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Cite This: J. Phys. Chem. Lett. 2019, 10, 4010−4016
Direct Mapping of the Gate Response of a Multilayer WSe2/MoS2 Heterostructure with Locally Different Degrees of Charge Depletion Jeongwoo Park,† Dohyeon Jeon,† Yebin Kang,† Young-Jun Yu,‡ and Taekyeong Kim*,† †
Department of Physics, Hankuk University of Foreign Studies, Yongin 17035, Korea Department of Physics, Chungnam National University, Daejeon 34134, Korea
‡
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S Supporting Information *
ABSTRACT: Understanding the interlayer charge coupling mechanism in a twodimensional van der Waals (vdW) heterojunction is crucial for optimizing the performance of heterostructure-based (opto)electronic devices. Here, we report mapping the gate response of a multilayer WSe2/MoS2 heterostructure with locally different degrees of charge depletion through mobile carrier measurements based on electrostatic force microscopy. We observed ambipolar or unipolar behavior depending on the degree of charge depletion in the heterojunction under tip gating. Interestingly, the WSe2 on MoS2 shows gating behavior that is more efficient than that on the SiO2/Si substrate, which can be explained by the high dielectric environment and screening of impurities on the SiO2 surface by the MoS2. Furthermore, we found that the gateinduced majority carriers in the heterojunction reduce the carrier lifetime, leading to the enhanced interlayer recombination of the photogenerated carriers under illumination. Our work provides a comprehensive understanding of the interfacial phenomena at the vdW heterointerface with charge depletion.
T
system with interlayer coupling.11−15 Various electrical transport measurements are also used to characterize 2D-materialbased HS devices with charge depletion layers.1,11,16−18 While these measurements and techniques are well-suited to investigating the gating behavior of 2D HS devices and the interlayer recombination at the heterointerface, the corresponding optical and electrical properties depending on the charge depletion are averaged over a large sample area rather than spatially analyzed. Furthermore, the degree of charge depletion in the 2D vdW HS is determined by various factors, such as the doping concentration, flake thickness, degree of annealing, interlayer coupling strength, and surface inhomoheneities.1,11,14−16,19,20 Hence, even a single HS device has spatially distributed charge depletion regions of different degrees inside the 2D vdW heterojunction. Therefore, spatially resolved characterizations of 2D vdW HSs with locally different degrees of charge depletion on the nanometer scale are crucial to understanding the charge transport and recombination mechanism in 2D-HS-based (opto)electronic devices. In this study, we present spatially resolved maps of the gate response of a vertically stacked multilayer WSe2/MoS2 p-n HS with locally different degrees of charge depletion region. We use the second harmonic (2ω) signal of electrostatic force microscopy (EFM) to image the nanoscale mobile charge carrier distribution in the WSe2/MoS2 heterojunction under nondestructive conditions. Under external tip gate bias,
wo-dimensional (2D) layered transition-metal dichalcogenides (TMDCs) have attracted great interest because of their distinctive electrical, mechanical, and optical properties and provide promising candidates for future ultrathin electronic and optoelectronic devices. Recently, vertically stacked van der Waals (vdW) heterostructures (HSs) based on TMDC materials can offer basic building blocks for new designs of nanoelectronic devices such as light-emitting diodes (LEDs), field effect transistors (FETs), photodetectors, and solar cells. For example, a tunneling diode based on a WSe2/ SnS2 HS via mechanical stacking exhibits ultrahigh photodetectivity and photoresponsivity.1 The chemical vapor deposition (CVD) grown HS such as MoS2/graphene,2−4 MoS2/WSe2/graphene,5 MoTe2/MoS26 and WS2/MoS2,7 show enhanced electronic performance and multiple optoelectronic functionalities owing to the various stacking orders and energy band alignments.8,9 These exceptional characteristics and unique functionalities of electronic and optoelectronic devices based on the 2D HS are attributed to the lack of dangling bonds on the vdW material surface and the highquality heterointerface without the constraints of atomic lattice match compared to conventional semiconductors. 10,11 Although utilizing HSs of 2D materials allows for enormous enhancements of device performance capabilities, little is known about the underlying physical mechanisms of the gating behavior and recombination process depending on the degree of charge depletion in the vdW HS. Optical measurement methods such as X-ray photoelectron microscopy, Raman spectroscopy, and photoluminescence spectroscopy have been used to elucidate the interfacial phenomena in the 2D vdW HS © 2019 American Chemical Society
Received: April 26, 2019 Accepted: May 29, 2019 Published: May 29, 2019 4010
DOI: 10.1021/acs.jpclett.9b01192 J. Phys. Chem. Lett. 2019, 10, 4010−4016
Letter
The Journal of Physical Chemistry Letters
Figure 1. Optical properties of the WSe2/MoS2 HS. (a) Optical image of WSe2/MoS2 HS on a SiO2/Si substrate. Scheme of stacked structure of WSe2(blue)/MoS2(red) in inset. Scale bar, 20 μm. (b) Raman spectra of WSe2 (blue), MoS2 (red), and overlapped WSe2/MoS2 (green) regions. (c) Photoluminescence spectra of WSe2 (blue), MoS2 (red), and overlapped WSe2/MoS2 (green) regions (a.u., arbitrary units).
Figure 2. Experimental setup and formation of charge depletion layer. (a) Schematic illustration of the mobile charge carrier mapping of the WSe2/ MoS2 HS sample based on the second harmonic (2ω) EFM setup. (b) AFM topographic image and height profiles (inset) of WSe2/MoS2 HS on a SiO2/Si substrate. The blue and red boxes indicate the WSe2 and MoS2 flakes, respectively. The oblique line stands for the depletion layer inside the HS. The filled green and gray circles indicate the overlapped regions of the HS with different degrees of charge depletion. Scale bar, 5 μm. (c) Schematic illustration of the formation of depletion layer in WSe2/MoS2 HS. xp and xn are the depletion layer widths of the WSe2 and MoS2, respectively. Bottom: Energy band diagram of the WSe2/MoS2 HS with energy gap of 1.2 eV for each of the WSe2 and MoS2 multilayer flakes.
532 nm laser. Figure 1b shows the Raman spectra of the sample at different positions of individual WSe2 (blue), MoS2 (red), and WSe2/MoS2 HS (green) regions. In the MoS2 region, there are two dominant Raman peaks at 384 cm−1 (E12g, in-plane mode) and 409 cm−1 (A1g, out-of-plane mode), as reported previously.11,16,17,23 The frequency difference (25 cm−1) between the E12g and A1g indicates that the MoS2 flake can be regarded as having a multilayer structure. In the WSe2 region, there are three dominant Raman peaks at 251 cm−1 (A1g, out-of-plane mode), 258 cm−1 (2LA(M), second-order Raman scattering mode), and 310 cm−1 (B12g, inactive mode), as reported by others.16−18 In the Raman spectra of the HS region (green), all of the characteristic peaks are observed with a significant decrease in the intensity because of the quenching effect.10,13,15−17 It was also observed that the A1g peak of WSe2 in the HS region shows a weak red shift (∼1 cm) relative to that of the isolated WSe2 due to phonon softening, suggesting electron doping from MoS2 to WSe2.11,24,25 To elucidate the interlayer coupling at the WSe2/MoS2 HS further, photoluminescence (PL) measurements were taken with a 532 nm laser. Figure 1c shows the PL spectra of the individual WSe2 (blue), MoS2 (red), and HS (green) regions indicated in Figure 1a. We observe characteristic peaks at 1.35, 1.8, and 1.95 eV for the MoS2. These PL peaks correspond to indirect transitions (I), A excitons (A), and B exciton (B) for multilayer MoS2, respectively. For the WSe2, PL peaks at 1.4 and 1.59 eV are observed. These two PL peaks are related to the indirect transition (I) and A excitons (A) for the multilayer WSe2.15,18,26 We find that the PL emissions from MoS2 and WSe2 were strongly quenched in the HS region, maintaining
ambipolar or unipolar behavior depending on the degree of charge depletion in the overlapped heterojunction is observed in dark conditions. We also confirm that these measured results are in good agreement with the calculations by solving Poisson’s equation. We found that the more efficient gating behavior is observed with WSe2 on MoS2 than with WSe2 on a SiO2/Si substrate because of the high dielectric environment and screening of impurities on the SiO2/Si surface by the bottom MoS2 layer. In the overlapped region of partially depleted WSe2 and MoS2, positive and negative gate-induced majority carriers reduce the carrier lifetime, giving rise to enhanced interlayer bimolecular recombination of the photogenerated carriers under illumination. On the other hand, in the overlapped region of partially depleted MoS2 under fully depleted WSe2, only the electrons as majority carriers enhance the interlayer bimolecular recombination and reduce the photogenerated carriers under positive gate voltage. Figure 1a shows an optical microscopy image of a WSe2/ MoS2 HS on a SiO2/Si substrate. The WSe2/MoS2 HS was artificially fabricated by stacking a multilayer WSe2 flake vertically onto a multilayer MoS2 flake exfoliated in advanced on a SiO2/Si substrate using a three-dimensional alignmenttransfer process.21,22 The layer thicknesses of the WSe2 and MoS2 flakes were verified by atomic force microscopy (AFM), as shown in Figure 2. We thermally annealed the sample in a pure Ar environment at 300 °C for 3 h to eliminate the effects of adsorbates and residuals on the surface, which also gives rise to the strongly coupled interface in the heterojunction. To investigate the optical properties of the WSe 2 /MoS 2 heterointerfaces, we performed Raman measurements using a 4011
DOI: 10.1021/acs.jpclett.9b01192 J. Phys. Chem. Lett. 2019, 10, 4010−4016
Letter
The Journal of Physical Chemistry Letters
Figure 3. Gate response of the WSe2/MoS2 HS in dark conditions. (a) Normalized 2ω signal images of the WSe2/MoS2 HS at different Vtip. The filled color circles indicate WSe2 (blue), MoS2 (red), OR1 (green), and OR2 (gray) positions. Scale bars, 5 μm. (b) Normalized 2ω signal of the nonoverlapped WSe2 (blue) and MoS2 (red) flakes as a function of Vtip. (c) Normalized 2ω signal of the overlapped WSe2/MoS2 region (OR1) as a function of Vtip. (d) Normalized 2ω signal of the overlapped WSe2/MoS2 region (OR2) as a function of Vtip. The colored boxed and oblique lines in the inset indicate the TMDC flakes and depletion layer, respectively. The solid curves are the calculated conductivities.
applied to the tip to modulate the n (p) of the sample depending on the type of majority carrier. Figure 2b shows an AFM topographical image of the WSe2/ MoS2 HS on the SiO2/Si substrate, specifically the area denoted by the rectangle in Figure 1a. The scaled and colored boxes represent the WSe2 (blue), MoS2 (red), and overlapped HS with vertical stacking in z-axis. The overlapped area consists of two distinct regions: overlapped region 1 (OR1) of the thick WSe2 (8.2 nm) on the thin MoS2 (4.6 nm) and overlapped region 2 (OR2) of the thin WSe2 (5 nm) on the thick MoS2 (13 nm), based on the height profiles in the inset. Note that a charge depletion layer (oblique line) can arise on either side of the junction in the overlapped regions by means of free charge carrier transfers at the interface due to the strong coupling between the WSe2 and MoS2 layers. The spatial extent of the charge depletion region in the heterojunction with a staggered band gap (indirect gap of 1.2 eV) depends on the doping concentration of each layer, as shown in Figure 2c.1,11,18,36,37 We calculate the depletion layer widths located on the WSe2 (zp) and MoS2 (zn) sides based on conventional
their original peak positions. We attributed the strong decrease in the PL spectra (PL quenching effect) to the separation of photoinduced charge carriers at the heterojunction, meaning efficient electron transfer at the interface of the HS. All of these observations of the shift of the Raman spectra and quenching in the Raman and PL spectra at the HS region reveal the strong interlayer coupling between WSe2 and MoS2, indicating a typeII staggered band alignment with charge separation and the formation of charge depletion at the vdW heterointerface.10,11,27 On the basis of this understanding of the interlayer coupling mechanism, we subsequently explored the local charge carrier mapping of the WSe2/MoS2 HS through the 2ω response of EFM. The 2ω signal measurements with a double-pass imaging operation were performed by a commercial AFM instrument (XE-7, Park Systems) under ambient conditions using PPPNCSTAu (Nanosensors) with tip curvature Rtip = 20−40 nm (spring constant k = 7.2 N/m). In the first scan, a sample topography profile is obtained by standard noncontact mode imaging. In the second scan, the tip retraces the same topography profile with a fixed lift height (h ≈ 10 nm) above the topographical baseline. An AC voltage (2 V amplitude) of Vac sin(ωt) is applied to the tip only in second scan by a homebuilt control circuit, where the electrical excitation frequency ω is set to 17 kHz, far below the cantilever resonance frequency of 280 kHz to avoid interference. The 2ω component of the 1 dC electrostatic force F2ω = − 4 dz Vac2 is measured by an external lock-in amplifier (SR 830, Stanford Research), where C is the total capacitance of the tip−sample system and z is the tip− sample distance.28,29 The experimental setup for our local charge carrier mapping is schematically illustrated in Figure 2a. The 2ω signal results from the local mobile charges built up by Coulomb force, as shown in the inset of Figure 2a, and is proportional to the sample local conductivity (σ) consisting of the carrier density (n or p) and carrier mobility (μ) oscillated at the ω frequency.30−35 An additional DC gate voltage (Vtip = −4 V to +4 V) with an offset voltage (∼0.15 V to nullify the contact potential difference between the tip and the sample) is
p−n junction heterostructures zp = 2Naε1ε2Vbi qNd(ε1Na + ε2Nd) −3
2Ndε1ε2Vbi qNa(ε1Na + ε2Nd)
and zn =
. We use Na = 1 × 1019 cm−3 and Nd = 5 × 1018
cm correspondingly for the hole and electron doping concentrations, ε1 = 11.7 and ε2 = 12.8 likewise for the dielectric constants of WSe2 and MoS2, and Vbi = 0.3 V for the built-in potential at the junction.11,18,38 We determine charge depletion widths of zp = 6.8 nm and zn = 3.4 nm by using those parameters. This implies that the WSe2 and the MoS2 in OR1 (green circle) are partially depleted, while the WSe2 is fully depleted and the MoS2 is partially depleted in OR2 (gray circle), as presented by the oblique lines and colored boxes in Figure 2b. To characterize the gate response of the WSe2/MoS2 HS with locally different degrees of the charge depletion region, we perform the spatially resolved mapping of the gate response of the sample. Figure 3a shows normalized 2ω signal images of the WSe2, MoS2, and WSe2/MoS2 HS regions at different Vtip gate voltages from −4 to 4 V in dark conditions to eliminate 4012
DOI: 10.1021/acs.jpclett.9b01192 J. Phys. Chem. Lett. 2019, 10, 4010−4016
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The Journal of Physical Chemistry Letters photogenerated carriers. Bright and dark regions correspond to the high and low local conductivity of the area, respectively. Obvious evolution of the brightness is observed depending on the tip gate voltage in the 2ω signal images, which indicates the electrostatic field induced local charge carrier mapping on each individual and overlapped region. To investigate quantitatively the gate response of the WSe2/MoS2 HS region depending on the degree of charge depletion, the gate-dependent 2ω signal changes of each region are plotted as a function of Vtip in Figure 3b−d. Each data point is obtained from the positions marked by the colored circles in the 2ω signal image of Figure 3a. The filled circles are the measured 2ω signal (left axis) and the solid curves are the conductivity (right axis) calculated by solving Poisson’s equation (for a detailed calculation, see Figures S1 and S2 in the Supporting Information). The colored boxes and oblique line in the inset indicate each flake and the degree of the charge depletion layer, respectively. Figure 3b shows the normalized 2ω signal of the nonoverlapped WSe2 (blue) and MoS2 (red) regions as a function of Vtip. We clearly observe that the WSe2 shows p-type behavior and the MoS2 shows n-type behavior with the tip gate voltage. Figure 3c shows the normalized 2ω signal of OR1 (green) with partially depleted WSe2 and MoS2 as a function of Vtip, as indicated by the colored box and oblique line in the inset. We clearly observe ambipolar behavior, which is attributed to the electron carriers in the undepleted MoS2 layer and the hole carriers in the undepleted WSe2 layer of the overlapped region. For positive tip bias, the conduction band (CB) minimum and valence band (VB) maximum shift downward, which results in the accumulation of electron carriers in the undepleted MoS2 layer and the depletion of hole carriers in the undepleted WSe2 layer, leading to n-type behavior. On the other hand, for negative tip bias, the CB minimum and VB maximum shift upward, which results in the depletion of electron carriers in the undepleted MoS2 layer and the accumulation of hole carriers in the undepleted WSe2 layer, leading to p-type behavior.16,17 Figure 3d shows the normalized 2ω signal of OR2 (gray) with fully depleted WSe2 and partially depleted MoS2 as a function of Vtip, as indicated by the colored box and the oblique line in the inset. In this case, only majority carrier electrons in the undepleted MoS2 layer can be attributed to the 2ω signal as mobile carriers because the WSe2 is completely depleted, leading to n-type unipolar behavior with the tip gate voltage. Next, we explore the substrate effect on the gate response of the WSe2 flakes. Figure 4 shows the normalized 2ω signal of WSe2 flakes with different bottom substrates as a function of Vtip; one (light blue) is the SiO2/Si substrate and the other (yellow) is the MoS2 with a depletion layer indicated in the inset. The thickness of the WSe2 flake on the SiO2/Si substrate is 1.4 nm (Figure S3 in the Supporting Information). The 2ω signal denoted by the yellow circle is obtained by subtracting the 2ω signal of overlapped region 3 (OR3) from that of OR1, where OR3 consists of the fully depleted WSe2 on the partially depleted MoS2 with a thickness identical to that of MoS2 in OR1 (Figure S3 in the Supporting Information). The thickness of the undepleted portion of WSe2 in OR1 is ∼1.4 nm, as determined by calculation of the width of the depletion layer. From these findings, we obtain the 2ω signal of the 1.4 nm WSe2 on the MoS2 with a depletion layer. The WSe2 flakes on both the SiO2/Si substrate and the MoS2 with a depletion layer show p-type behavior; however, each gating efficiency of the WSe2 flakes differs in accordance with the bottom substrate.
Figure 4. Substrate effect on the gate response of the WSe2 flake. Normalized 2ω signal of the WSe2 flakes of 1.4 nm thickness on the SiO2/Si substrate (light blue) and on the MoS2 with a depletion layer (yellow) as a function of Vtip. The black solid lines are the fits to the gate response in the negative Vtip region.
The 2ω signal of WSe2 on MoS2 with a depletion layer shows a higher slope on the negative tip voltage than that of WSe2 on the SiO2/Si substrate, as indicated by the black solid lines in Figure 4. This indicates that the mobility is higher for WSe2 on MoS2 with a depletion layer than WSe2 on the SiO2/Si substrate. This is most likely because MoS2 with a depletion layer under WSe2 serves as a high dielectric (ε ≈ 11−12) substrate compared to SiO2 (ε ≈ 3.9). Moreover, it can screen the charged impurities at the WSe2/SiO2 substrate, leading to increased mobility of WSe2 on MoS2 with the depletion layer.21,22,39 It is worth noting that the local conductivity is not spatially uniform over the nonoverlapped or overlapped regions of the WSe2/MoS2 HS, despite the fact that the corresponding topographical image shows a homogeneous surface. The fact that the 2ω signal can locally vanish in some parts of OR1 at negative tip gate bias, as indicated by the white arrows in Figure 3a, implies that the part of OR1 behaves as an insulator because of the absence of mobile charge carriers. This inhomogeneous and local disappearance of the 2ω signal comes from the combined effects of random charges or structural defects of individual flakes and impurities and charge trapping across the WSe2/MoS2 or MoS2/SiO2 interface.20,40,41 Hence, our 2ω mapping of the WSe2/MoS2 HS can provide information about the quantitative local conductivity and mesoscopic electrical inhomogeneity in the sample. To investigate the gating behavior of photogenerated charge carriers and their recombination depending on the degrees of charge depletion, we perform the 2ω signal mapping of the WSe2/MoS2 HS under laser illumination. The sample is illuminated with a wavelength of 532 nm and a power level of 80 mW/cm−2. Figure 4a shows the normalized 2ω signal images as photogenerated charge carrier maps of the sample at different Vtip values from −4 to 4 V. The images are obtained by subtracting the 2ω signal image recorded in dark conditions (Figure 3a) from that recorded under illumination (Figure S4 in the Supporting Information). Bright and dark regions correspond to the high and low photoconductivity of the area. Figure 5b shows the normalized 2ω signal of OR1 (green) and OR2 (gray) of the WSe2/MoS2 HS as a function of Vtip. The 2ω signal is maximized near Vtip = 0 V and decreases with other gate voltages for OR1, which consists of partially depleted WSe2 and MoS2. On the other hand, the 2ω signal increases with a decrease of Vtip for OR2 which is fully depleted WSe2 on partially depleted MoS2. This indicates that the gating 4013
DOI: 10.1021/acs.jpclett.9b01192 J. Phys. Chem. Lett. 2019, 10, 4010−4016
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The Journal of Physical Chemistry Letters
Figure 5. Gate response of the WSe2/MoS2 HS under illumination. (a) Normalized 2ω signal images of the WSe2/MoS2 HS at different Vtip. The colored circles indicate OR1 (green) and OR2 (gray) positions. Scale bars, 5 μm. (b) Normalized 2ω signal of OR1 (green) and OR2 (gray) from the indicated positions in panel a as a function of Vtip. (c) Schematic illustration of energy band diagram of the WSe2/MoS2 HS with photogenerated carrier (i), exciton dissociation with diffusion (ii), and recombination (iii). Left panel, partially depleted WSe2 and MoS2; right panel, fully depleted WSe2 and partially depleted MoS2. In the upper part, the colored boxes indicate the WSe2 (blue) and MoS2 (red) flakes with a depletion layer (oblique line).
V.47 On OR2, however, the photogenerated electrons rarely have an opportunity to recombine with majority hole carriers because of the fully depleted WSe2 layer, as shown on the right side of Figure 5c, leading to an increase of the 2ω signal even in negative gate bias. In conclusion, we have demonstrated that the gate response of the OR in the WSe2/MoS2 vdW HS depends on the degrees of the charge depletion layer by direct mapping of mobile charge carriers. In dark conditions, the OR with partially depleted WSe2 and MoS2 exhibits ambipolar behavior; on the other hand, the OR with fully depleted WSe2 and partially depleted MoS2 exhibits n-type behavior. In particular, the more efficient gating behavior of WSe2 on MoS2 with a depletion layer compared to WSe2 on a SiO2/Si substrate is observed. This confirms that the bottom MoS2 layer protects the WSe2 layer from Coulomb scattering by charged impurities on the SiO2 surface and acts as a high dielectric substrate. Under illumination, the recombination rate is reduced in negative gate bias for the OR with fully depleted WSe2 on partially depleted MoS2 because of the lack of majority hole carriers which can recombine with photogenerated charge carriers. Our work helps to clarify the interlayer coupling and recombination mechanism in vdW interfaces. It will potentially lead to optimized high-performance electronic and optoelectronic devices based on stacked 2D materials with a depletion layer by providing information about spatially resolved inhomogeneities related to the degree of charge depletion in vdW-based 2D heterojunctions.
behavior of the photogenerated charge carrier under illumination depends on the degree of charge depletion in the HS. To understand the distinct gate response depending on the degree of charge depletion of the HS under illumination, we use the energy band diagram of the WSe2/MoS2 HS in Figure 5c, which shows the photogenerated carriers and majority carriers in each region. Owing to the type-II band structure, the photogenerated electrons and holes (i) are diffused and spontaneously separated to the CB edge of MoS2 and the VB edge of WSe2 (ii), respectively, as indicated by the arrows. After charge separation, the majority charge carriers in each WSe2 and MoS2 layer can participate in the recombination (iii) process with the photogenerated carriers via inelastic tunneling.10,12,42 Because of the binding energy of interlayer excitons in the vdW HS at levels lower than the hundreds of millielectronvolt range, we can assume that bimolecular recombination (Langevin) is the dominant mechanism compared to Shockley−Read−Hall (defect-assisted) recombination.10−12,43,44 The bimolecular recombination rate is p n n written as Rbi = βnp = 1 / βp = 1 / βn = τ , where n and p are
the electron and hole charge concentrations; β is the recombination coefficient, and τ is the concentration-dependent carrier lifetime.10,45,46 The generation rate is independent of the gate bias; therefore, the 2ω signals from the photogenerated charge carriers are determined by the recombination rate which depends on Vtip, as follows. Vtip controls the majority charge carrier concentrations by shifting the Fermi level in the band gap. The photogenerated carriers are mainly recombined with other types of majority carriers. On OR1, for example, photogenerated electrons (holes) are able to recombine with the majority hole (electron) carriers of WSe2 (MoS2), as shown in the left side of Figure 5c. In this case, the recombination increases with |Vtip| due to the reduced carrier lifetime stemming from the accumulation of majority carriers, giving rise to the maximum 2ω peak at a gate bias of 0
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01192. Experimental details, theoretical calculations, and additional experimental data (PDF) 4014
DOI: 10.1021/acs.jpclett.9b01192 J. Phys. Chem. Lett. 2019, 10, 4010−4016
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The Journal of Physical Chemistry Letters
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Taekyeong Kim: 0000-0003-0776-6204 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea (Grant No. 2016R1D1A1B03931148) and Hankuk University of Foreign Studies Research Fund.
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