Hihly efficient ate-tunable photocurrent eneration in vertical heterostructures of layered materials Woo Jon Yu, Yuan Liu, Hailon Zhou, Anxian Yin, Zhen Li, Yu Huan, and Xianfen Duan. Schematic illustration of the device fabrication process. (f) GrG (e) (d) HfO GrT GrT GrB GrB Fiure SI Schematic illustration of the device fabrication procedures. a, CVD rown monolayer raphene was transferred onto a 3-nm SiO covered silicon substrate -4. b, Bottom raphene was patterned by oxyen plasma etchin usin photo resist as a etchin mask. c, MoS layer was exfoliated onto the raphene throuh a micromechanical cleavae approach 5. d, The top raphene electrode was transferred and patterned on the MoS to overlap with MoS and bottom raphene. e, For the dualate heterostructures, a 6-nm of HfO dielectric layer was deposited by e-beam evaporation, followed by the transferrin of another raphene layer as the top-ate electrode.. Temporal photoresponse in the vertical raphene-mos -raphene device..5 I sc.4.3. 3 4 5 Time (s) on off 5 V oc (mv) 5 35 4 49 56 63 Time (s) on off V oc (mv) 8 4.4.8..6. Time (ms) Fiure SI Temporal photoresponse in the vertical raphene-mos -raphene device. a, Time dependent photocurrent response and b, Photovoltae response under a lobal laser (478 nm) illumination with alternatively laser on- and off-periods.. c, Time dependent photovoltae measurement under a lobal laser (478 nm) illumination throuh a.6 khz chopper. The periodic photoresponse characteristics exhibit the same frequency as that of the laser on-off cycles. The observed photocurrent on-off transition is less than 5 μs, which is only limited by the speed of the mechanical chopper and our measurement capability. The intrinsic speed of the photocurrent eneration is likely much hiher. off on NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnoloy 3 Macmillan Publishers Limited. All rihts reserved.
3. Electrical characteristics of raphene-mos -raphene device. 4 3 V V 3 V BG = +8 V I ds I ds -8-4 4 8 V s (V) - - - V ds (V) V BG = -8 V Fiure S3I Electrical characteristics of raphene-mos -raphene device. a, Transfer characteristics of raphene-mos -raphene device with silicon substrate as the back-ate at V ds = V and V. b, Output characteristics of the same device. The back-ate voltae is varied from 8 V (top) to 8 V (bottom) in the step of V. 4. Simulation of the band diaram of the vertical heterostructure device..6 V.6 V BG, V TG.6-6 V V, -6 V.4 V.4 V, V.4 6 V V, 6 V 6 V, -6 V Gr B MoS Gr T Gr B MoS Gr T Gr B MoS Ti - 3 4 5-3 4 5-3 4 5 Fiure S4I Simulation of the band diarams. a, Simulated band diaram of sinleated raphene-mos -raphene (GMG) stack. b, Simulated band diaram of dualated GMG stack. c, Simulated band diaram of sinle-ated raphene-mos -Ti (GMM) stack. To calculate the band slope and chare distribution, the confiurations of GMG and GMM are considered in our simulation. Since MoS is a semiconductor, we adapted the depletion approximation, which means MoS is uniformly chared, and the chare density equals to its dopin level. Therefore, bands of MoS are parabolic instead of linear. This model is reasonable when the device channel is short, as in our cases. For the simulation of GMG, we consider the electric field induced by ate as E, E = V / D () where V is the ate voltae and D is the thickness of SiO dielectric. Then we consider electric fields in the two rapheme-mos interfaces as E and E. The carrier density n and n for bottom raphene and top raphene, respectively, can be iven by: ε E + ε E = ne () l s E s ε = ne (3) where ε and ε are the dielectric constant for SiO and MoS, respectively. E and E satisfy: V -6 V V 6 V NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnoloy 3 Macmillan Publishers Limited. All rihts reserved.
Ned E = E + (4) ε s where N is the dopin level in MoS. In our simulation, N is chosen to be 7 cm -3, which can be rouhly estimated from transport characteristics of MoS used in our study. If we have E and E, we can et the potential drop ΔV in MoS as: Δ V = ( E+ E ) d (5) For raphene, we have the relation between carrier density n and chemical potential μ (Dirac point as zero) as: h μ = v F π n n (6) π where h is the plank constant and v F is the Fermi velocity, n is the fixed chare raphene. As we know, intrinsic raphene in air is p-doped, and the fix chare can come from SiO substrate or absorbed H O and O molecules. When a bias voltae V b is applied, we et: ev = eδ V + μ μ (7) b For GMM, it will be different because the band of MoS are pinned in the metal side. Equation (7) should be replaced by: ev = eδv μ + W W (8) b m where W is the potential drop from the vacuum level to Dirac point of raphene, W m is work function of metal. Another difference is that n should be the chare density in the interface of MoS and metal. Equations above are solved self-consistently to obtain the band diarams. 5. Simulation of ate dependant depletion width of raphene-mos schottky contact. Deplesion width (nm) 84 8 8 78 76 74 7 7 68-6 -4-4 6 Gate voltae (V) Fiure S5I Simulated depletion width in raphene-mos schottky contact at the various ate voltaes. In the raphene-mos -raphene device, the total depletion width at both contact is doubled to be around 4-7 nm. The electric field induced by ate E can be written as the followin: E = V / D () where V is the ate voltae and D is the thickness of SiO dielectric. NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnoloy 3 3 Macmillan Publishers Limited. All rihts reserved.
Considerin the electric field in the raphene-mos interface as E, the carrier density n for the raphene can be iven by: εε E + εε E = ne () where ε and ε are the dielectric constant for SiO and MoS, respectively. E satisfies: Ned E = (3) εε where N is the dopin level in MoS, and d the depletion lenth. In our simulation, N is chosen to be 7 cm -3, which can be rouhly estimated from transport characteristics of MoS used in our study. We can et the potential drop ΔV in MoS as: Δ V = Ed (4) For raphene, we have the relation between carrier density n and chemical potential μ (Dirac point as zero) as: h μ = v F π n n (5) π where h is the plank constant and v F is the Fermi velocity, n is the fixed chare raphene. As we know, intrinsic raphene in air is p-doped, and the fix chare can come from SiO substrate or absorbed H O and O molecules. ev = eδv μ + W W (6) b MoS where W is the potential drop from the vacuum level to Dirac point of raphene, W m is the work function (4.55 ev) 6 of MoS. d is calculated by solvin above equations. p- doped raphene (5. ev) was used in this calculation. 6. Optical absorption spectroscopy of multi-layer MoS flake. Absorbance (%) 36 3 8 4 54 nm 6 8 4 Absorbance (%) Power (μw) 48 54 6 66 7 Wavelenth (nm) Fiure S6I Optical absorption spectroscopy of multi-layer MoS flake. a, Power dependent optical absorption of a multi-layer MoS flake. The absorption was determined by comparin the focused laser (54 nm) transmission throuh the same lass substrate on and off an MoS flake. The thickness of the MoS flake is about 56 nm. b, Absorption spectra of a multi-layer MoS flake on lass substrates. The thickness of MoS flake is about 6 nm. The absorption spectra of MoS was determined from the reflectance measurements 7,8. To obtain the absorption spectrum, the reflectance spectrum from the MoS flake on a lass substrate (R m+s ) and that from 4 NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnoloy 3 Macmillan Publishers Limited. All rihts reserved.
the same bare lass substrate (R s ) were measured usin an optical microscope coupled with a spectrometer and CCD camera. The fractional chane in the reflectance, δ R, can be determined as the difference of these two quantities divided by the Rm+ s Rs δ R = reflectance from the bare lass substrate ( Rs ). The absorbance of MoS (A) 4 δ R = A can then be determined by usin the relation: nsub, where n sub is the refractive index of the lass substrate. 7. Schematic illustration of the fabrication procedure of raphene-mos -metal (Ti) device. Fiure S7I Schematic illustration of the fabrication procedure for raphene-mos - metal (Ti) device. a, CVD rown monolayer raphene was transferred and patterned on the ITO lass with 3-nm Al O 3 dielectric layer -4. b, MoS layer was exfoliated onto the raphene throuh a micromechanical cleavae approach 5. c, As a final step, Ti/Au (5 nm/ 5 nm) was patterned as a top electrode and contact electrode for raphene usin e-beam lithoraphy. References. Liu. L. et al. A systematic study of atmospheric pressure chemical vapor deposition rowth of lare-area monolayer raphene, J. Mater. Chem.,, 498-53 ().. Li, X. et al. Lare-area synthesis of hih quality and uniform raphene films on copper foils. Science 34, 3-34 (9). 3. Reina, A. et al. Lare area, few-layer raphene films on arbitrary substrates by chemical vapour deposition. Nano Lett. 9, 3-35 (9). 4. Zhou, H. et al., Chemical vapour deposition rowth of lare sinle crystals of monolayer and bilayer raphene, Nat. Commun. 4:96, doi:.38/ncomms396 (3). 5. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V., Kis, A. Sinle-layer MoS transistors. Nature Nanotechnol. 6, 47-5 (). 6. Williams, R. H. & Mcevoy, A. J. Photoemission studies of MoS. Phys. Stat. Sol. 47, 7-4 (97). 7. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz & T. F. Atomically thin MoS : A new directap semiconductor. Phys. Rev. Lett. 5, 3685 (). 8. Mak, K. F. et al. Measurement of the optical conductivity of raphene. Phys. Rev. Lett., 9645 (8). NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnoloy 5 3 Macmillan Publishers Limited. All rihts reserved.