Fig. 1. Observation of quantum-confined Stark effects (QCSEs). (a) Illustration of Wannier–Mott exciton in the absence (left panel) and presence (right panel) of a vertically built-in electric field (Fbi). (b) Calculated chemical potential difference among various TMDCs (WSe2, WS2, MoS2, MoSe2) and graphene monolayers, showing a maximum of ∼750.44meV between WSe2 and graphene. The inset is the schematic of a 1L WSe2/1L graphene heterostructure (HS), demonstrating the formation of built-in electric fields. Here, the direction from top to bottom is designated as the direction of positive electric fields. (c) Optical image (i) after heterostructures showing 1L graphene, 1L WSe2, and HS. Scale bar: 8μm. (ii) PL image after heterostructures showing 1L graphene, 1L WSe2, and HS. Scale bar: 8μm. (iii), (iv), Atomic force microscopy image (iii) and Kevin probe force image (iv) of the dotted rectangular region in panel (i). The measured contact potential difference of graphene and 1L WSe2 is ∼81.79mV. Scale bar: 2μm. (d) PL spectra of 1L WSe2 and HS at room temperature, showing an energy difference of 15.20 meV. The inset is the differential reflectance spectra (ΔR/R) of 1L WSe2 and HS, exhibiting an energy difference of 28.4 meV. (e), (f) Calculated orbital-resolved band structure of 1L WSe2 (e) and HS (f), demonstrating that the bandgaps of 1L WSe2 and HS are 2.079 and 2.061 eV, respectively.

Fig. 2. Electrical control of built-in electric field enabled QCSEs. (a), (b) PL intensity mappings of 1L WSe2 (a) and HS (b) as a function of emission energy and gate voltage (VG) measured at room temperature. The dotted white line works as a guide to the eye for the neutral exciton (A) for the two structures. (c) Redshifts of HS and 1L WSe2 at back gate voltages ranging from −50 to 50 V. (d) PL spectra of 1L WSe2 and HS at −50V, showing a redshift of ∼56.97meV. (e) Charge distributions in an HS metal oxide semiconductor (MOS) device at high positive (top panel) and negative (bottom panel) back gate voltages. The vacuum layer acts as the blocking layer that inhibits the efficient charge transfer between WSe2 and graphene. (f) Calculated partial density of states (DOS) of HS with VG>0 (i) and VG<0 (ii), respectively. The insets display the energy band alignment of HS under VG>0 (top panel) and VG<0 (bottom panel). The dashed lines denote the Fermi level of graphene under different doping conditions. EC and EV represent the minimum conduction band energy and the maximum valence band energy, respectively. As VG>0, the majority of electrons are induced in graphene and WSe2, leading to an upshift of the Fermi levels of graphene and band bending of WSe2. For VG<0, the Fermi level moves downward due to the numerous injections of holes.

Fig. 3. Optical power tunability of QCSEs. (a) Redshifts (left Y axis) and PL quenching factor (η) (right Y axis) as a function of excitation powers. (b) The exciton density as a function of excitation powers in a log-log plot. The black and red lines are the power-law fit with a slope of ∼0.99 for WSe2 and 1.02 for HS. Notably, the exciton density of 1L WSe2 demonstrates a saturated trend at high excitation powers, implying the occurrence of exciton-exciton annihilation (EEA). (c) Band alignment of HS under small power (top panel) and high power (bottom panel). EC and EV represent the minimum conduction band energy and the maximum valence band energy, respectively. The dashed black line represents the Fermi level without illuminations, whereas the blue and red dashed lines represent the Fermi level with illuminations. (d) Measured radiative lifetime (τ) of 1L WSe2 and HS as a function of excitation powers. (e), (f) Contour plot of PL intensity versus emission energy and space of exciton diffusion for 1L WSe2 and HS at 1.15μW (e) and 92.08μW (f). The middle of the laser excitation spot is at x=0.

Fig. 4. Interlayer-distance dependence of QCSEs. (a) Calculated interlayer distance within HS as the temperature varies from 298 to 0 K, showing a clear decreasing trend with the decrease of temperature. Notably, the interlayer distance reduces slowly below 183 K. The inset indicates the initial interlayer distance of ∼5.62Å at 298 K. (b) Redshifts (left Y axis) and PL quenching factor (right Y axis) as a function of temperature, showing a reversed tendency. (c) Measured time-resolved PL traces of 1L WSe2 and HS at 298 K and 83 K. IR denotes the instrument response curve. According to the deconvolution with the instrument response, a double exponential equation I=Aexp(−tτ1)+exp(−tτ2)+c is employed to extract the short lifetime τ1 and long lifetime τ2. Here, τ1 and τ2 represent the nonradiative and radiative lifetime, respectively. (d) Measured radiative lifetime (left Y axis) of HS and the lifetime ratio (right Y axis) of 1L WSe2 to HS as a function of temperature.

Fig. 5. Built-in electric field-driven high-performance HS photodetector. (a) Responsivity for the HS photodetector versus excitation powers under VG=−60, −65, and −70V. (b) Comparison plots of temporal photocurrents in 1L WSe2 (top panel) and HS (bottom panel) under 15.9μW. The rise (decay) time τr (τd) is defined from 10% (90%) to 90% (10%) of the maximum photocurrent. It shows a τd of ∼370μs and a τr of ∼353μs for 1L WSe2. For HS, the τd and τr of HS are 120 and 100μs, respectively. (c) Response time as a function of responsivity for previously reported devices, HS and 1L WSe2. It demonstrates that the HS photodetector achieves a balance between responsivity and response speed due to built-in electric fields. “2D/2D” refers to architecture where all constituent components exist at the nanoscale in two dimensions.^45–54" target="_self" style="display: inline;">^–54 “Hybrid structure” typically integrates components from multiple dimensionalities, such as 2D materials combined with 3D matrices.^55–60" target="_self" style="display: inline;">^–60 (d) Scanning photocurrent images of HS at VD=5V. Scale bar: 10μm. The inset is the optical image of the HS photodetector. (e) Band diagram of 1L WSe2 (top panel) and HS (bottom panel) under illumination. Photogenerated electron-hole pairs are created in the WSe2 and HS channels. Nevertheless, more currents contribute to the ON-state of the HS devices. EC, EV, and EF represent the minimum conduction band energy, maximum valence band energy, and Fermi level of 1L WSe2, respectively. (f) Schematic representation of a single-pixel imaging measurement system. (g) Measured photocurrent image of a satellite under 532 nm laser at 1000 Hz modulated frequency. Scale bar: 20μm.