Fig. 3: Nanoscale photo-thermoelectric response of graphene modulated by moiré ferroelectricity in proximal t-WSe₂.

a, b Images of the gate-controlled photocurrent (\({I}_{{{{{{\rm{pc}}}}}}}\)) acquired at excitation energy \(\omega\) = 880 cm⁻¹. The back gate voltage is denoted above each image. c, d Images of the near-field scattering amplitude \({s}_{4}\) simultaneously acquired with a and b, respectively. The scale bar in a also applies to Panels b-d. The dashed lines in a and c mark domain boundaries. e The photocurrent as a function of gate voltage, \({V}_{g}-{V}_{{CNP}}\), measured along the blue line in Fig. 2c. f Line profiles of the photocurrent \({I}_{{{{{{\rm{pc}}}}}}}\) and of the scattering amplitude \({s}_{4}\) for several back gate voltages. Data were collected at the location of the white arrow in d. The photocurrent acquired at \({V}_{g}-{V}_{{CNP}}=5{\:}\;{V}\) displays sign flipping. g The Seebeck coefficient of graphene as a function of Fermi energy. h The Seebeck coefficient forms a checkerboard pattern. Thus, a photocurrent can be formed. Inset: The Seebeck coefficient profile, along the pink line, in graphene engineered by adjacent ferroelectric AB and BA domains. A nonzero gradient of the Seebeck coefficient is formed at the domain wall. i The photocurrent forms near the domain boundary due to the Seebeck coefficient gradient (schematic). The neighboring boundaries display sign flipping of the photocurrent, which originates from the opposite signs of the Seebeck coefficient gradient. All the photocurrent data were acquired by demodulation at the second harmonic of tip tapping frequency. Data in (e) were acquired for Device B. All other data were acquired for Device A. For Device A, we measured a series of photocurrent/nano-IR images, with a back gate voltage step of 5 V. We collected a detailed series of images for this device, yet the back gate dependence data are only fragmentary. Thus, we acquired this additional information for Device B. The data presented for Device B take the form of a line scan, with a fine back gate step size of 0.8 V. In order to present more comprehensive photocurrent evolution data, data acquired on Device B are used in (e).