Supporting Information

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1 Supporting Information Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo 2 C/MoS 2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide Jaeho Jeon, Yereum Park, Seunghyuk Choi, Jinhee Lee, Sung Soo Lim, Byoung Hun Lee, Young Jae Song, Jeong Ho Cho, Yun Hee Jang,*, and Sungjoo Lee,* SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon , Korea Department of Energy Systems Engineering, DGIST (Daegu Gyeongbuk Institute of Science & Technology), Daegu 42988, Korea School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju , Korea [Thermal annealing time-dependent Mo 2 C conversion from MoS 2 ] The dependence of the synthesized Mo 2 C area (from MoS 2 ) on the annealing time was characterized by measuring the XPS before and after annealing the samples under different annealing times. The resulting samples included different fractions of the molybdenum sulfide and carbide states. The doublet peaks at ev and ev corresponded to the Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 binding energies, respectively, in the thermodynamically stable form of 2-H MoS 2. The XPS spectra collected before annealing displayed two obvious peaks corresponding to Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2, as shown in Figure S1. Additional shoulders at ev and ev were observed in the XPS Mo 3d spectra (Figures 1b, c, and d) due to the presence of the synthesized Mo 2 C and remnant MoS 2. Mo 2 C yielded Mo binding energy peaks centered at 3d 5/2 and 3d 3/2 due to the Mo 2+ oxidation state. Deconvoluting each spectrum using Gaussian functions clearly revealed that the area under the Mo 2+ binding peaks increased with the annealing time, and only the Mo 2+ binding peaks remained after 4 h thermal annealing. The areas of the synthesized Mo 2 C gradually increased with the annealing time, and Mo 2 C and MoS 2 thermodynamically coexisted. Furthermore, the C 1s spectra collected from each sample after annealing, as shown in Figures S2b, c, d, and e, revealed binding peaks at ev, corresponding to Mo-C bonds, and the areal intensity of the Mo-C binding peak increased with the annealing time. Peak shoulders were not observed prior to annealing (Figure S2a). The peak at ev was attributed to adventitious carbon contamination, as reported previously. 1

2 Figure S1. Mo 3d XPS spectra obtained from samples prepared with different areal fractions of the synthesized Mo 2 C over different thermal annealing times: (a) Pristine MoS 2 before annealing, (b) Mo 2 C-MoS 2 sample after 1 h annealing, (c) Mo 2 C-MoS 2 sample after 2 h annealing, (d) Mo 2 C-MoS 2 sample after 3 h annealing, and (e) fully converted Mo 2 C sample after 4 h annealing. Figure S2. C 1s XPS spectra obtained from samples having different areal fractions of the synthesized Mo 2 C over different thermal annealing times: (a) pristine MoS 2 before annealing, (b) Mo 2 C-MoS 2 sample after 1 h annealing, (c) Mo 2 C-MoS 2 sample after 2 h annealing, (d) Mo 2 C-MoS 2 sample after 3 h annealing, and (e) and fully converted Mo 2 C sample after 4 h annealing.

3 The dependence of the synthesized Mo 2 C area on the annealing time was determined from the OM images. Figures S3a, b, and c display OM images of a single flake before (upper images) and after (bottom images) annealing under different annealing conditions. The red dotted lines indicate the regions in which MoS 2 was converted into Mo 2 C. These regions increased in area with the annealing time, consistent with the results obtained from the Raman mapping (Figures 1f h) and KPFM (Figures 1i k) results. We calculated the percentages of the converted Mo 2 C area based on contrast differences in the OM images, as shown in Figures S3d, e, and f. After 1 h annealing, 19 25% of MoS 2 was converted into Mo 2 C, and longer annealing times produced larger conversion percentages, such as 53 62% after 2 h annealing and 83 95% after 3 h annealing. Figure S3. OM images before (upper images) and after (bottom images) annealing over different annealing times: (a) 1 h annealing, (b) 2 h annealing, and (c) 3 h annealing, and percentages of the synthesized Mo 2 C area and remnant MoS 2 area after different annealing times: (a) 1 h annealing, (b) 2 h annealing, and (c) 3 h annealing. (Scale bars: 10 μm)

4 [Raman spectra with wide Raman shift range] The Raman spectra with 600 g/nm grating were measured at the synthesized Mo 2 C (position 1 in Figure S4) and the remaining MoS 2 (position 2 in Figure S4). From position 1 (red color), we obtained the Raman vibration peaks at 140 cm 1 and 240 cm 1 as the 2E g (in-plane) and A 1g (out-ofplane) modes, respectively. Any additional peaks such as G (~1300 cm -1 ) or D (~1600 cm -1 ) modes of graphite carbon 2 were not observed from the wide range of Raman shift. The Raman spectrum from the remaining MoS 2 region (position 2) exhibited the distinct E 1 2g (382 cm -1 ) and A 1 g (406 cm -1 ) modes of MoS 2, also without any G or D modes. Figure S4. Raman Spectra of the synthesized Mo 2 C and the remaining MoS 2 with broad Raman shift range [Mo 2 C conversion by thermal annealing at higher temperatures] Higher annealing temperatures increased the conversion rate of MoS 2 into Mo 2 C by accelerating the reaction speed. Figures S5 and S6 show OM images obtained after annealing at different temperatures: 850 C or 950 C, respectively. The 850 C annealing temperature provided a four-fold faster Mo 2 C conversion rate than the 820 C annealing temperature. The 950 C annealing temperature provided an even faster rate. We utilized the 820 C conditions to obtain more stable Mo 2 C crystals. Figure S5. OM Images of the Mo 2 C synthesis from MoS 2 flakes annealed at 850 C. OM images were obtained from MoS 2 (a) before annealing, after (b) 10 min, (c) 20 min, and (d) 30 min annealing. The

5 insets of each OM image shown an enlargement of the area at each time, and the yellow dotted lines indicate the Mo 2 C area synthesized from MoS 2. Figure S6. OM images of a sample (a) before and (b) after 5 min annealing at 950 C, and (c) the corresponding Raman spectra measured at the red points indicated in (a) and (b). [Dependence of the converted Mo 2 C layer thickness on the MoS 2 thickness] Figure S7. AFM images of MoS 2 flakes of various thickness before and after annealing (a d), and (e) plot of the thicknesses of the MoS 2 and converted Mo 2 C samples, extracted from the AFM height profiles. The thickness of the converted Mo 2 C was determined by the thickness of the MoS 2 flake. Figures S7a d show AFM height mapping images collected before (upper images) and after annealing (bottom images) samples of different thicknesses. The white dotted lines indicate the area in which MoS 2 was converted to Mo 2 C. After the Mo 2 C conversion, the Mo 2 C thickness decreased from the initial MoS 2 flake thickness. This trend was also observed in the cross-sectional TEM images shown in Figure 2c. The synthesized Mo 2 C area was 70 nm thick, less than the remnant MoS 2 thickness of 83

6 nm. A plot of the thickness values revealed a clear linear thickness dependence between the MoS 2 and Mo 2 C thicknesses, with a slope of 0.83 (Figure S7e), this thickness ratio is similar to that obtained from the Mo 2 C/MoS 2 TEM results (0.84, Figure 2c). Previous studies reported a Mo 2 C lattice constant of 0.48 in the a direction 3,4 with a ratio of 0.79 between the lattice parameters of MoS 2 in the c direction. This lattice parameter ratio agreed with the extracted linear thickness dependence obtained from the AFM data for converted Mo 2 C and pristine MoS 2. Figure S8. (a) OM image obtained from flakes with various MoS 2 thicknesses before anneali ng. (b) A 1g, MoS 2, and E 2g peak differences in the MoS 2 Raman mapping. (c) KPFM image a nd (d) OM image of the same flake after 2 h annealing. (e) A 1g, MoS 2, and 2E g peak differe nces in the MoS 2 Raman mapping. (f) KPFM image and schematic illustration of the DFT-cal culated layer-dependent stability of the Mo 2 C structure. The trends in the Mo 2 C synthesis as a function of the MoS 2 crystal thickness were explored using Raman mappings and KPFM measurements applied to partially converted Mo 2 C after 2 h annealing. Figures S8a, b, and c show OM images, A 1g, MoS 2, E 2g, peak differences in the MoS 2 Raman mapping, and KPFM results obtained before annealing the MoS 2. This flake included MoS 2 layers of various thicknesses, from monolayer to bulk, as confirmed by the peak difference Raman mapping. The brightest area in Figure S8b indicated mono-layer MoS 2 with an 18.6 cm 1 peak difference. The other dark-colored regions represented thicker MoS 2 layers, such as bi-, tri-, tetra-, penta-, and bulk materials with peak differences of 22, 23.5, 24.6, 25, and 25.5 cm 1, respectively. The corresponding KPFM measurements are shown in Figure S8c, and the calculated MoS 2 work function was ev, calculated according to e V CPD = Φ tip Φ sample, with a V CPD of V and Φ tip = 4.8 ev which is calibrated by HOPG. This range agreed well with the work function values measured previously. 5

7 The annealing process was conducted using this MoS 2 sample over 2 h. An OM image, Raman mapping, and KPFM image are shown in Figures S8d, e, and f respectively. The flake did change significantly over the OM images; however, two MoS 2 Raman modes corresponding to A 1g and E 2g were not observed in the green dotted area of Figure S8e due to the preferential Mo 2 C conversion process from MoS 2. A comparison of Figures S8b and e show that the carbide conversion process proceeded preferentially in the thicker penta-layer MoS 2 regions rather than in the thin MoS 2 regions. The green dotted regions shown in the KPFM images of Figures S8f show work functions of ev, whereas other thin MoS 2 regions displayed slightly higher work function values of 4.3 ev compared to the case before annealing. These results could not be obtained from our measurements; however, the energies calculated for each phase using the DFT method provided evidence for the layer-dependent carbide conversion shown in Figure S8g. Multilayered Mo 2 C formed an energetically more stable phase, 3.3 ev/unit cell, compared to the monolayer structure because the binding energy between each Mo 2 C layer provided greater stability. The Mo 2 C conversion process proceeded preferentially in the thicker MoS 2 region. [Large area Mo 2 C Synthesis from Bulk MoS 2 ] To determine the maximum Mo 2 C flake size that could be synthesized using this method, a bulk MoS 2 crystal 2 mm 2 mm in size was subjected to 820 C annealing. After 20 h annealing, we obtained a fully converted bulk Mo 2 C crystal from the bulk MoS 2, as shown in Figure S9a. The obtained Mo 2 C crystal could be cleaved using the tape exfoliation method (Figures S9b and c). As shown in Figure S9d, Raman peaks from the exfoliated Mo 2 C flakes were similar to those obtained from Mo 2 C converted from MoS 2. Figure S9. (a) OM image of the Mo 2 C sample converted fully from a bulk MoS 2 crystal. (b) Exfoliated Mo 2 C lifted on tape from the bulk Mo 2 C. (c) Exfoliated Mo 2 C transferred onto the SiO 2 substrate, and (d) Raman spectra obtained from the bulk Mo 2 C and exfoliated Mo 2 C flakes.

8 [EELS line profile measurement at the MoS 2 Mo 2 C Junction] We carried out the EELS line profile measurement to provide clear elemental composition from the MoS 2 Mo 2 C junction (Figure S10). Figure S10b shows the EELS line profile result obtained along the blue dotted line in Figure 10a. Abrupt decrease of the S peak intensity and increase of the C peak intensity at the atomically sharp MoS 2 Mo 2 C interface were observed. Figure S10. (a) STEM image of the Mo 2 C MoS 2 junction, and (b) EELS line profiles along the blue dotted line in Figure S10a: peak intensities of the S L-edge (black line) and the C K-edge (blue line).

9 [Contact resistance between Mo 2 C and the metal electrode] Figure S11. (a) The current voltage characteristics obtained from a Ti contact Mo 2 C device p repared with different channel lengths (inset: OM image of a Mo 2 C device prepared with a T LM structure, scale bar: 3 µm), and (b) extracted resistance vs. channel length. The TLM method was used to measure the sheet resistance (R sh ) of Mo 2 C and the contact resistance (R c ) between Mo 2 C and the Ti electrode. We fabricated a multi-terminal Mo 2 C device with a channel width of 3 µm using electron beam lithography. The Ti (5 nm) contact metal and Au (50 nm) top metal were deposited using electron beam deposition. Figure S10a shows the current voltage curves obtained from different channel lengths: 1, 2, 4, 5, and 7 µm, and the total resistance (R T ) was plotted as a function of the channel length in Figure S10b. The total resistance values displayed a linear dependence. The total resistance channel length slope indicated the value of R Sh /W, and the y- intercept value was 2 R c, extracted from the equation 2. The contact resistance between Mo 2 C and the Ti electrode was measured as Ω µm, similar to the contact resistance between Mo 2 C and the Ti electrode, as shown in Figure 4a.

10 [Charge transfer characteristics from top-contacted Mo 2 C/MoS 2 FET device] Figure S12. (a) I DS V G transfer curves of vertical Mo 2 C/MoS 2 contact FET and (b) electron barrier heights extracted from the ln(i DS 2 /T) q/k B T curves (inset) for vertical Mo 2 C/MoS 2 contact We fabricated a Mo 2 C top-contacted MoS 2 field effect device and measured the barrier height between the fully converted Mo 2 C and semiconductor MoS 2 (Figure S11). Figure S11a shows the schematic diagram of the device and drain current vs. gate bias transfer curves measured at different temperatures. Figure S11b shows the extracted Φ SB from the top-contacted Mo 2 C/MoS 2 FET from the slopes of the ln(i DS /T 2 ) vs. q/(k B T) curves, as shown in the inset. Φ SB of the vertical Mo 2 C/MoS 2 junction is estimated to be ~110 mev, which is higher than that of a lateral-contact, which is probably due to the unavoidable impurities introduced during the transfer process and/or the presence of a vdw gap at the interface, unlike in the lateral junction case. Therefore, by using the fully converted Mo 2 C, a vertical Mo 2 C/MoS 2 junction can be formed in the top-contact structure like another reported MXene electrodes. 6,7

11 [Schottky barrier heights of MoS 2 with top metal contacts (Ti and Pd contacts)] Figure S12. Temperature-dependent I DS V G curves obtained from a MoS 2 FET device with a top (a) Ti contact or (b) a Pd contact, and electron barrier heights extracted from the ln (I DS 2 /T) q/k B T curves (inset) for (c) the Ti contact, and (d) the Pd contact. We measured the temperature-dependent I DS V G curves using a MoS 2 FET with Ti contacts or Pd contacts (Figures S12a and b). Figures 12c and d shows the gate bias-dependent barrier height (Φ B ), calculated from the slopes of the ln(i DS /T 2 ) vs. q/(k B T) curves (insets in Figures 12c and d). At a flat band voltage, where Φ B deviated from the linear trend, the effective barrier height could be extracted. The MoS 2 Ti Schottky barrier height was estimated to be 63 mev, similar to the value reported for few-layer MoS 2 with a Ti electrode 8. The barrier height extracted from the MoS 2 -Pd contact (140 mev) was similar to the value reported previously 9. These results were attributed to the strong Fermi level pinning effect of MoS 2 due to the interface interactions between the metal and the top S atoms in MoS 10 2.

12 [Contact resistance calculations for the Mo 2 C MoS 2 lateral junction device] Figure S14. (a) I 14 V 14 curves obtained at different gate biases, (b) voltage drop measurements obtained from V 14, V 23, V 12 as a function of the gate bias, and (c) calculated contact resistance between Mo 2 C and MoS 2 as a function of the channel resistivity. We fabricated a lateral contact Mo 2 C MoS 2 Mo 2 C device with 4-terminal electrode contacts (inset in Figure S13). The electrodes deposited onto Mo 2 C are indicated as electrodes 1 and 4, and electrodes 2 and 3 contacted the MoS 2 channel regions. Ohmic contact was observed in our lateral contact device based on the linear dependence of the drain current (I 14 ) versus drain bias (V 14 ) over various back gate biases, as shown in Figure S13a. The contact resistance (R c ) between Mo 2 C and MoS 2 was extracted by substituting the channel sheet resistance of MoS 2 (R MoS2 ) and the sheet resistance of Mo 2 C (R Mo2C ) into the total resistance (R T ) with consideration for the channel aspect ratio. Figure S13b shows the voltage drop at each electrode as a function of the gate bias. The voltage drop increased as the gate bias decreased due to back gate-induced channel resistivity modulation. The values of R T between the lateral contacts in the Mo 2 C MoS 2 Mo 2 C device were extracted according to R T = V 14/ I 14, and the MoS 2 channel sheet resistivity (ρ channel ) was calculated according to ρ channel = (W 23 /L 23 ) (V 23 /I 14 ), where W 23 (= 8.5 µm) and L 23 (= 1.5µm) are the MoS 2 width and length between electrodes 2 and 3. Finally, we calculated R c using equation R c = R 12 R 12, MoS 2 R 12,Mo2C = (V 12 /I 14) (L MoS2 12/W MoS2 12) ρ channel (L Mo2C 12/W Mo2C 12) R Mo2C, where L MoS2 12 and W MoS2 12 are the length and width of the MoS 2 region between electrodes 1 and 2 (L MoS2 12 = 3 µm, W MoS2 12 = 8.5 µm). The value of R 12,Mo2C was extracted from the sheet resistance of Mo 2 C (123.6 Ω sq 1 in Figure 4a ) with L Mo2C 12/W Mo2C 12 = The Ti contact resistance (R C,Ti ) was extracted in a similar way, according to 2R c,ti = R T -R channel = V 14 /I 14 (L 14 /W 14 ) R sh = V 14 /I 14 (L 14 /W 14 ) (W 23 /L 23 ) (V 23 /I 14 ) = V 14 /I 14 (L 14 /L 23 ) (V 23 /I 14 ). Figure S13c shows the R channel dependence of R c,mo2c extracted from the results presented in Figure S13b. R c,mo2c scaled according to R channel, similar to the results of R c,ti and reported previously. 11

13 [The relationship between junction thickness and transport properties] Figure S15. AFM Images of devices with different junction thicknesses: (a) 3.8 nm, (b) 8.5 nm, and (c) 11 nm. Blue lines indicate the thicknesses of the MoS 2 on the SiO 2 substrate, an d green lines indicate the thicknesses of the Mo 2 C on the SiO 2 substrate. (d) Extracted contac t resistance (R c ) and Schottky barrier height (Φ SB ) as a function of junction thickness. The relationship between junction thickness and transport properties is experimentally investigated by fabricating Mo 2 C/MoS 2 junctions with different junction thicknesses, such as 3.8 nm (4.5 nm of MoS 2 ), 8.5 nm (10.3 nm of MoS 2 ), and 11 nm (13 nm of MoS 2 ). AFM profiles of these junctions are shown in Figure S14a-c. Figure S14d shows the measured contact resistance (R c ) and Schottky barrier height (Φ SB ) from 3.8, 8.5, and 11 nm-thick Mo 2 C/MoS 2 junctions. R c between Mo 2 C and MoS 2 are extracted as 1.2, 4.1, and 3.8 kω µm from 3.8, 8.5, and 11 nm-thick junctions, respectively, at the same channel sheet resistivity (ρ channel = Ω/ ). In addition, the values of Φ SB are extracted as 26, 43, and 27 mev from the junction thicknesses of 3.8, 8.5, and 11 nm, respectively.

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