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1 Imaging the motion of electrons across semiconductor heterojunctions Michael K.L. Man, Athanasios Margiolakis, Skylar Deckoff-Jones, Takaaki Harada, E Laine Wong, M Bala Murali Krishna, Julien Madéo, Andrew Winchester, Sidong Lei, Robert Vajtai, Pulickel M. Ajayan, Keshav M. Dani* * kmdani@oist.jp Content Figure S1 Photoluminescence, structural, electronic and work function characterization of the InSe/GaAs heterostructure Figure S2 Fitting of the energy distribution curves gives the offset of conduction band minima between InSe and GaAs Figure S3 Energy and spatial distribution of photoexcited electrons of another InSe/GaAs sample Figure S4 Temporal evolution of the photoexcited electrons in space in another InSe/GaAs sample Supplementary Movie 1 Video of the motion of photoexcited electrons in InSe/GaAs Supplementary Movie 2 Video showing the motion of photoexcited electrons in another InSe/GaAs sample NATURE NANOTECHNOLOGY 1

2 Supplementary Figure 1 Photoluminescence, structural, electronic and work function characterization of the InSe/GaAs heterostructure. a, PEEM image of the InSe/GaAs heterostructure. b, Low energy electron different pattern of the InSe flake taken at 50eV. c, PL spectra from the InSe flake and the GaAs substrate. d, PL spectra of the InSe flake show the small, but reliable, difference in peak positions between the thin and thick part of InSe. e, Photoemission spectra show the offset of the conduction band mimimum between InSe and GaAs at equilibrium. f, Electron reflectivity versus incident electron energy curves indicate work function of InSe is about 0.2eV lower than that of GaAs. 2 NATURE NANOTECHNOLOGY

3 SUPPLEMENTARY INFORMATION Supplementary Figure 2 Fitting of the energy distribution curves gives the offset of conduction band minima between InSe and GaAs. Figure shows the fitting of the EDC curves taken at the instant of photoexcitation for a, InSe and b, GaAs, respectively. 2D maps on the right plot the goodness of fittings for the conduction band edge (Ec), Fermi Level on photoexcitation (Ef * ) and instantaneous sample temperature (kt) around the best-fit parameters. NATURE NANOTECHNOLOGY 3

4 Supplementary Figure 3 Energy and spatial distribution of photoexcited electrons of another InSe/GaAs sample. a, Energy-resolved image of photoexcited electrons taken at the instant of photoexcitation of another typical InSe/GaAs sample. It shows that electrons in GaAs sit at higher energies than in InSe, which is similar to the sample we shown in figure 2. b, LEEM image of the InSe flake. c, Energy distribution curves of the photoexcited electrons taken at different positions indicated in (b). 4 NATURE NANOTECHNOLOGY

5 SUPPLEMENTARY INFORMATION Supplementary Figure 4 Temporal evolution of the photoexcited electrons in space in another InSe/GaAs sample. a, Sequence of images showing the change in photoemission intensity after photoexcitation. A reference image taken at time-delay of 500fs (after the departure of the pump pulse) is subtracted as in Fig. 3. After the first few picoseconds, thick parts of InSe exhibit an increase in photoemission intensity, while thinner parts of InSe and GaAs lose electron density. b, Optical image of the InSe/GaAs heterostructure. c, Plot of normalized photoemission intensity versus pump-probe delay taken at different positions as indicated in (b). (See also Supplementary Movie 2) NATURE NANOTECHNOLOGY 5

6 Supplementary Movie 1 Video of the motion of photoexcited electrons in InSe/GaAs. This video shows the complete sequence of images taken during and after photoexcitation (few stills from the video are shown in figure 3 of the manuscript). At zero-time delay (pumpprobe overlap), we see the near-instantaneous creation of electrons in the conduction band of InSe and GaAs by the pump pulse. Thereafter, we see electrons accumulate (red) in all parts of InSe at early time delays due to electron transfer from the higher energy GaAs states. After ~10ps, depletion of electrons (blue) occurs in thinner regions of InSe, while thicker regions with lower conduction band minimum continue to accumulate electrons. The depletion of electrons (blue) in GaAs and thin InSe is due to electron transfer as well as recombination. Finally, at much longer time delays (~100ps), thicker parts of InSe also lose most of the photoexcited electrons, as the sample returns to the ground state. The video captures some of the most fundamental opto-electronic processes in a type-ii semiconductor heterostructure photoexcitation, charge separation and transfer, and eventual decay back to the ground state. Supplementary Movie 2 Video showing the motion of photoexcited electrons in another InSe/GaAs sample. This video shows the complete sequence of images taken during and after photoexcitation (few stills from the video are shown in Figure S4). Similar to Supplementary Movie 1, this movie captures electron migration and accumulation in InSe from the higher energy GaAs state. Electrons also migrate from thinner parts (with higher conduction band minimum) to thicker parts of InSe (with lower conduction band minimum). At the end, recombination processes eventually return the sample to the ground state. 6 NATURE NANOTECHNOLOGY