Supporting Information
Ultrahigh Vacuum Self-Assembly of Rotationally Commensurate
C8-BTBT/MoS
2/Graphene Mixed-Dimensional Heterostructures
Xiaolong Liu1, Itamar Balla2, Vinod K. Sangwan2, Hakan Usta3, Antonio Facchetti4, Tobin J. Marks1,2,4, and Mark C. Hersam1,2,4,5,*
1Applied Physics Graduate Program, Northwestern University, Evanston, IL 60208, USA 2Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA
3Department of Materials Science and Nanotechnology Engineering, Abdullah Gül University, 38080, Kayseri, Turkey
4Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
5Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, USA
*Correspondence should be addressed to: [email protected]
Experimental Methods
Synthesis of Epitaxial Graphene (EG) on SiC. A 4H-SiC (0001) (Cree Inc.) wafer was degassed at 550 °C under UHV conditions (~6 × 10-10 Torr) for ~6 hours and heated at 1270 °C for 20 min to induce graphitization.
Synthesis of MoS2 on EG. Rotationally commensurate MoS2 is grown via chemical vapor
deposition following our previous published procedure.1 Briefly, the graphitized SiC wafer was placed downstream (Si face up) of an Al2O3 boat with 10 mg of molybdenum trioxide powder (MoO3, 99.98% trace metal, Sigma-Aldrich) in a 1 inch cylindrical quartz tube furnace (Lindberg/Blue). Another Al2O3 boat with 150 mg of sulfur powder (Sigma-Aldrich) was placed upstream of the MoO3 boat. The sulfur source was heated with a heating belt wrapped around the quartz tube. The quartz tube was purged with argon gas at ~50 mTorr. Following the purge, the furnace was heated to 150 °C for 20 min and then heated to 800 °C in 55 min, maintained at 800 °C for 15 min, and cooled down to room temperature. The sulfur was maintained at 50 °C for 49 min,
temperature. The tube was maintained at 40 Torr under inert conditions using argon as a carrier gas at a flow rate of 25 sccm.
Scanning Tunneling Microscopy and Spectroscopy (STM/STS). STM and STS measurements were carried out in a home-built UHV STM system (~1× 10-10 Torr).2 The microscope is based on the Lyding design.3 The MoS2/EG sample was degassed at ~200 °C for 6 hours prior to deposition of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) molecules from an Al2O3 crucible. Electrochemically etched PtIr tips (Keysight) were used for imaging after thorough degassing in UHV. A lock-in amplifier (SRS model SR850) was used with an RMS amplitude of 30 mV and a modulation frequency of ~8.5 kHz for STS measurements.
Atomic Force Microscopy (AFM). Tapping mode AFM was performed on an Asylum Cypher AFM with NCHR-W Si cantilevers (NanoWorld). The resonant frequency of the cantilevers was ~300 kHz, and the scanning rate was 1-1.5 Hz. Gwyddion software was used for both STM and AFM data processing. First-order plane-fitting was performed to level images. Line profiles were also extracted using Gwyddion.
Figure S1. (a) AFM height and (b) phase images of C8-BTBT deposited on MoS2/EG. The red arrows indicate the fuzzy boundaries of the depressed regions (i.e., exposed EG). This apparent fuzziness is likely from molecules being displaced by the scanning AFM tip. In contrast, other step edges, such as the MoS2 (green arrow) and graphene (blue arrows) step edges covered uniformly by C8-BTBT, remain abrupt. (c) AFM height image of an area with multiple layers of C8-BTBT. The red arrows indicate triangular MoS2 domains where multi-layer C8-BTBT is absent.
Fig. S2. Step height measurements from atomic force microscopy (AFM). (a) AFM image as in Fig. 1d. (b-d) Extracted profiles (blue curves) along the 6 lines marked in (a). Each of the line profiles is fitted with a step function. The step heights and their averaged values <h> are labeled in each panel. For each step edge type, the measured step heights at various locations show small fluctuations. The averaged step heights for first layer C8-BTBT, second layer C8-BTBT, and monolayer MoS2 are 0.457 0.025 nm, 1.649 0.031 nm, and 0.692 0.047 nm, respectively. The uncertainties in the step height measurements are all below 0.05 nm, which is much smaller than the step height differences.
Figure S3. (a) STM image of a pristine monolayer MoS2 surface showing the presence of protruding (blue arrow) and depressed (white arrows) point defects. (b) STM image of a pristine EG surface showing not only the atomic lattice and the 6√3 reconstruction, but also the presence of protruding point defects (blue arrow). (c,d) STM images of 1L C8-BTBT on (c) MoS2 and (d) EG, where the molecular self-assembly is not perturbed by underlying point defects (white and blue arrows). (e) 1L C8-BTBT covers the step edge (white arrow) of graphene and point defects (blue arrow) as a continuous overlayer.
Figure S4. (a) STM image of 1L C8-BTBT on EG with 60° rotations between different domains. (b) A pristine defect free region of EG showing both the honeycomb graphene lattice and the larger-scale 6√3 reconstruction. (c) Left: STM image of 1L C8-BTBT on EG. Right: The Fourier transform of the image on the left that shows not only spots corresponding to the ordered molecular stripes (blue circles), but also points corresponding to the 6√3 reconstruction (yellow circles) that are rotated by 16.6° from the molecular stripes. By choosing only the points corresponding to the 6√3 reconstruction, the filtered image (bottom right) indeed confirms the 6√3 reconstruction. (d) Given the fact that the lattice of graphene is rotated by 30° with respect to that of the 6√3 reconstruction, the orientation of 1L C8-BTBT on EG can be determined as schematically shown.
Figure S5. STS spectra taken on C8-BTBT/MoS2 at different stabilization conditions. For semiconductors, tip induced band bending (TIBB) has the potential to influence spectroscopy results due to the presence of the electric field induced by the tip, particularly for large electric fields. In such cases, the amount of TIBB varies as a function of the tunneling set point. However, under the measurement conditions used here, the two spectra show similar conductance onsets, indicative of minimal TIBB. Moreover, TIBB typically leads to larger bandgap measurements due to the band edges being bent upward and downward at positive and negative sample biases, respectively. Since our measured gap size of ~3.92 eV closely matches the gap of C8-BTBT (3.84 eV) in literature reports, TIBB is playing a minimal role in our spectroscopy measurements.4 References
(1) Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2015, 10, 1067-1075.
(2) Foley, E. T.; Yoder, N. L.; Guisinger, N. P.; Hersam, M. C. Cryogenic Variable Temperature Ultrahigh Vacuum Scanning Tunneling Microscope for Single Molecule Studies on Silicon Surfaces. Rev. Sci. Instrum. 2004, 75, 5280–5287.
(3) Brockenbrough, R. T.; Lyding, J. W. Inertial Tip Translator for a Scanning Tunneling Microscope. Rev. Sci. Instrum. 1993, 64, 2225–2228.
(4) Kobayashia, H.; Kobayashi, N.; Hosoi, S.; Koshitani, N., Murakami, D.; Shirasawa, R.; Kudo, Y.; Hobara, D.; Tokita, Y.; Itabashi, M. Hopping and Band Mobilities of Pentacene, Rubrene, and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from First Principle Calculations.
