition annealing. Conversely, on
bulk MoS2, a thick overlayer of TiOPc was deposited at 373 K for 3 min, then postdeposition annealing was employed at 573 K for 6 min to thin the layer to 1ML. As a result, four-fold symmetry in TiOPc ML can be observed in fig. S8(a). It is known that the crystallinity of organic layers can be altered by both different deposition conditions and different interactions with substrates (27, 33, 57, 58). As shown in fig. S8(b), STS of a TiOPc ML deposited on bulk MoS2 reveals a Fermi level close to the highest energy occupied molecular orbital (HOMO) (i.e the VB). Consequently, it can be concluded that for TiOPc/bulk MoS2, negative charge transfer occurs from TiOPc to bulk MoS2, opposite to TiOPc/MoS2 ML. This inverse charge transfer results from the altering of the band structure in TMDs with the number of layers, inducing changes in the band gaps and work functions (59, 60). fig. S8. STM image and STS recorded in bulk MoS2 deposited TiOPc molecules. (a) STM image showing the adsorption of two TiOPc molecule on bulk MoS2. (Vs = 2 V, It = 80 pa) (b) di/dv/i/v measured near the TiOPc molecule and away from the TiOPc molecule.
fig. S9. Full ML of TiOPc on bulk MoS2 and corresponding STS of a TiOPc ML. (a) STM image showing the TiOPc monolayer on bulk MoS2. (Vs = 2 V, It = 20 pa) (b) di/dv/i/v of TiOPc ML deposited on bulk MoS2. 10. Thermal stability of TiOPc ML on MoS2. The full TiOPc ML on MoS2 surface is thermally stable, as shown in fig. S9. After annealing at 573 K for 10 min, the TiOPc ML can be observed with hexagonal symmetry. As the annealing temperature is increased to 673 K, corrugation is observed in the TiOPc ML, but the TiOPc ML still maintains its crystallinity. However, after annealing at 773 K for 30 min, partial decomposition of a TiOPc layer can be observed; note, it is the layer structure which decomposes, not the individual TiOPc molecules. This thermal stability of TiOPc ML results from a strong charge transfer with MoS2 ML and a relatively large size of TiOPc molecules resulting in a large binding of the TiOPc to the MoS2 ML. The present DFT calculations reveal the binding energy of TiOPc molecules with MoS2 ML is about 3.1 3.2 e, consistent a high binding energy. Consequently, the it is possible that present TiOPc ML can serve as a thermal protection layer from ambient conditions, while being compatible with standard industry high temperature fabrication processing.
fig. S10. Thermal stability of a TiOPc ML on an MoS2 ML. (a) After annealing at 573 K for 10 min. (b) After annealing at 673 K for 30 min. (c) After annealing at 773 K for 30 min. 11. DFT calculations of net charge in TiOPc/MoS2 ML To elucidate the amount of charge transfer from MoS2 ML to TiOPc, the net charge of each site is calculated by DFT, as shown in 3X4 neutral super cell (no net electrons) of fig. S10. Three different locations are selected as shown wine circles. The results of net charge are summarized, as shown in table 1. Although there are tiny variations in the values of each location, the net charge of TiOPc molecules is in - 0.48-0.58 e, while the MoS2 has net charge in 0.51 0.54 e, consistent with about 0.5 e electron transfer from MoS2 ML to the TiOPc molecule. For the 2e MoS2 system, the charge of MoS2 is 1.2 e, while a single TiOPc molecule has - 0.8 e charge, consistent with 0.8 e charge transfer from MoS2 to the TiOPc molecule, as shown in table 2.
fig. S11. DFT calculations of net charge in a TiOPc/MoS2 ML; three different locations in MoS2 ML are selected, as shown in the circles. table S1. Summary of relative net charge of TiOPc and MoS2 (neutral) from three different locations. TiOPc MoS 2 ML Location 1-0.48 e 0.53 e Location 2-0.58 e 0.54 e Location 3-0.53 e 0.51 e table S2. Net charge of TiOPc and MoS2 ( 2e). TiOPc MoS 2 ML Net charge at - 2 e -0.8 e -1.2 e
12. Device fabrication methods and electrical measurement techniques a of single layer TiOPc/MoS2 FET Standard electron beam lithography (EBL) and e-beam evaporation methods were used to define and deposit the source and drain metals consisting of 20/40 nm Ag/Au. The 4pt. device active area was defined with EBL and etched with Cl2/O2 plasma. Electrical DC characterization was done on a Cascade Microtech Summit 11000B-AP probe-station using an Agilent B1500A parameter analyzer. The measurements were taken at room temperature, in ambient atmosphere, and in the dark. During electric measurements of bare, UHV annealed and TiOPc/MoS2, identical parameters were employed in all cases, as shown the nearly same level of back gate leakage current in fig. S11. Additional electric measurements of single layer MoS2 FETs are shown in fig. S12 with 0.1 VD. Similar with the device results of Fig. 4, UHV annealing of the MoS2 ML FET induces degradation of the ION/IOFF ratio, while deposition of TiOPc ML results in a decrease of the OFF-state current by more than two orders magnitude, and a positive VTH shift of 20 V, consistent with the deactivation of defect states at the interface of the TiOPc ML and MoS2 ML. fig. S12. Back-gated leakage current of a single-layer MoS2 FET, with VD = 1 V before and after deposition of a TiOPc ML.
fig. S13. Back-gated transfer characteristics of a single-layer MoS2 FET, with VD = 0.1 V before and after deposition of a TiOPc ML.