Nanoneedle decoration of Bi2Te3 nanoparticles

Before Bi2Te3 nanoparticle decoration, an acetone solution containing the nanoparticles (Sigma-Aldrich Inc.) was ultrasonicated for 3 to 5 hours by low power to obtain much smaller particle diameters (e.g., <1 nm) by crushing them, avoiding introduction of defects to nanoparticles. Then, a low amount of acetone solution containing Bi2Te3 nanoparticles (e.g., 0.01 mg/8 ml) was dropped from the top end of the nanoneedle (Saito Medical Instruments Inc.) on the two neighboring Hall bar patterns of graphene (section S1 and fig. S1C). This droplet on graphene was then absorbed by the nanoneedle (see movie S1). We repeated this dropping and absorbing 10 to 20 times, resulting in the controlled decoration of Bi2Te3 nanoparticles with very low density (<5% coverage).

The obtained density and distribution of nanoparticles on the patterned graphene was sensitive to the density of nanoparticles contained in acetone solution, time for ultrasonication of the solution, position to absorb the solution by a nanoneedle from a container right after the ultrasonication, and the number of times to repeat the dropping and absorbing as mentioned above. Because the deposited nanoparticles were spread over a large area of the sample (e.g., over the region for section S1 and fig. S1B) depending on this optimized condition, a passivation film with an open window only on the graphene surface (e.g., section S1 and fig. S1C) was needed. After the decoration, each sample was annealed at 400°C for 10 to 15 min under a high vacuum (10−6 torr) for surface cleaning. These annealing conditions were the upper limit to keep the quality of the nanoparticles. Nanoparticles annealed at 450°C with the same time and vacuum degraded.

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