Supporting Information for Tunable Ambipolar Polarization-Sensitive Photodetectors Based on High Anisotropy ReSe 2 Naonosheets

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1 Supporting Information for Tunable Ambipolar Polarization-Sensitive Photodetectors Based on High Anisotropy ReSe 2 Naonosheets Enze Zhang 1 Peng Wang 2, Zhe Li 1, Haifeng Wang 3,4, Chaoyu Song 1, Ce Huang 1, Zhi-Gang Chen 5, Lei Yang 5, Kaitai Zhang 1, Shiheng Lu 1, Weiyi Wang 1, Shanshan Liu 1, Hehai Fang 2, Xiaohao Zhou 2, Hugen Yan 1, Jin Zou 5,6, Xiangang Wan 3,4, Peng Zhou 7 *, Weida Hu 2 * and Faxian Xiu 1 * * s: faxian@fudan.edu.cn, wdhu@mail.sitp.ac.cn, pengzhou@fudan.edu.cn Content 1. Layer-dependent Raman spectroscopy and Lorentzian analysis 2. Details of angular-dependent polarized Raman measurements on ReSe 2 3. Identification of crystalline orientation and anisotropic field-effect mobilities 4. Schematic configuration of CVD setup for ReSe 2 nanosheet growth on hbn/sio 2 /Si substrates 5. Optical images of CVD-grown ReSe 2 nanosheets on SiO 2 substrates 6. Optical images of CVD-grown ReSe 2 nanosheets on hbn substrates 7. Additional top-gate ReSe 2 FET devices and their characterizations 8. Temperature-dependent conductance of four-terminal devices 9. Extraction of the activation energy of ReSe 2 nanosheets 1. Polarization-sensitive photodetection of semicircle-shaped device 11. Gate-tunable polarization-sensitive photodetection of semicircle-shaped device 12. Polarization-sensitive photodetection with different laser wavelength 1

2 13. Polarization-sensitive photodetection for a 7 nm-thick ReSe 2 device 14. Photocurrent generation mechanism in ReSe 2 photodetectors 15. Band structure for monolayer ReSe References 2

3 1. Layer-dependent Raman spectroscopy and Lorentzian analysis During experiments, we have measured the Raman spectra of ReSe 2 nanosheets with the thickness of 1~12 layers. As shown in Figure S1a and b, when the thickness increases, the peak position slightly shifts towards smaller wavenumber (within 2 cm -1 ). Due to the weak interlayer coupling, the shift is not as prominent as in other TMDs such as MoS 2. 1 To minimize the contribution of shear mode and extrinsic effects like defects, a careful Lorenzian lineshape fitting 2,3 was performed to the Raman signals to determine the peak positions and peak intensities. Also, all the measurements were performed under the same condition, i.e., the same laser wavelength (532nm), power density and integration time, to avoid possible influence of the peak position by the thermal effect 4 induced by the laser. a b 3 c Wavenumber (cm -1 ) Raman shift(cm -1 ) Layers Figure S1. Thickness-dependent Raman spectra of ReSe 2 and Lorenz fitting. (a) Raman spectra of different thickness ReSe 2, which show a shift towards the low Peak psition (cm -1 ) Original Fitting 1 Fitting 2 Fitting 3 Fitting 4 Fitting 5 Fitting 6 Fitting 7 Cumulative Number of layers 285 cm cm cm cm cm cm cm cm cm -1 3

4 wavenumber as the thickness increases. (b) Deduced peak positions as a function of layers. (c) Lorenz fitting of the Raman signals, from which the peak position and peak intensity can be determined. 2. Details of angular-dependent polarized Raman measurements on ReSe 2 The observed scattered light intensity for active Raman vibrational modes is denoted by the equation, 5, (1) where R is the Raman tensor of a given mode, e i and e s are the polarization vectors of the incident and scattered light, respectively. With the incident light perpendicular to the sample layer plane, the Raman tensor of ReSe 2 can be written as, 6 The transformation of the Raman tensor is where A and A T are rotation matrices, =. (2) =, (3) α α = α α, α α =. (4) α α α is the angle of the vibration of the incident light polarization direction to the sample edge. After performing the transformation to the Raman tensor, we can obtain the following equations 7 to fit the Raman intensities under three circumstances. I α+ α+2 + α α+ (5) α 2α+ α (6) 2α+ 2α (7) 4

5 a P I b P I P S c P I P S Raman shift (cm -1 ) Raman shift (cm -1 ) Raman shift (cm -1 ) Figure S2. Raman spectra obtained under different measurement configurations. (a) Angular-dependent Raman spectra of ReSe 2 sample (from the main text) with the polarization direction of the incident light varying from º to 36º, where no polarizer was used. (b), (c) Angular-dependent Raman spectra of the same ReSe 2 sample with the polarization of the scattering light parallel and perpendicular to the polarization direction of the incident light, respectively. 5

6 a b c P I Fitting P I P S Fitting P I P S Fitting d e f P I Fitting P I P S Fitting P I P S Fitting g h i P I Fitting P I P S Fitting P I P S Fitting j 9 k 9 l 12 6 P I P I P S 4.2 Fitting Fitting P I P S Fitting Figure S3. Deduced polar plots showing different Raman modes of the sample (a-i are from the sample described in the main text whereas j-l are from a 5 nm-thick ReSe 2 nanosheet sample) (a-i are from the sample described in the main text whereas j-l are from a 5 nm-thick ReSe 2 nanosheet sample) (a)-(c) Polar plots of peak 124 cm -1 extracted from Figure S2. Equations (5)-(7) are used to fit the data as shown in (a)-(c), respectively. (d)-(f) and (g)-(i) Polar plots of peak 219 cm -1 and peak 16 cm -1, respectively. (j)-(l) Polar plots of peak 16 cm -1 from another sample with a thickness of 5 nm which reveals a similar behavior as the sample described in the main text. 6

7 3. Identification of crystalline orientation and anisotropic field-effect mobilities From the angular-dependent Raman spectra measurement, we find that the peak 124cm -1 always displays a maximum intensity when the polarization direction of incident laser is parallel to one of the edges of sample (Figure S2a). Based on the pervious report, 6 we can confirm that this edge is the b-axis of the crystal. Using this method, we conclude that our CVD synthesized samples always crystalize with well-defined edges parallel to b-axis or a-axis (Figure 1b). Also, from the angular-dependent electrical measurements (Figure S3), the maximum value of two-terminal field-effect mobility occurs when the electric field is parallel to b-axis while the minimum mobility is developed when parallel to a-axis. This is consistent with the previous study based on bulk materials. 8,9 We also performed ab-initio calculations to acquire the room temperature carrier mobilities along the a and b-axes of ReSe 2. For 2D materials, the carrier mobility (µ) can be calculated by: 1 μ= 2 ħ 3 where C is the elastic modulus defined as = /, in which E is the total energy of the system, and δ is the applied uniaxial strain, and A is the area of the optimized 2D structure. m * is the effective mass in the transport direction and it is (1) given by = ħ (in which ħ is the Planck s constant and k is the magnitude of the wave-vector in momentum space), T is the temperature, and E 1 is the deformation 7

8 potential constant, which is proportional to the band edge shift induced by the strain. E 1 is defined as E=E 1 ( l/l ), in which E is the energy shift of the band edge position with respect to the lattice dilation l/l along the direction of the a or b-axis. Electron type E 1 (ev) C (J/m 2 ) m * (m ) µ (cm 2 V -1 s -1 ) Along a Along b Table S1. Calculated deformation-potential constant (E 1 ), 2D modulus(c), effective mass (m*), and electron mobility (µ) in a and b-axis directions of the monolayer ReSe 2 at 3 K, indicating an anisotropy electron mobility along the a and b-axes, which is consistent with our experimental results. Figure S4. Polar plots of normalized filed-effect mobility of the back-gate FET device. (a) An optical image of the back-gate field-effect transistor. The angle between each pair of adjacent electrodes is 3º. Scale bar, 7 µm. (b) Polar plot of normalized filed-effect mobility of the device, in which the maximum and minimum values are obtained when the electrical field is parallel to b-axis and a-axis of the 8

9 ReSe 2 channel, respectively. 4. Schematic configuration of CVD setup for ReSe 2 nanosheet growth on hbn/sio 2 /Si substrates Se powder Ar/8 sccm ReO 3 powder/hbn hbn/s /SiO 2 /Si P=76 Torr 25 ºC 74 ºC Figure S5. Schematic structure of growing ReSe 2 nanosheets onto hbn/sio 2 /Si SiO 2 substrates. The hbn was first mechanically exfoliated onto SiO 2 /Si substrates, then ReO 3 power was placed onto hbn/sio 2 /Si substrates which together they were put into the center region of the tune furnace. 5. Optical images of CVD-grown ReSe 2 nanosheets on SiO 2 substrates Figure S6. Optical images of ReSe 2 nanosheets synthesized on SiO 2 substrates. (a)-(h) Images of ReSe 2 crystals grown on SiO 2 substrates with different thickness and shapes, AFM data shows that monolayer ReSe 2 has a thickness of ~.7nm. All the 9

10 scale bars are 5 µm. 6. Optical images of CVD-grown ReSe 2 nanosheets on hbn substrates Figure S7. Optical images of ReSe 2 nanosheets synthesized on hbn substrates. (a)-(d) Images of ReSe 2 crystals grown on hbn substrates with different thicknesses and shapes. The thickness of exfoliated hbn also varies. Scale bars, 5 µm. 1

11 7. Additional top-gate ReSe 2 FET devices and their characterizations a I DS (A) c x1-13 S = 98 mv dec -1 I DS (A) V DS =.5 V V BG (V) V TG (V) V DS 1 V 2 V 3 V b I DS (na) d x V DS (V) V TG -1 V -9 V -8 V -7 V -6 V -5 V -4 V -3 V -2 V -1 V V 1 V 2 V 3 V 2.x x1-1 I TG (V). -2.x1-13 I BG (V). -2.x x x V TG (V) V BG (V) Figure S8. Performance of another top-gate ReSe 2 FET device and leakage current. (a) Transfer curves (I DS -V TG ) of another top-gated ReSe 2 FET device with a channel thickness of 5 nm. The device shows an on/off current ratio of more than 1 6 at V DS =3 V. The subthreshold swing is 98 mv dec -1 and the extracted mobility is 6 cm 2 V -1 s -1. Inset, an I DS -V BG curve. (b) I DS -V DS curves of the device which show a current saturation behavior. (c) Leakage current of the top-gate ReSe 2 FET from the main text during I DS -V TG measurements, indicating an excellent quality of Al 2 O 3. (d) Leakage current of the four-terminal back-gate device during the low-temperature measurements described in the main text. 11

12 8. Temperature-dependent conductance of four-terminal devices a b G (µs) K G (µs) K B BG (V) 2 K V BG (V) 2 K Figure S9. Linear plot of the sheet conductance as a function of V BG. (a)-(b) Linear plot of G-V BG of the devices on SiO 2 and hbn substrates, respectively. Both devices are from the main text. 12

13 9. Activation energy extraction of the ReSe 2 nanosheet. As depicted in Figure S9a, at the high temperature regime (1~3 K), the activation energy can be obtained through fitting the temperature-dependent G to the thermally activated equation, 11,12 =, (8) where E a is the activation energy, k B is the Boltzmann constant and G is a constant. The activation energy E a corresponds to the energy needed to activate the charge carriers at the Fermi energy into the conduction band. As shown in Figure S9b, the extracted E a becomes smaller as the Fermi level moves towards the conduction band at higher V BG. Also, we note that the 2D variable range hopping model 12 provides an excellent description for the electrical transport of ReSe 2 at 3-3 K, σ exp. (9) The extracted T under different V BG is summarized in Figure S9d, which is consistent with the T values in MoS 2 and ReS 2. 13,14 13

14 a G (S) c ln (G) /T (K -1 ) T -1/3 (K -1/3 ) V BG 1 V 15 V 2 V 25 V 3 V 35 V 4 V 45 V 5 V 55 V 6 V 65 V 7 V 75 V V BG 1 V 15 V 2 V 25 V 3 V 35 V 4 V 45 V 5 V 55 V 6 V 65 V 7 V 75 V b E a (mev) d T (K) V BG (V) V BG (V) Figure S1. Activation energy extraction and the variable hopping model. (a) Arrhenius plots of the sheet conductance versus 1/T in the high-temperature range (1~25 K). (b) The calculated activation energy E a as a function of back-gate voltage V BG. (c) Logarithmic plots of sheet conductance as a function of T -1/3 for different V BG. (d) Extracted hopping parameter T under different V BG. 14

15 1. Polarization-sensitive photodetection of semicircle-shaped device Figure S11. Polarization-sensitive photodetection of another device with a semicircular contact. (a) An optical image of the device. The electrical contact is fabricated to be a semicircular shape 15 to avoid the geometric edge effect between the electrodes and ReSe 2 ; the sample thickness is 3 nm. Scale bar, 5 µm. (b) Photocurrent mapping of the device under different polarization directions of the incident light, which shows consistent results with the device described in the main text. The º and 9º correspond to the polarization direction of incident light being parallel and perpendicular to b-axis of the crystal, respectively. 15

16 11. Gate-tunable polarization-sensitive photodetection of a semicircle-shaped device Figure S12. Gate-tuned polarization-sensitive photocurrent mapping of the device with semicircular contact. (a) Photocurrent mapping of the device under different V BG at the polarization direction of the incident laser parallel and perpendicular to b-axis, which shows an ambipolar behavior. 16

17 12. Polarization-sensitive photodetection with different laser wavelengths Figure S13. Polarization-sensitive photocurrent mapping of the device under the incident laser wavelength of 52 nm. (a) An optical image of the device; the channel thickness is 3 nm. Scale bar, 6 µm. (b) Photocurrent mapping of the device at different incident light (λ=52 nm) polarization directions, which shows consistent behavior with the incident light at λ=633 nm. 17

18 13. Polarization-sensitive photodetection for a 7 nm-thick ReSe 2 device Figure S14. Polarization-sensitive photocurrent mapping of the device with a 7 nm-thick ReSe 2. (a) An optical image of the device, the channel thickness is 7 nm. Scale bar, 5 µm. (b) Photocurrent mapping of the device at different incident light polarization directions; º and 9º correspond to the polarization direction of incident light being parallel and perpendicular to b-axis of the crystal, respectively. 18

19 14. Photocurrent generation mechanism in ReSe 2 photodetectors. There are three main types of photocurrent generation mechanisms in layered materials based photodetectors: photoconductive effect, photovoltaic effect and photo-thermoelectric effect. 16 A typical feature of the photo thermoelectric current is the localized photoresponse near the electrodes and a zero current crossover around the channel. 15 As shown in Figure S15b, when V DS <.5 V, there is a zero current crossover around the channel, indicating that the photo thermoelectric effect is dominating the photocurrent generating process. 15 When V DS.5 V, the current directions from the entire channel and near the electrodes are the same, suggesting that the device works beyond the photo-thermoelectric regime. Typically for the photoconductive effect, a main signature is the linear increase of photocurrent with the incident layer power, 17. While for the photovoltaic effect, the increase of photocurrent with the P IN follows the relation 18,19, <1. By fitting the power law to the I PH and P IN curve, we can extract the η values under different V BG. As shown in Figure S16d, utilizing V BG, we can tune the dominant photocurrent generation effect from the photoconductive effect (positive V BG, η~1) to the photovoltaic effect (negative V BG, η<1 ) due to the tuning of the Fermi level by the V BG (Inset of Figure S16d).Note that both the photoresponsivity and the external quantum efficiency (EQE)(Inset of Figure S16a) of our device is lower than the previous studies 2,21 being measured in O 2 and NH 3 atmosphere, where ReSe 2 channel can absorbed a large amount of O 2 and NH 3 molecules, leading to the increase of the carrier density and higher photoresponsivity and EQE of the 19

20 photodetector 16,22. Figure S15. Photocurrent mapping of the device under different V DS. (a) A schematic plot of the photocurrent measurement system. (b) Photocurrent mapping of the device at different V DS. The definition of positive current is that the current flows from the source to drain electrode. The laser has a wavelength of 633 nm and P IN = 5 µw. 2

21 Figure S16. Photocurrent generation mechanism and the characterizations of another device. (a) Logarithmic plot of I DS -V BG of the device from the main text under different P IN. EQE (defined as = φ=, where q is the electron charge, φ is the number of incident photon and v is frequency of incident laser.) of the device from the main text under various V BG, showing ambipolar gate tunability (b), (c) I PH -P IN characteristics of the device under different V BG, from which α can be determined by fitting the curves to the power law. (d) Extracted η as a function of V BG, η~1 at negative V BG while η<1 at positive V BG. Inset, a schematic band diagram of the device at bias voltage V DS =1V under V BG =3 V and V BG =-3 V, respectively. The barrier between the channel and electrodes becomes larger as the V BG tunes the Fermi level of the ReSe 2 channel from the hole regime to the election regime, which changes the dominant photocurrent generation effect from the photoconductive (positive V BG, η~1) to the photovoltaic effect (negative V BG, η<1 ). (e) 21

22 I DS -V BG performance of another photodetector (channel thickness=15 nm) under different laser power illumination-p IN changes from to 6µW, power density changes from to.76µw/µm 2. (f) Time-resolved photoresponse of the device with and without laser illumination at different V DS, P IN = 1 µw, power density.12µw/µm Band structure for monolayer ReSe 2. Similar to ReSe 2 bulk material, the conduction band minimum at the Gamma point and the valence band maximum located close to Gamma point forms the indirect band gap of monolayer ReSe 2. Monolayer Energy (ev) ev Γ M1 K1 Γ M2 K2 Γ M3 K3 Γ Figure S17. Calculated band structure for monolayer ReSe 2. Calculated band structure for monolayer ReSe 2, the indirect band gap value is 1.24 ev. 22

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