Modulating Optoelectronic Properties of Two-Dimensional Transition

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Modulating Optoelectronic Properties of Two-Dimensional Transition Metal Dichalcogenide Semiconductors by Photoinduced Charge Transfer Jungwook Choi, Hanyu Zhang, and Jong Hyun Choi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07457 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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Modulating Optoelectronic Properties of Two-Dimensional Transition Metal Dichalcogenide Semiconductors by Photoinduced Charge Transfer

Jungwook Choi, Hanyu Zhang, and Jong Hyun Choi*

School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States *

Email: [email protected]

Keywords: two-dimensional semiconductor, transition metal dichalcogenide, photoluminescence, photoinduced charge transfer, photoconductive AFM, optoelectronics

ABSTRACT Atomically thin transition metal dichalcogenides (TMDCs) have attracted great interests as a new class of two-dimensional (2D) direct bandgap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we demonstrate a modulation of the optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules: nickel Pc (NiPc) and magnesium Pc (MgPc). We show that the photoluminescence (PL) emission can be selectively and reversibly engineered through energetically favorable electron transfer from photo-excited TMDCs to MPcs. NiPc molecules, whose reduction potential is positioned below the conduction band minima (CBM) of monolayer MoSe2 and WSe2, but is higher than that of 1

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MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2, as its reduction potential is situated below the CBM of WSe2, but above those of MoS2 and MoSe2. The quenched PL emission can be fully recovered when MPc molecules are removed from the TMDC surfaces which may be re-functionalized and recycled multiple times. We also find that photocurrents from TMDCs, probed by photoconductive atomic force microscopy (PC-AFM), increase over two-fold, only when the PL is quenched by MPcs, further supporting the photoinduced charge transfer mechanism. Our results should benefit design strategies for 2D inorganic-organic optoelectronic devices and systems with tunable properties and improved performances.

Recent progress in two-dimensional (2D) materials has shown that they can be a versatile platform to study new physics and develop novel optical/electrical nanodevices.1 Among diverse emerging 2D materials, semiconducting transition metal dichalcogenides (TMDCs), such as MoS2, MoSe2, WS2, and WSe2, have attracted significant interests due to their sizable bandgap (~1–2 eV), in contrast to graphene which lacks a bandgap.2 These TMDCs have layer-number dependent properties, for example, transitioning from indirect to direct bandgap as their thickness decreases from bulk to monolayer.3 The monolayer-thin, direct bandgap semiconductors exhibit unique optical properties, including strong photoluminescence (PL),4 high absorption in the visible range,5 valley polarization,6,7 and strongly bound excitons and charged excitons.8–10 The ability to control these properties is critical for novel optoelectronic applications.11 The optical and optoelectronic properties of monolayer TMDC semiconductors are dominated by excitonic transitions which originate from their direct gaps at the K point of the Brillouin zone.3,12 Due to strong Coulomb interaction in the TMDC monolayers, large exciton 2


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binding energies of ~0.6 eV for MoS28 and ~0.37 eV for WSe210 have been reported. Such excitonic transitions can be modulated by regulating the carrier concentrations, ultimately controlling the optical properties of TMDCs. For example, applying electric fields on MoS2, MoSe2, and WSe2 using back-gated field-effect transistors (FETs) allows one to control either electron or hole carrier density. The external fields can electrostatically adjust positively or negatively charged excitons, providing PL tunability.8,9,13,14 While electrostatic doping provides effective means to modulate emission characteristics of TMDCs, it necessarily requires a FET architecture fabricated by complicated processes. Several alternative methods based on chemical functionalization have been proposed, including covalent chemistry,15,16 defect engineering,17,18 quantum dots deposition,19,20 and physical and chemical adsorption of molecules21–24. Among these strategies, chemical doping, which induces surface charge transfer between adsorbates and TMDCs (with no external field such as photo-irradiation), has been intensively investigated to modify electrical transport behaviors by controlling carrier type and density. 25–29 This approach shows promise given the simplicity and scalability of the process and wide range of selection of chemical groups. The adsorption of chemical species can modulate the optical properties of TMDCs. For examples, p-type dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) withdraw excess electrons from n-type MoS2 and WS2 such that their PL intensity increases by reducing negatively-charged excitons (i.e., trions) and thus enhancing excitonic recombination.21,24 When electron-donating molecules (e.g., cesium carbonate) are interfaced with monolayer MoS2, however, formation of charged excitons is promoted, resulting in the PL suppression.23 Previous studies indicate that the competition between excitons and charged excitons dominates the overall PL and photo-properties of TMDCs. Thus, most chemicaldoping methods rely on regulating abundant electrons that are naturally present in the TMDCs. The abundant electrons may also be depleted by oxygen molecules in ambient air,22 3


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which is unavoidable unless the TMDCs are protected by surface passivation or vacuum packaging. It could also result in unreliable PL responses and performances of TMDC-based optoelectronic devices. Therefore, it is important to find a viable route that modulates the TMDC properties beyond the control of excess electrons, and to understand the underlying mechanism of optoelectronic tunability. Here we present a controllable, reversible modulation of optoelectronic properties of the TMDCs via photoinduced excited-electron transfer from TMDCs to metal-centered phthalocyanine (MPc) molecules, while preserving their intrinsic properties. Photoinduced charge transfer in organic/inorganic hybrid systems has been exploited to tailor the properties of inorganic nanomaterials by selecting the organic molecules whose redox potentials are properly aligned with band energies of the nanomaterials.30 One promising candidate molecule for such purpose is MPc due to its high stability and rich redox properties such as tunable reduction potential depending on the metal cation species bound at the core.31 Phthalocyanines have been widely used in light-harvesting systems, where MPcs serve as either an electron donor or an acceptor for carbon nanotubes and graphene.32–34

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Figure 1. 2D semiconducting TMDCs and MPcs with their band energies and relative potentials. (a) Molecular model of TMDCs: MX2, where M is Mo or W and X is S or Se. Metalcentered phthalocyanine or MPc consists of planar, aromatic systems with a metal cation (Ni2+ or Mg2+) at the core, forming NiPc or MgPc. In case of NiPc, a ligand (R = NaO3S) is conjugated to improve water solubility. Under light illumination, the ground state electrons in TMDCs are excited to the conduction band and can be subsequently transferred to the functionalized MPcs. Such photoinduced excited-electron transfer modulates the optical and optoelectronic properties of TMDCs. (b) Band alignment of MoS2, MoSe2, and WSe2 along with reduction potentials of NiPc and MgPc. The first reduction potentials of NiPc ([NiPc]/[NiPc]–) and MgPc ([MgPc]/[MgPc]–) are located between the CBMs of MoS2 and MoSe2 and between the CBMs of MoSe2 and WSe2, respectively. Accordingly, photoexcitedelectron transfer from WSe2 to NiPc and MgPc is energetically favorable, whereas the electron transfer from MoSe2 to NiPc is allowed, but not to MgPc. Charge transfer from MoS2 to both phthalocyanines is prohibited. Once the photoexcited-electrons are transferred, the radiative 5


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recombination of electron-hole pairs decreases significantly, thus leading to PL quenching and photocurrent enhancement. The relative energy offset between TMDCs and MPcs enables a selective and controllable modulation of optical and optoelectronic properties of TMDCs.

In this work, we engineered the optical and optoelectronic properties of MoS2, MoSe2, and WSe2 by using MPcs as photoexcited electron acceptors, thereby modulating PL and photoresponse of TMDCs (Figure 1). The TMDCs, hexagonal planes of three atoms in a trigonal prismatic structure, are functionalized with either nickel- or magnesium-centered phthalocyanines, termed NiPc or MgPc, respectively. Once the pristine TMDC monolayers absorb photons and become excited, they exhibit strong PL through radiative recombination of electron-hole pairs. When MPcs are functionalized on the surface of TMDCs, photoexcited electrons are transferred to the MPcs, which decreases radiative recombination and results in drastic PL quenching (Figure 1). The reduction potential of MPcs, a measure of tendency to accept an electron, is -4.07 eV ([NiPc]/[NiPc]–) for NiPc35 and -3.79 eV ([MgPc]/[MgPc]–) for MgPc.36 Considering the position of reduction potential of NiPc relative to the band edges of MoS2, MoSe2, and WSe2,37 the photoexcited-electron transfer from the conduction band minimum (CBM) of MoSe2 and WSe2 to NiPc is energetically favorable, as described in Figure 1b. However, the electron transfer is not expected to occur between MoS2 and NiPc, because the reduction potential of NiPc is situated higher than the CBM of MoS2. In case of MgPc whose reduction potential is positioned lower than CBM of WSe2, but higher than those of MoS2 and MoSe2, electron transfer from WSe2 is favorable, but not allowed for MoSe2 and MoS2 (Figure 1b). The dependence of electron transfer on MPc species would result in a selective, controllable PL modulation of the TMDCs. Although similar behaviors on photoexcited-electron transfer from inorganic nanomaterials to organic molecules were reported previously,38,39 this work reports the first observation of such mechanism with TMDCs, to the best of our knowledge. 6


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Along with PL studies, we have investigated the properties of MPc-TMDC hybrids using x-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, atomic force microscopy (AFM), and photoconductive AFM (PC-AFM). In particular, PC-AFM reveals a significant enhancement of photoconductivity when TMDCs are functionalized with MPcs, and only the phthalocyanines, which quench the PL of TMDCs. This observation not only provides insights into the relationship between PL and photocurrent, but also suggests potential applications of TMDCs as high-performance 2D optoelectronic systems.

RESULTS AND DISCUSSION Intrinsic Properties of TMDCs and their Interaction with MPcs. TMDC flakes were mechanically exfoliated from their bulk crystals onto a SiO2/Si substrate, followed by annealing in an inert gas environment at 250 °C for removal of residues and contaminants. We find that the thermal annealing is a crucial step to prepare the sample for photoinduced charge transfer experiments as their PL emission properties strongly depend on the annealing process. For example, the PL emission of monolayer MoS2 enhances significantly with increased exciton peak intensity after annealing and subsequent air exposure (Supporting Information, Figure S1). Once the annealed TMDC samples are exposed to ambient air, oxygen molecules can adsorb on the fresh surface of TMDCs and deplete abundant electrons in TMDCs given their strong electronegativity.22 As a result, the recombination rate of excitons and charged excitons change drastically; we observe a prominent enhancement of PL emission with n-type semiconductors MoS2 and MoSe2 layers due to reduced negative trions, and thereby, increased excitonic recombination. In contrast, WSe2, a p-type semiconductor, exhibit decreased emission characteristics after annealing. Through the annealing process, the effects of excess electrons may be minimized, and it allows us to observe photoinduced charge transfer, ultimately modulating the optoelectronic properties of TMDCs with functional chemical 7


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groups. In our experiments, both NiPc and MgPc were dissolved in DI water (pH = 6.7) at desired concentrations ranging from 0.01 to 1 mM. The TMDC layers were incubated in the MPc solutions for 20 min, followed by rinsing with DI water and blow-drying with air. We note that other solvents, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), which are widely used for dissolving MPcs and other organic molecules, significantly change the PL of TMDCs, even without MPcs. It could be attributed to charge transfer interactions between the TMDCs and the chemical functional group (e.g., -CH3) of the solvents. In contrast, DI water displays no apparent effects on TMDC emission properties (Supporting Information, Figure S2 and S3). Since the excess electrons in TMDC layers are already depleted by molecular oxygens after annealing and air exposure, there is no significant effect of water molecules, which is consistent with the previous reports on MoS2.20,22 Thus, selection of solvents should be taken into account for chemical doping of TMDCs in order to avoid any undesirable influence on the TMDC properties. Throughout this work, we used DI water for dissolving the MPcs and rinsing the TMDCs.

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Figure 2. Functionalization of NiPc on the TMDCs. (a) AFM height image of pristine WSe2 exfoliated onto a SiO2/Si substrate. Measured thickness of ~0.8 nm indicates the monolayer. (b) AFM height image of the WSe2 monolayer after NiPc functionalization. NiPc molecules adsorb on the surface of WSe2, and thickness of WSe2-NiPc measures to be approximately 0.9 nm. Insets in (a) and (b) show AFM images of the entire flake before and after functionalization. Scale bars in the both insets are 1 μm. (c) Raster-scanned Raman intensity map of NiPc at 1560 cm-1, which corresponds to the shape of entire WSe2 flake. It clearly indicates that the NiPc molecules are uniformly and selectively functionalized on the WSe2. (d–g) XPS spectra of WSe2 and MoS2 with and without NiPc. W 4f (d), Se 3d (e), Mo 3d (f), and S 2p (g) core level XPS spectra show little deviation (around 0.06–0.07 eV) after NiPc functionalization, demonstrating negligible chemical doping effects.

Figures 2a and b show AFM images of WSe2 before and after NiPc functionalization. The thickness of as-exfoliated WSe2 was measured to be approximately 0.8 nm indicating a monolayer, while that of NiPc-functionalized WSe2 was ~0.9 nm. The roughness of the WSe2 9


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layer also increased from ~0.2 to ~0.4 nm after functionalization. It is clearly seen that the NiPc was deposited on the surface of the WSe2 layer in Figure 2b. In order to verify the uniformity of NiPc functionalization, raster-scanned micro-Raman spectra of NiPc were collected with 633 nm excitation (Figure 2c). NiPc molecules have several distinct Raman signatures ranging from 450 to 1650 cm-1, which do not overlap with vibration modes of TMDCs (Supporting Information, Figure S4). The reconstructed 2D image from the recorded NiPc Raman intensity at 1560 cm-1 corresponds to the entire shape of WSe2 (insets in Figure 2a and b), indicating that the NiPc molecules were selectively and uniformly adsorbed on the surface of the WSe2 layer, but not on the SiO2. The Raman intensity of NiPc at the edge of the WSe2 layer is slightly lower than the inner area due to the smaller overlap between the laser spot (diameter of ~1 μm) and the NiPc layer on the WSe2. We measured the chemical structure of WSe2 and MoS2 before and after the NiPc functionalization using XPS (Figure 2d–g). W 4f core level XPS spectrum of the pristine WSe2 has a doublet of 4f7/2 and 4f5/2 at binding energy of approximately 32.29 and 34.43 eV, respectively. The NiPc-functionalized WSe2 shows a similar W 4f doublet at ~32.23 and ~34.37 eV (Figure 2d). The binding energy of Se 3d5/2 and 3d3/2 are approximately 54.55 and 55.36 eV for the pristine WSe2 and about 54.48 and 55.30 eV for the NiPc-functionalized WSe2, respectively (Figure 2e). The XPS spectra of pristine WSe2 are consistent with previous studies.40 A shift of binding energy may be correlated with a shift of Fermi level; thus, blue or red shifts can originate from n- or p-doping of the TMDCs. In addition, a formation of built-in potential by surface charge transfer at TMDC-metal interface may induce a shift of XPS spectra according to the work function difference between the TMDCs and the metal.41 Given that the binding energy shift in chemically doped-TMDCs is in the range of 0.3–0.8 eV,23,26,28 slight deviation (~0.06 eV) after NiPc functionalization of WSe2 indicates little doping effect, which could otherwise change the physical properties of WSe2 considerably. This could be attributed 10


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to minimal surface charge transfer and negligible potential barrier at the interface between WSe2 and NiPc, resulting in no significant changes in kinetic energy of electrons and corresponding binding energies. Similarly, the core level spectra of Mo 3d and S 2p reveal minimal doping effects with a binding energy shift less than 0.07 eV before and after NiPc functionalization on MoS2 (Figure 2f and g). Additional XPS spectra on pristine MoSe2 and NiPc-functionalized MoSe2 consistently support our observation (Supporting Information, Figure S5). All XPS binding energies of TMDCs with and without NiPc molecules are summarized in the Supporting Information (Table S1).

Figure 3. Optical characterization of TMDCs before and after MPc functionalization. (a,b) PL spectra of MoS2, MoSe2, and WSe2 before and after functionalization of NiPc (a) and MgPc (b). The emission signatures are centered around 660 (1.88), 790 (1.57), and 750 nm (1.65 eV) for MoS2, MoSe2, and WSe2, respectively. PL quenching behaviors strongly depend on the relative positions between reduction potentials of MPcs and the CBM of TMDCs. The NiPc functionalization quenches PL of both MoSe2 and WSe2 (a), whereas the MgPc quenches only WSe2 PL (b), indicating the photoinduced charge transfer mechanism. (c,d) Raman spectra of MoS2, MoSe2, and WSe2 before and after functionalization of NiPc (c) and MgPc (d). The inplane (E12g) and out-of-plane (A1g) modes are observed with MoS2 (385 and 405 cm-1) and MoSe2 (291 and 240 cm-1), while WSe2 displays second-order longitudinal acoustic mode (2LA(M)) at 259 cm-1. No observable shifts of Raman signatures after MPc functionalization indicate little chemical doping effects. (e) Optical microscope image of an exfoliated WSe2 11


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flake with various layer numbers from monolayer to bulk (> 22L). (f,g) Spatially-resolved PL intensity maps of the WSe2 flake shown in (e) with (f) and without (g) NiPc. Strong PL signature at 749 nm is observed in the monolayer region of pristine WSe2 (f), while it is significantly quenched after NiPc functionalization (g). Note that the intensity scale bar in (g) is one-tenth of that in (f). (h,i) Spatially-resolved Raman intensity profiles at 259 cm-1 with (h) and without (i) NiPc. The Raman signatures of WSe2 are not changed even after NiPc functionalization.

Optical Modulation by Photoinduced Charge Transfer. Figure 3 presents PL and Raman spectra of monolayer MoS2, MoSe2, and WSe2 with and without NiPc and MgPc molecules. All spectra before functionalization were recorded after immersing the TMDCs in DI water with no MPcs and drying by air as a control, which were identical conditions except for the presence of MPcs. The pristine monolayers of MoS2, MoSe2, and WSe2 excited at 633 nm in the ambient environment exhibit strong PL signatures at ~660, 790, and 750 nm, respectively (Figure 3a and b).10,21,22,42 This direct bandgap emission is attributed to the recombination of excitons. For MoSe2 and WSe2, the exciton and charged exciton emissions can be distinctively observed only at low temperature; for example, MoSe2 has two emission peaks at ~747 and ~762 nm at 20 K, which correspond to excitons and negatively charged excitons, and the charged exciton peak disappears above 55 K.9 Similar emission characteristics are observed with WSe2.13 In contrast, MoS2 shows two distinct emission features even at room temperature at approximately 650 and 670 nm due to excitons and negatively charged excitons, respectively.8,21,43 Pristine MoS2 exhibits strong recombination of charged excitons, which is comparable to that of excitons due to the presence of abundant electrons. When MoS2 is exposed to electron-withdrawing molecules such as oxygen, the formation of negatively charged excitons is suppressed due to the depletion of excess electrons, resulting in a drastic increase of excitonic emission and overall PL intensity.21,22 As discussed above, we also observed the significant change of PL intensity of the MoS2, MoSe2, and WSe2 layer after 12


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annealing and subsequent exposure to ambient air (Supporting Information, Figure S1). The effect of air exposure is especially important for electron-rich n-type MoS2 and MoSe2 semiconductors, and thus, we completed all optical measurements within 1 hour after annealing. Once the PL is stabilized after annealing and air exposure, the PL emission is examined with NiPc and MgPc whose relative positions of the reduction potential against TMDCs are shown in Figure 1b. Figure 3a shows that PL signals of both WSe2 and MoSe2 are quenched after 1 mM NiPc functionalization, whereas PL of MoS2 remains unchanged. This observation is consistent with the photoinduced electron transfer mechanism from the TMDCs to NiPc, given that the reduction potential of NiPc is located below the CBM of MoSe2 and WSe2, but above that of MoS2. Photoexcited electron transfer from MoS2 to NiPc is energetically unfavorable. Further, the degree of quenching in WSe2 is more prominent than that in the MoSe2 layer, possibly due to a larger energy difference between the CBM of WSe2 and the reduction potential of NiPc than that between MoSe2 and NiPc. When functionalized with 1 mM MgPc, the TMDC layers show distinct and different PL emission behaviors; while a significant PL quenching is observed with WSe2, the emission signatures of both MoS2 and MoSe2 are unaltered (Figure 3b). Because the reduction potential of MgPc is positioned above the CBM of MoS2 and MoSe2, but below that of WSe2, the photoexcited electron transfer is allowed only for WSe2. The proposed photoinduced charge transfer mechanism is further verified with two additional molecules: sodium 2-naphthalenesulfonate and F4TCNQ. Both molecules were dissolved in DI water to avoid any solvent effects and interfaced with monolayer WSe2 and MoS2 after thermal annealing, whose PL signals were monitored under identical conditions. The naphthalene derivative has a reduction potential well above the CBM of monolayer WSe2, and thus, the photoinduced charge transfer from WSe2 to naphthalene is not expected. Indeed, we observed no significant change in the PL signature of the WSe2 layer before and after 13


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functionalization (Supporting Information, Figure S6). In parallel, F4TCNQ has a reduction potential below the CBM of monolayer MoS2, and accordingly, we observe a significant quenching from F4TCNQ-functionalized MoS2 (Supporting Information, Figure S7). This observation seemingly contrasts a previous work that reported an increased exciton recombination of F4TCNQ-doped MoS2 by depleting free electrons in the conduction band of MoS2.21 The discrepancy may be understood considering that our MoS2 samples were undergone annealing and air exposure that deplete excess electrons. Without annealing, we also observed the PL enhancement from as-exfoliated MoS2 layers after F4TCNQ functionalization (Supporting Information, Figure S8). We also investigated Raman spectra of the MoS2, MoSe2, and WSe2 layers before and after MPc functionalization. Figure 3c and d show Raman spectra excited at 633 nm. Several Raman signatures are observed in monolayer MoS2 including first-order in-plane mode (E12g) at ~385 cm-1 and out-of-plane mode (A1g) at ~405 cm-1.42 Such E12g and A1g peaks are also observed in MoSe2 at ~291 and ~240 cm-1, while WSe2 exhibits a distinctive second-order longitudinal acoustic mode at M point (2LA(M)) at ~259 cm-1. In contrast to the PL spectra, which are significantly influenced by the presence of NiPc and MgPc, the Raman spectra remain nearly identical after functionalization. The results suggest negligible doping effects which would otherwise cause the peak shifts in Raman spectra due to an increase or decrease of carrier concentration.23,27,29 Thus, our findings from Raman spectra are consistent with the XPS data. Additionally, full range Raman spectra (200–1800 cm-1) of NiPc and MgPc deposited on MoS2, MoSe2, and WSe2 layers were collected, and the Raman signatures from both MPcs and TMDCs verified the successful functionalization (Supporting Information, Figure S9). Spatially-resolved PL and Raman spectra are examined with pristine WSe2 and NiPcfunctionalized WSe2 layers. Figure 3e shows an optical microscope image of exfoliated WSe2 14


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which has layer number variation from monolayer to bulk (over ~22 layers). The layer number is verified by AFM. Corresponding raster-scanned PL intensity image of WSe2 at 749 nm is shown in Figure 3f, exhibiting strong PL only from the monolayer region where direct interband transitions take place at the K point.12 The PL is drastically quenched after NiPc functionalization (Figure 3g and Figure S10), consistent with what we observe in PL spectra of WSe2-NiPc in Figure 3a. It is noted that the PL intensity range in the Figure 3g is one tenth of that in the Figure 3f. Figure 3h and i show raster-scanned Raman intensity images (at 259 cm1

) of the WSe2 layer before and after NiPc functionalization. In contrast to the PL modulation,

no significant change in Raman signatures is observed across the entire flake after NiPc functionalization, and a thicker layer of WSe2 generates higher Raman intensity. We also measured AFM, PL and Raman images for other MPcs and TMDCs. For example, both MoS2NiPc and MoSe2-MgPc show no changes in PL and Raman images before and after functionalization (Figure S11 and S12), while a significant PL quenching is observed with WSe2-MgPc with no apparent changes in Raman spectra (Figure S13), as anticipated.

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Figure 4. Tunable PL of TMDCs by MPcs functionalization. (a) PL spectra of monolayer WSe2 upon consecutive 10 μM NiPc functionalization steps. Gradual PL quenching along with a blue shift is observed. (b) Normalized PL intensity follows the fractional surface coverage with NiPc molecules. Greater surface coverage leads to increased probability of photoinduced electron transfer, resulting in gradual PL quenching. (c) Gradual blue shift of the PL peak position with stepwise functionalization due to dielectric screening. Filled objects with error bars are experimental data, and solid lines are curve-fits from the Langmuir adsorption model. (d) Reversible modulation of WSe2 PL. The quenched PL is fully recovered after removing NiPc molecules by dissolving the WSe2 layer in DI water for 16 hour. (e) Two consecutive quenching and recovery cycles, demonstrating active control of emission properties without significantly altering intrinsic properties of WSe2.

Controllable and Reversible Optical Modulation. A gradual tunability of PL spectra is studied by increasing the surface coverage of quencher molecules through stepwise 16


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functionalization. Figure 4a–c show the evolution of PL quenching under successive functionalization where the PL intensity and peak position of WSe2 change with number of NiPc functionalization steps. The WSe2 monolayer was incubated in low-concentration NiPc solution (10 μM) for 5 min, followed by rinsing with DI water and drying for each functionalization step. The PL intensity gradually decreases and is nearly saturated after 9 steps with an apparent blue shift in the peak position. As a control, we also measured the WSe2 PL after identical stepwise treatment with DI water (no NiPc); the PL spectra remain unchanged, indicating negligible effects of DI water after successive immersing and drying (Supporting Information, Figure S14). The changes in the PL intensity and peak position as a function of functionalization steps are well described by the Langmuir adsorption model (Figure 4b and c):44 𝜃𝜃 =

𝐾𝐾𝐾𝐾[MPc] (1 + 𝐾𝐾𝐾𝐾[MPc])

where θ is the fractional surface coverage, K is the association constant, and n is the number of functionalization steps. With [NiPc] = 10 μM, the estimated association constant is approximately 2.6 × 104 M-1. In parallel, the PL quenching behavior can be also expressed as static quenching:45 𝐼𝐼0 = 1 + 𝐾𝐾𝐾𝐾[MPc] 𝐼𝐼

where I and I0 are the PL intensities with and without quencher molecules. From the slope of the linear relation between I0/I and functionalization step (Supporting Information, Figure S15), the association constant is estimated to be K = ~2.3 × 104 M-1 which is comparable to the value obtained from the Langmuir adsorption model. These results imply that the WSe2 PL emission is dominated by the extent of surface coverage by NiPc molecules; the increase of NiPc coverage enhances the probability of photo-excited electron transfer, thereby decreasing the emission. 17


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The gradual blue shift of PL energy up to 15 meV in Figure 4c can be attributed to the localized dielectric screening of Coulomb interaction.43 Given the high dielectric constant of NiPc (εNiPc = ~6–8),46 the exciton binding energy decreases with increasing surface coverage. By assuming that the WSe2 layer is fully covered with the NiPc and the PL is dominated by excitonic transition, the electron self-energy bandgap renormalization in vacuum (EBGR) can be estimated from the shift of PL peak position, following the method proposed by Lin et al.:43 Δ𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 = 𝐸𝐸𝑓𝑓 − 𝐸𝐸𝑝𝑝 = �𝐸𝐸𝑆𝑆𝑆𝑆 + = 𝐸𝐸𝐵𝐵𝐵𝐵𝐵𝐵 �

1

𝛽𝛽

�𝜖𝜖𝑓𝑓 �

−

𝐸𝐸𝐵𝐵𝐵𝐵𝐵𝐵

(𝜖𝜖𝑓𝑓 1

)𝛽𝛽

�𝜖𝜖𝑝𝑝 �

𝛽𝛽

−

𝐸𝐸𝑏𝑏


𝛼𝛼

𝐸𝐸

− � − �𝐸𝐸𝑆𝑆𝑆𝑆 + (𝜖𝜖𝐵𝐵𝐵𝐵𝐵𝐵 )𝛽𝛽

� + 𝐸𝐸𝑏𝑏 �

1

𝛼𝛼


−

1


𝛼𝛼

𝑝𝑝

𝐸𝐸𝑏𝑏

𝛼𝛼


�

�.

Here ΔEOpt is the difference between PL peak positions (i.e., optical bandgap) before (Ep) and after functionalization (Ef), ESP is the single-particle bandgap, and Eb is the exciton binding energy in vacuum (~0.9 eV).47 α and β are the empirical scaling factors (~0.7),43 while εp and εf are the effective relative dielectric constants before and after functionalization, respectively. From the fitting to the experimental data, the estimated EBGR of WSe2 is approximately 828– 839 meV, which is comparable but slightly lower than the previously predicted value of monolayer WSe2 (~1230 meV)47 and MoS2 (~720–1220 meV).47–49 This large renormalization energy should be considered in designing the TMDC-based optoelectronic systems that may be operated in diverse dielectric environments. Of the utility in our TMDC engineering is that the quenched PL is fully recoverable. We removed NiPc molecules from the WSe2 surface by dissolving the TMDC layer in DI water for 16 hours. As shown in Figure 4d, the PL is quenched after NiPc functionalization and fully recovered by removing the NiPc from the surface of WSe2. The nearly identical PL spectra of pristine and recovered WSe2 suggest that its intrinsic properties are well retained even after functionalization, but only the PL is quenched by photoinduced charge transfer. After PL recovery, subsequent annealing and second NiPc functionalization of WSe2 induces PL 18


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quenching again, which can then be fully recovered again (Figure 4e). The reversible modulation of optical properties offers an on-demand control of PL as well as potential utility as optical nanosensors.

Figure 5. Photoconductive AFM (PC-AFM) of TMDCs before and after MPc functionalization. (a) Schematic of PC-AFM configuration where the TMDCs are exfoliated onto a conductive, transparent ITO substrate. The current flow between the ITO and the conductive Pt-Ir probe is recorded in dark and under illumination from the bottom (λex = 658 nm), while height profile is simultaneously measured. (b,c) Current–voltage (I–V) curves obtained by measuring currents at various bias voltage values. Photogenerated carriers increase current flow in pristine WSe2 under illumination (b). Functionalized NiPc facilitates separation of photogenerated electronhole pairs, and thus, increases photocurrents significantly (c). Filled objects with error bars are experimental data, while solid lines are curve-fits from the thermionic emission model. (d) AFM height image of a WSe2 bilayer exfoliated on the ITO substrate. (e,f) Spatially resolved 2D photocurrent profiles of pristine WSe2 (e) and NiPc-functionalized WSe2 (f) at bias voltage of -0.5 V. Photoresponsivity improves over 2 times after functionalization due to effective charge separation by MPcs.

Photocurrent Measurements. The PL quenching of TMDCs after MPc 19


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functionalization is predominantly due to excited-electron transfer before the recombination of photogenerated electron-hole pairs. Therefore, it is expected that such photo-processes should promote photoconductivity in TMDCs. We investigated the photoresponse of TMDCs using PC-AFM50 which measures local currents as functions of bias voltage as well as wavelength and intensity of incident light. TMDC layers were exfoliated onto a conductive, transparent ITO-coated glass substrate, and currents flow through the TMDCs and the ITO was measured using a Pt-Ir coated silicon nitride probe as a function of bias voltage between them (Figure 5a). During the measurement, the Pt-Ir probe was held at ground while the bias voltages were applied on the ITO, and light (λex = 658 nm) was illuminated from the bottom of the substrate. Figure 5b and c show the measured current–voltage (I–V) curves of WSe2 before and after NiPc functionalization in dark and under illumination. Because the Pt-Ir probe has large work function (~5.5 eV)51 and intrinsic WSe2 behaves as p-type material,25 they would form an ohmic contact where the work function of Pt-Ir is aligned in the valence band of WSe2 (Figure S16). The difference between the valence band edge of WSe2 (~4.9 eV)37 and the work function of ITO (~4.7 eV)50 leads to formation of the Schottky barrier with an estimated height of ~0.2 eV, resulting in nonlinear I–V behaviors. The experimental I–V data can be fitted to the thermionic emission model52 as shown in blue and red solid lines of Figure 5b and c. The current I and saturation current I0 are given by: 𝐼𝐼 = 𝐼𝐼0 �exp �

𝑞𝑞𝑞𝑞

𝜂𝜂𝑘𝑘𝐵𝐵 𝑇𝑇

� − 1� and 𝐼𝐼0 = 𝐴𝐴𝑒𝑒 𝐴𝐴∗ 𝑇𝑇 2 exp �−

𝑞𝑞Φ𝐵𝐵 𝑘𝑘𝐵𝐵 𝑇𝑇

�,

where q is the electronic charge, V is the applied bias voltage, η is the ideality factor, kB is the Boltzmann constant, ΦB is the barrier height, Ae is the effective contact area, A* is the Richardson constant, and T is the temperature. From the curve-fit of our experimental data (shown as solid curves in Figure 5b), the barrier heights of pristine WSe2 are estimated to be 0.17 and 0.21 eV for forward and reverse bias in dark, and 0.15 and 0.18 eV for those under illumination, respectively (see Supporting Information). These barrier heights are slightly 20


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underestimated compared to those from the band alignment because this model does not consider tunneling currents.50 Similar I–V behaviors, but significantly increased photocurrents are observed after NiPc functionalization on the WSe2 (Figure 5c). The NiPc functionalization facilitates the separation of photogenerated carriers, which results in increased current flow. It is noted that the I–V curves measured in dark with and without NiPc are similar. The current increase after functionalization is more clearly seen in semi-log I–V plots with typical p-type behavior (Figure S16). These results further support the mechanism of photoinduced charge transfer and are consistent with our observation of the PL quenching. Large resistance (~70 MΩ) in I–V curves may originate primarily from the small contact area between the Pt-Ir probe and the WSe2 flake. We also visualized spatially resolved photocurrent maps of WSe2 before and after NiPc functionalization. To reconstitute the 2D photocurrent profiles, we first obtained the current maps of WSe2 and NiPc-functionalized WSe2 both in dark and under illumination at a fixed bias voltage of -0.5 V (Figure S17). The dark map was then subtracted from the illuminated current map. The AFM height image in Figure 5d shows bilayer WSe2 on the ITO substrate with a measured thickness of ~1.4 nm. It is clearly seen that the WSe2 layer generates currents under illumination compare to the bare ITO, and the currents after functionalization (Figure 5f) are significantly greater than those before functionalization (Figure 5e). We extract the photoresponsivity (R), which is defined by the photocurrent output per optical power input:53 𝑅𝑅 =

𝐼𝐼𝑝𝑝ℎ 𝑃𝑃𝐴𝐴𝑝𝑝

where Iph is the measured photocurrent, P is the incident optical power density, and Ap is the photon-incident area. The average photoresponsivity under 658 nm illumination increases from 2.66 ± 0.73 to 5.95 ± 1.30 A W-1 after NiPc functionalization. To confirm the correlation between photoresponse and PL, we measured the PL spectra on the same flake before 21


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photocurrent measurements of the WSe2 and WSe2-NiPc; the PL of WSe2 considerably quenched after NiPc functionalization, as expected (Figure S18). Additionally, we performed the PC-AFM and PL measurements for other TMDCs and MPcs. We find that photocurrents are enhanced with the TMDC samples that demonstrate the PL quenching due to charge transfer at the interface with MPcs. For example, the photocurrents significantly increase in both cases of WSe2-MgPc and MoSe2-NiPc (Figure S19 and S20). However, the photocurrent enhancement is not observed with MoS2-NiPc which does not exhibit photoinduced charge transfer (Figure S21). The photocurrent has dependency on the number of layers; thicker TMDCs (> 3L) show higher photocurrents due to the increased amount of photon absorption (Figure S19–21). Given the layer number dependent optoelectronic properties of TMDCs, one can expect that the photoresponse may vary even with the identical organic molecules because of different band edge positions and origin of optical transitions in the TMDCs. Thus, the layer number of TMDCs would be an important factor in designing organic-inorganic hybrid 2D systems. In turn, it offers a wide range of tunability to modulate optoelectronic properties, together with diverse choice of organic molecules that have distinct redox and electronic properties.

CONCLUSIONS We have demonstrated the photoinduced charge transfer from atomically thin 2D TMDCs to the organic acceptor molecules, thus modulating the optical and optoelectronic properties of TMDCs in a controllable manner. Engineering relative positions between the band edges of TMDCs and redox potentials of MPcs enables optical modulation of TMDCs, while maintaining their intrinsic electronic structures. The photo-excited electron transfer results in a drastic PL quenching across the entire TMDC flakes due to decreased recombination of excitons. Importantly, the electron transfer promotes effective charge separation of photo22


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generated electron-hole pairs, which significantly increases photoconductivity of TMDCs, as verified by PC-AFM. Given the simplicity and scalability of functionalization process, it could be used for manufacture of large-area, high-performance 2D optoelectronic devices such as redox-active molecular sensors, phototransistors, light emitting diodes, and light energy harvesters.11 With careful design and engineering of redox properties, other organic compounds and chromophores may be adopted to further modulate the optoelectronic properties of TMDCs as charge donors or acceptors. Our approach is not only applicable to homogeneous TMDCs, but could also be explored to control excitonic transitions in heterogeneous 2D materials.54,55

METHODS Preparation of Atomically Thin TMDCs. TMDCs were mechanically exfoliated from their bulk crystals (MoS2 from SPI Supplies and MoSe2 and WSe2 from 2Dsemiconductors Inc.) onto Si wafers with 285-nm-thick oxide layer or ITO/glass substrates. Before exfoliation, the substrates were cleaned by acetone and isopropyl alcohol for 5 min, followed by mild oxygen plasma treatment for 60 s. The TMDC-exfoliated substrates were annealed in argon environment at atmospheric pressure at 250 °C for 1 h in order to remove residues and surface adsorbates and improve contact quality between the TMDCs and the substrates. Preparation of MPc Solutions. Nickel phthalocyanine-tetrasulfonic acid tetrasodium salt and magnesium phthalocyanine were purchased from Sigma-Aldrich and used as received. Both NiPc and MgPc were dissolved in DI water at desired concentrations. While NiPc molecules are well solubilized in aqueous solution due to hydrophilic ligands (R = NaO3S), the solubility of MgPc is lower at approximately 180 mg/L. Functionalization of TMDCs with MPcs. TMDC-exfoliated substrates were immersed in MPc solution for 20 min, rinsed with DI water, and dried by air, unless specified otherwise. Approximately 1 mM NiPc and MgPc were used for XPS and Raman measurements. For PC23


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AFM studies, we used 100 μM NiPc and MgPc. For the PL reversibility experiment, the WSe2 sample functionalized with 100 μM NiPc was immersed in DI water for 16 h to remove the NiPc from the surface of WSe2. Optical Characterization. PL and Raman spectra were recorded with a Renishaw inVia confocal Raman microscope using a 633 nm HeNe laser and a 100× objective lens at room temperature. Laser spot diameter was approximately 1 μm. We used sufficiently low laser power to avoid undesirable heating or damage of the samples. AFM and PC-AFM Characterization. AFM and PC-AFM measurements were conducted using a Bruker Dimension Icon with a PF-TUNA module. We used a SCANASYST-AIR probe and a PF-TUNA probe for AFM and PC-AFM measurements, respectively. In order to prevent damage of the tip and the sample by a highly localized current, a current range was set to ±24 nA. For PC-AFM, a contact force between the tip and the sample was ~20 nN, and a 658 nm diode laser (Laserglow Technologies) was used for photo-excitation. A Newport power meter measured the laser power intensity at approximately 33.6 mW/cm2. To evaluate the photoresponsivity, we averaged the photocurrent of each data point measured by PC-AFM across the TMDCs flake. The size of the WSe2 flake within the PC-AFM scan window (~5.7 μm2) was used as the photon-incident area. XPS Characterization. XPS measurements were carried out using a Kratos Axis Ultra DLD XPS system with a monochromatic Al Kα X-ray source (1486.6 eV). Due to relatively large X-ray spot diameter of ~15 μm, the XPS data were obtained from thick layers of TMDCs exfoliated on SiO2/Si substrates.

Conflict of Interest: The authors declare no competing financial interest.

Supporting Information Available: PL spectra of TMDCs before and after annealing; effect of 24


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solvents on optical characteristics of TMDCs; Raman spectra of MPcs; XPS spectra of TMDCs with and without NiPc; AFM, PL, and Raman studies on functionalized TMDCs; Raman spectra of TMDCs and MPc-functionalized TMDCs; PL quenching of multilayer WSe2 after NiPc functionalization; AFM, PL, and Raman maps of MPc-functionalized TMDCs; static PL quenching; analysis of I-V curves measured using PC-AFM; photocurrent map and PL spectra of MPc-functionalized TMDCs. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENT This work was supported by the National Science Foundation. The XPS data was obtained at the Surface Analysis Facility of the Birck Nanotechnology Center at Purdue University.

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Modulating Optoelectronic Properties of Two-Dimensional Transition

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