Effects of Organic Molecules with Different Structures and Absorption

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The effects of organic molecules with different structures and absorption bandwidth on modulating photoresponse of MoS photodetector 2

Yanmin Huang, Wei Zheng, Yunfeng Qiu, and PingAn Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06968 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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ACS Applied Materials & Interfaces

The effects of organic molecules with different structures and absorption bandwidth on modulating photoresponse of MoS2 photodetector Yanmin Huang,b Wei Zheng,b Yunfeng Qiu,b* PingAn Hua, b* a

State Key Laboratory of Robotics and System (HIT), Harbin, 150080, P.R. China.

b

Key Lab of Microsystem and Microstructure, Ministry of Education, Harbin Institute of

Technology, No. 2 YiKuang Street, Harbin 150080, PR China. KEYWORDS：Photodetector, MoS2, dye-sensitized, n-doping, charge transfer

Abstract: Organic dye molecules possessing modulated optical absorption band-width and molecular structures, can be utilized as sensitizing species for the enhancement of photodetector performance of semiconductor via photoinduced charge transfer mechanism. MoS2 photodetector were modified by drop-casting of Methyl orange (MO), Rhodamine 6G (R6G), and Methylene blue (MB) with different molecular structures and extinction coefficients, and enhanced photodetector performance in terms of photocurrent, photoresponsity, photodetectivity, and external quantum efficiency were obtained after modification of MO, R6G, and MB, respectively. Furthermore, dyes showed different modulating ability for photodetector performance after combination with MoS2, mainly due to the variation of molecular structures

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and optical absorption band-width. Among tested dyes, deposition of MB onto monolayer MoS2 grown by CVD resulted in photocurrent ~20 times as high as pristine MoS2 due to favorable photoinduced charge transfer of photoexcited electrons from flat MB molecules to the MoS2 layer. Meanwhile, the corresponding photoresponsivity, photodetectivity, and an external quantum efficiency are 9.09 A W-1, 2.2 × 1011 Jones, 1729% at 610 nm, respectively. Photoinduced electron-transfer measurements of the pristine MoS2 and dye modified MoS2 indicated the n-doping effect of dye molecules on the MoS2. Additionally, surface enhanced Raman measurements also confirmed the direct correlation with charge transfer between organic dyes and MoS2 taking into account the chemically enhanced Raman scattering mechanism. Present work provides a new clue for the manipulation of high-performance of two-dimensional (2D) layered semiconductor based photodetector via the combination of organic dyes.

Introduction High performance photodetectors based on 2D layered nanomaterials have shown promising potential for safety monitoring, environmental sensing, motion detection, video imaging, and biochemical detection.1-2 2D nanomaterials showed superior properties considering the high light absorption ability, flexibility and mechanical properties comparing to traditional zerodimensional or one-dimensional nanomaterials, also possess compatibility with Si-based manufacturing process.3 Up to date, graphene as one of the most appealing 2D nanomaterials, has illustrated fascinating potential for the fabrication of photodetector with ultra-broadband ranging from ultraviolet to the teraherz because of its high carrier mobility and wide band absorption.4 Additionally, the presence of an internal field near the interfaces between the graphene and metal electrodes endows graphene photodetector ultrafast photoresponse.5


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Nevertheless, the practical application of pristine graphene based photodetector has been severely limited due to the unsatisfactory photoresponsivity (10-3−10-1 A W-1). Such drawback is related to the absence of photogain, which means the ability to induce multiple electrical carriers upon irradiation by single incident photon. It is proved that graphene’s zero band gap and short photocarrier lifetime is responsible for this.6 Tremendous efforts have been applied toward addressing these issues via the design of atomically thin materials, such as transition metal dichalcogenides (TMDCs).7-12 TMDCs have offered appealing optoelectronic properties, such as large bandgap in the range of 1-2 eV, showing very sensitive photodetection ability and high on/off ratios modulated by back-gate voltage.13-15 Very recently, MoS2 proved to be a promising candidate for high-performance photodetector, and may provide an alternative solution to surpass the disadvantages of graphene.9, 16 Single layer MoS2 possesses a direct band gap of ~1.9 eV comparing to its bulk counterpart with indirect band gap of ~1.2 eV.17 Field effect transistor consisting of monolayer MoS2 using HfO2 as top-gate displayed carrier mobility of ~0.1-10 cm2 V-1 S-1 at room temperature and ultrahigh on/off-ratios of 108, attributing to the suppression of the Coulomb scattering because of the highκ dielectric environment.18 Single- and multi-layer MoS2 photodetectors have been successfully fabricated, and illustrated high gain, fast photo response down to milliseconds, and responsivity up to 880 A W-1.19 However, the relatively long decay time of thousands of seconds needs further optimization. Up to date, some promising routes have been developed to enhance the performance of MoS2 photodetector via enhancing its light absorption ability and extending its response spectral range. The photodetector based on graphene/MoS2 heterostructure has been reported to show an extremely high photogain greater than 108 due to the presence of a perpendicular effective electric field.20 The synergism of n-type 2D MoS2 and p-type 0D PbS


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quantum dots semiconductors was found to beneficial to improve the responsivity of phototransistor with orders of magnitude higher than that of individual component, attributing to effective separation of electron-hole pairs at the p-n interface.21 Very recently, dye-sensitized MoS2 photodetectors exhibited enhanced responsivity, detectivity, EQE, and spectral range, because the enhanced light absorption and matched band diagram.22 Most organic π-conjugated molecules are showing p-type semiconductor characteristics, and possessing rational option for the modification of n-type MoS2 based photodetector or phototransistor.23 It is found that the hybrid MoS2/rubrene based thin-film transistors demonstrated increased charge carrier mobility from 34.3 to 42.3 cm2 / V s comparing to pristine MoS2, and the photoresponsivity was ~10 times as high as that of pristine MoS2.24 A very simple drop-casting method for the combination of R6G and MoS2 based photodetector was successfully developed for acquiring much better optical performance, exhibiting a maximum photoresponsivity of 1.17 AW-1, a photodetectivity of 1.5 × 107 Jones, and a total EQE of 280% at 520 nm.25 It is worth noting that the photoresponse extended to the infrared (λ < 980nm) due to the effective photoinduced charge transfer from dye to MoS2, much wider than the band gap of pristine MoS2 (λ < 681 nm). To the best of our knowledge, systematic evaluation of πconjugated organic dyes with various optical absorption band-width and molecular structures on the performance of MoS2 photodetector are lack of investigation.23 In view of the positive role of organic dyes in enhancing the performance of MoS2 photodetector, herein three dyes of Methyl orange (MO), Rhodamine 6G (R6G), and Methylene blue (MB) with different molecular structures and extinction coefficients were drop-cast onto MoS2 photodetector to improve the optical parameters in terms of photoresponsivity, photodetectivity, and EQE. Once modification of MB, the resulted photodetector not only


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showed photocurrent ~20 times as high as pristine MoS2, but also high photoresponsivity, photodetectivity, and an external quantum efficiency of 9.09 A W-1, 2.2 × 11 Jones, 1729% at 550 nm (Vg = 0 V, Vd = 1 V), respectively. Given the photoinduced charge transfer of photoexcited electrons from dye molecules to the MoS2 layer are varied in the cases of three dyes, the hybrid photodetector displayed different performance. n-doping effect of dye molecules on the MoS2 and direct charge transfer among them are responsible for the observed enhancement of optical performance of hybrid photodetector, showing promising route for the fabrication of high performance photodetector via dye-sensitized method with respect to their optical absorption band-width and molecular structure. Experimental section Materials and methods The triangle monolayer MoS2 nanosheets were characterized by optical microscopy (Leica DM4500P), atomic force microscopy (Nanoscope IIIa Vecco), transmission electron microscopy (TEM, Tacnai–G2 F30, accelerating voltage of 300 kV) and Raman spectroscopy (LabRAM XploRA, power 0.15 mW, excitement wavelength 532 nm), UV-Vis spectra (U-4100, Hitachi). Fabrication of MoS2 or dye/MoS2 FETs and photodetectors: Triangle single layer MoS2 nanosheets were grown on p-doped silicon substrates bearing a 300 nm thick SiO2 layer by a double temperature zones furnace CVD method according to our previous work.26 The upstream zone was kept at 250 oC, while the MoO3 powder was put into the downstream zone elevating to 700 oC in a speed of 15 oC/min. When the downstream zone’s temperature reached 700 oC, put the sulfur into the upstream zone quickly and keep another 25


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min. During the whole process of the reaction, Argon was utilized as the carrier gas at a flow rate of 15 sccm. Then the furnace cool down naturally, and the triangle single layer MoS2 at the size of ~30 µm was successfully prepared. Cr/Au electrodes were fabricated using film–plating machines (ZHD–300) by a shadow mask. 10 µL MO, R6G, or MB aqueous solution at 1 mM was directly drop-cast onto the single layer MoS2 device, then the device was kept in an oven at 70 oC for about 10 min to dry. Characterizations of optoelectronic properties: Optoelectrical characterizations of triangle monolayer MoS2 devices were performed by using semiconductor characterization system (Keithley 4200 SCS) with a Lakeshore probe station. Mono-chromatic lights from 700 to 490 nm were obtained by using optical filters using a 500 W xenon lamp as the light source. And the illumination power were normalized to 0.29 mW/cm2 at different wavelength by changing the lamp current. The intensities of incident light source were identified by a power and energy meter (Model 372, Scienteck). Results and discussion 1. Optical performance of photodetectors MoS2 is a highly promising semiconductor for building optoelectronic devices, such as FETs, photodetectors, photovoltaic cells, etc., mainly because of its direct band-gap of 1.9 eV at room temperature and flat nanostructure for matching well-established Si-based processing technology.27 MoS2 herein were prepared via CVD method using MoO3 and elemental sulfur as precursors according to our previous work. High resolution TEM in Figure S1a gives the interlayer spacing of 0.271 nm, corresponding to the lattice constant of (100) direction. Selected


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area electron diﬀraction pattern (SEAD) in Figure S1b confirmed the single crystalline structure of MoS2.26 As shown in Figure 1a, the triangular shape of MoS2 is determined by AFM measurement, and the thickness is ~0.9 nm corresponding to monolayer structure. Raman spectrum in Figure 1c displayed the typical E2g mode at 386 cm-1 and A1g mode at 408 cm-1, further indicative of the successful preparation of monolayer MoS2.28-29 The photoluminescence spectrum of MoS2 in Figure 1d showed the characteristic peak at 1.83 eV. Three dyes are selected to evaluate the effect after modification onto the surface of MoS2 with respect to optical absorption band-width and different molecular structure.25, 30 We first measured the UV-vis spectra of MO, R6G, and MB water solution in Figure S2, giving maximum absorption wavelength at 465, 520, and 660 nm, respectively. However, MO showed obviously lower absorption coefficient than those of R6G and MB. Dyes are separately drop-cast onto the surface of MoS2, which can be examined by surface enhanced Raman spectra. As reported in previous work, the chemical contact between organic molecules and 2D materials including graphene and TMDs can effectively result in the Raman enhancement.31 Previous work have confirmed that both charge transfer and dipole–dipole coupling may be responsible for the enhanced Raman signals of the dye molecules.31 It is worth noting that these charge-transfer transitions are possibly between the valence band of monolayer MoS2 (−5.6 eV) and unoccupied π* orbitals in the adsorbates.32 To our knowledge, it is reasonable to assume that chemically enhanced Raman scattering (CERS) is responsible for the Raman enhancement on MoS2, rather than electromagnetic enhancement mechanism. Raman scattering of R6G on MoS2 became much stronger in Figure 1d comparing to that on SiO2/Si substrate. The Raman enhancement factor of the R6G molecules adsorbed onto MoS2 film was ~11 at 1365 cm-1, comparable to previous values.33-35 Raman mapping of R6G peak at 1365 cm-1 is illustrated in Figure 1f, in which the


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blue triangle represents the distribution of R6G on the corresponding MoS2 triangle. This Raman mapping result further confirmed the strong adsorption of R6G molecules on the surface of MoS2. Meanwhile, such CERS was further observed in the case of MB in Figure S3. The Raman scattering of MB on SiO2/Si substrate was silent, however, strong Raman signals corresponding to MB molecules are clearly observed on MoS2. And corresponding Raman mapping at 1637 cm1

of MB in Figure S3c further indicated the deposition of MB on the surface of MoS2. We failed

to observe the CERS phenomenon in the case of MO, which might be due to the relatively weak chemical interaction between MO molecules and MoS2. Furthermore, AFM measurement of naked MoS2 and dye molecules modified MoS2 are shown in Figure S4, in which nano-sized irregular aggregates are observed after modification (More clear in Figure S5: AFM phase images). The roughness of pristine MoS2 and dye molecules modified MoS2 are summarized in Table S1, and it is obvious that the roughness of MO molecules modified surface was almost twice times (1.05 nm vs 0.52 nm) as high as those on pristine MoS2. The roughness of R6G (0.59 nm vs 0.54 nm) and MB (0.59 nm vs 0.47 nm) modified MoS2 also showed slight increase comparing to the pristine MoS2. In addition, similar roughness changes were observed on the SiO2/Si substrate in the same trend. Briefly, the above results solidly confirmed that dye molecules strongly adsorbed on the surface of MoS2, and the chemical interactions between dyes and MoS2 are varied depending on different structural features of tested dyes. Basically, dye molecules modification was favorable to the improvement of light absorption.23 As reported in dye-sensitized solar cells (DSSCs), many natural or synthesized dyes are successfully utilized to enhance the DSSCs performance due to its profound optical characteristics.36 Although MoS2 photodetectors have shown intriguing performance in terms of high photoresponsivity, photogain, and fast light response, it is still far away the substantial


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application due to the harsh requirements for practical photodetector. In our system, dye molecules could facilitate the light absorption of MoS2 photodetector due to its optimal light absorption range, as indicated in Figure S2. And the photoinduced carriers, such as excited electrons, could effectively transfer from the LUMO level with higher energy of dye molecules to the conduction band with relatively lower energy of MoS2.37 Thus, such electron transfer process will lead to the high concentration of free carriers in MoS2, leading to the increase of carriers in the circuit, and finally causing the enhancement of photocurrent. Taking into account the variations of light absorption band-width of MO, R6G, and MB, also the molecular structure, we will systematically study the photodetector performance after modification of the above three dyes in the following part. The single-layer MoS2 based photodetectors is fabricated using Ti/Au (5 nm/35 nm) contacts on Si substrates covered with 285 nm silicon dioxide (SiO2/Si). The device configuration was depicted in Scheme 1, in which 10 µL dye aqueous solutions with 1 mM was drop-cast on the channel and rinsed with distilled water twice. Figure S6 are optical images of some representative devices. All photoelectrical measurements are performed at room temperature under ambient condition. The photocurrents were measured as a function of different irradiated wavelength from 490 to 700 nm. The dark currents (IOFF) and illumination photocurrents (ION) were recorded with a source-drain bias voltage Vds = 1 V, the gate voltage Vg = 0 V, and light density of 0.29 mW/cm2. The photocurrent vs. time plots of pristine MoS2 and MO, R6G, or MB modified MoS2 photodetectors under alternating dark and light irradiation in Figure S7 clearly displayed the stable increase of photocurrent upon light illumination and the decrease in dark in all tested wavelength. As illustrated in Figure 2a and 2c, the photocurrent (∆I = ION - IOFF) at each wavelength for MO or R6G modified MoS2 was 7-11 times as high as that of pristine MoS2.


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Interestingly, the photocurrent of MB modified MoS2 photodetector in Figure 2e was about 20 times higher than that of MoS2. The photocurrent vs. light power density were recorded in Figure S8a, 8c, and 8e of pristine MoS2 and MO, R6G, or MB modified MoS2 photodetectors at 490, 550, or 610 nm, respectively. The selection of illumination wavelength was based on the largest extinction coefficient position of individual dye. It is widely accepted that the generated photocurrent of channel material is mainly controlled by the illumination intensity when the photon energy higher than the Eg of semiconductor.38 Here, The Eg of MoS2 is 1.83 eV (~680 nm), lower than the selected illumination wavelength. Meanwhile, as discussed in above text, the selected illumination wavelength was very close to corresponding optical band-width of MO (465 nm), R6G (520 nm), and MB (660 nm), suggesting the effective excitation of both MoS2 and dye molecules. The photocurrent increased gradually when the light power density increased in all cases. The almost linear change of ∆I vs. light intensity is attributed to the quantity of photogenerated carriers under illumination. The photocurrent vs. bias was performed, and the data in Figure S8b, 8d, and 8f of pristine MoS2 and MO, R6G, or MB modified MoS2 photodetectors showed almost linear change as increasing the bias. The significant increase of source-drain current mainly resulted from the improvement of carrier drift velocity and the related decrease of carrier’s transit time.39 The time response of the photodetector can be characterized by the rise time (τr) and decay time (τd), which are defined as the time interval for the response to rise and decay from 10 to 90% ro 90% to 10% of its peak value.40-41 As seen in Figure S9 and table S2, the time constant τr and τd are calculated to be 19.1 and 48.2 S for pristine MoS2, comparable to our previous work.26 After modification of dyes, the time constant τr and τd are 23.9 and 50.1 S for MO modified MoS2, showing relatively slow response comparing to pristine MoS2. In other two cases of R6G


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and MB, the time constant τr and τd only showed faint changes. The photocurrent of pristine MoS2 and three dyes modified MoS2 all decreased gradually as a function of wavelength. As expected, there is no observable photocurrent when the illumination light reached 700 nm, consistent with the limitation of optical band gap of the single layer MoS2 (1.83 eV, λ = 680 nm). Such experimental phenomena are widely observed in pure MoS2 based photodetectors, which are explained as the photoinduced carriers generation from the valence band into the conduction band of MoS2 can only occur when the energy of incident photons are higher than 1.83 eV. In contrast, dye molecules sensitized MoS2 photodetectors showed obvious photocurrent at 700 nm in Figure 2a, 2c and 2e. Taking into account the matched energy bands of dyes and MoS2, the photoresponse at 700 nm might result from the photoexcited electron transfer from the HOMO level of dyes to the bottom of the conduction band of MoS2.25 Responsivity (Rλ), detectivity (D*) and EQE are critical parameters for evaluating the performance of a photodetector. Basically, Rλ represents the ability of the generation of photocurrent per unit power of the incident light on the effective area of a photodetector, which can be expressed as = ⁄ . Here, Iλ is the generated photocurrent, Pλ is the incident light intensity, S (120, 40, and 415 µm2 for three representatives, respectively) is the effective illuminated area. From our experimental results in Figure 2, the Rλ of pristine MoS2 photodetector is calculated to be 0.42 AW-1 under an illumination of 490 nm (Vds = 1 V, Vg = 0 V). In comparison, R490 nm of MO, R6G, and MB modified MoS2 photodetector are calculated to be 3.73, 1.49, and 10.87 AW-1 under identical measurement conditions, respectively. The responsivity of MB molecules sensitized MoS2 photodetector are superior to those of silicon,42 InGaAs,43 or GaSe44 based photodetectors, slight lower than that of GaS photodetector.15 If assumed that the contribution to total noise mainly arises from the shot noise from dark current,


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the specific detectivity of D* can be regarded as a measure of detector sensitivity. Generally, it is

defined as ∗ = 2 , where R is the responsivity in AW-1, A is the area of the photodetector channel in cm2, e is the electron charge, and Id is the dark current. The normalized detectivity of D* is calculated in units of Jones, namely, cm Hz1/2W-1. Figure 2b, 2d and 2f showed the measured D* of pristine MoS2 photodetector, as well as MO, R6G, and MB modified MoS2 photodetectors at different light wavelength (Vds = 1 V, Vg = 0 V). The D* is ~1010 Jones for pristine MoS2 photodetector, however D* is ~1011 Jones after dye molecules modification, which is comparable to those of recently reported Si,42 InGaAs,43 MoS2,19 and In2Se345 devices (~1011 -1012 Jones). EQE is defined as the number of carriers circulating a photodetectors per absorbed light photon and per unit time. It is calculated as = ℎ ⁄, where R represents the responsitivity, h is the Plank constant, c is the velocity of light, e is the charge of electron, and λ is the wavelength of incident light, respectively. MO, R6G, and MB decoration improved the corresponding EQE from 106% to 945% at 490 nm, from 31% to 316% at 550 nm, and from 75% to 1729% at 610 nm (Vds = 1 V, Vg = 0 V), respectively, indicative of an outstanding performance improvement for dye molecules-modified MoS2 photodetectors. Although present EQE of our hybrid photodetector is inferior to our previous work of GaS photodetector,44 it surpassed the EQE (6-16%) of graphene photodetector.46 Briefly, dye modified MoS2 photodetectors showed significant improvement of optical performance in terms of Rλ, D*, and EQE, in particular, MB illustrated the most effective modulation ability on the performance. Here, the EQE can be enhanced via using higher bias up to 12000% at 5 V in Figure S8b, 8d, and 8f, which was due to the enhancement of the carrier drift velocity and shortening of carrier transit time. 2. N-doping and charge transfer analysis


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As a next step, we further measure the transfer curves of pristine MoS2 and dye molecules modified MoS2 FETs devices. Such experiments are aiming for disclosing the mechanism of photoinduced electron transfer from dye molecules to the MoS2, and the variations of the three dye molecules on modulation of optical performance of MoS2 phohotdetector. FET devices are prepared using the same device configurations of photodetectors in above experiment. As shown in Figure 3, the fabricated MoS2 FETs displayed typical characteristic of n-type FET behavior. Meanwhile, the transfer curves of MO, R6G, and MB modified MoS2 FETs were also recorded, and the photocurrent at Vd = 0 V became obviously larger than that of pristine MoS2 FETs under dark condition (Vds = 1 V). Such results solidly indicated that dye molecules offered n-doping when in contact with MoS2.23 As confirmed in CERS, there are strong chemical interactions between dye molecules and MoS2, viz., such non-covalent interaction can be regarded as charge transfer between them due to the existence of S-π coordination interaction.30 Under the light illumination of 490, 550, and 610 nm, the photocurrent of MO, R6G, or MB modified MoS2 FETs was apparently higher than those of pristine MoS2 FETs in Figure 3a, 3b and 3c, respectively. To our knowledge, such photocurrents changes can be attributed to the effective photoinduced electron transfer from the LUMO level of dye molecules to the conduction band of MoS2 film.22, 25, 31 We further measured the photoinduced transfer curves of pristine and dye molecules modified MoS2 photodetectors under different light wavelengths in Figure 4a, 4c and 4e. Under different illumination wavelengths, all devices exhibited n-type behavior, consistent with the results in Figure 3 and previous work.25 Meanwhile, the photocurrents extracted from the transfer curve at Vg = 0 V were plotted versus different illumination wavelengths, showing the same increasing phenomena after modification of dye molecules. The photocurrent of MB modified MoS2 photodetector in Figure 4f was about 19 times higher than that of MoS2,


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showing the highest increasing extent than those from MO (~4 times) and R6G (~8 times) modified devices. Such increasing trends are agreeing with the results from photocurrent vs. time measurements in Figure S7 and Figure 2. In order to confirm the above hypothesis, carrier densities of pristine MoS2 and dye molecules modified MoS2 are calculated using the following equation: = × ×

!"#

%$, where

IDS is the drain source current at Vg = 0 V, Ci = 1.15× 10-8 F/cm2 is the capacitance of the 300 nm SiO2 dielectric layer, e is the electron charge, and

!"#

%$ is the slope of transfer curve at Vg = 0 V.

As summarized in Table 1, the carrier densities of dye molecules modified MoS2 were obviously larger than those of pristine MoS2 at each illumination wavelength or dark condition. More importantly, the carrier densities of dye molecules modified MoS2 under light illumination were also quite higher than those values under dark conditions, confirming the assumed photoinduced charge transfer from dyes under light irradiation.47 It is worth noting that MB showed the highest modulating ability towards the optical performance in terms of the highest photocurrents, responsivity, detectivity, and carrier density. We made a preliminary evaluation of the molecular structure using Chemdraw 3D software under minimized structural energy state. As depicted in Scheme 2, MO containing a N=N bond is an azobenzene derivative, showing trans-cis photoisomerization under alternative UV and white light irradiation or thermal treatment.48 Generally, the photoisomerization degree from cis to trans is not 100% complete, thus leading to poor flat structure on the surface of MoS2 due to the partial presence of cis-structure.49 R6G has steric hindrance of benzoic acid ethyl ester, which will affect the tight contact between R6G and MoS2. It is assumed that flat structure of dye molecules on the surface of MoS2 will be favourable to the photoinduced electron transfer on the basis of matching energy levels between


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them. From structural point, MB shows optimal flat molecular structure, which is quite important for the ideal stacking of MB on the surface of MoS2, facilitating the effective charge transfer between them. Meanwhile, sulphur atom in π-conjugated skeleton of MB will form S-S or S-Mo coordination interactions with MoS2, providing multiple photoinduced carrier transfer pathway. In addition, as confirmed in UV-vis measurement, absorption coefficient of MB is comparable to that of R6G, but obviously higher than MO, showing relatively strong light adsorption ability. Taken together, structural features and the excellent light adsorption ability of MB will be greatly helpful to accelerate the effective charge transfer from MB to MoS2 under light illumination because of the tight contact and synergistic light adsorption,23, 30 thus leading to the highest adjusting ability in terms of the optical performance of dye/MoS2 hybrid photodetectors among all tested dyes. Conclusions In summary, our results indicate that dye molecules can sensitize MoS2 based photodetector. Three dye molecules have different optical absorption band-width and molecular structures, leading to diverse modulation on the performance of photodetector. MB molecule not only shows the highest absorption coefficient at 610 nm, but also possesses flat configuration under minized structural energy. The strong chemical interaction between MB and MoS2 was confirmed by enhanced Raman scattering of MB on the surface of MoS2 via the CERS mechanism. MB displayed n-doping effect on the optoelectrical properties, further to enhance the photocurrent under light illunination because of the effective photoinduced electron transfer from the LUMO level of MB to the conduction band of MoS2. Overall, the responsitivity, detectivity, and EQE increased to 9.09 A W-1, 2.2 × 1011 Jones, 1729% at 610 nm (Vds = 1 V, Vg = 0 V) after depositing MB on the surface of MoS2 photodetector, respectively. It is worth noting that the


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optical performance of 2D materials based photodetector can be improved through the combination of organic π-conjugated molecules via facile drop-casting method. Considering all measurements are performed at ambient condition, the stability of organic molecules on photocatalytic MoS2 surface should be taken seriously in future study. The much improved optical performance of organic dye modified MoS2 photodetectors shortens the path towards substantial applications in touch panel, environmental sensor, biology detector, etc. ASSOCIATED CONTENT Supporting Information. Electronic Supplementary Information (ESI) available: [High resolution TEM of MoS2 nanosheet and SAED pattern; UV-vis absorption spectra of dyes; Raman spectra and mapping; AFM height images and phase images of naked MoS2 and MO, R6G, and MB modified MoS2; Roughness of the surface of pristine MoS2 and dye molecules modified MoS2; Optical images of field effect transistors (or photodetectors); Photocurrent vs. time plots; Photocurrents as a function of light power densities and bias. EQE at different bias; Representative rise and decay time.]. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: E-mail: [email protected] (Qiu Y.), [email protected] (P. Hu), Phone: +86 451 86403583, Fax: +86 451 86403583. Author Contributions


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Y.M. Huang, Y.F. Qiu, and P.A. Hu conceived the experiments, Y.M. Huang and W. Zheng designed and conducted the experiments. Y.M. Huang, Y.F. Qiu, and P.A. Hu analyzed the results, Y.F. Qiu and P.A. Hu wrote the manuscript. All the authors reviewed and approved the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900, National Natural Science Foundation of China (NSFC, 61390502, 21373068), Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No.51521003) and by Self-Planned Task (NO. SKLRS201607B) of State Key Laboratory of Robotics and System (HIT). REFERENCES 1.

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Figure 1. (a) AFM image of MoS2 triangles. Inset of a is the height profile. (b) Raman spectrum of MoS2. (c) Photoluminescence spectrum of MoS2. (d) Raman spectra of R6G molecules on SiO2/Si substrate (red line) and MoS2 (black line). The wavelength and the power of Raman excitation beam were 532 nm and 0.15 mW, respectively. (e) Optical image of R6G modified MoS2 triangles. (f) Corresponding Raman mapping at 1376 cm-1 of R6G.

Scheme 1. General procedure of drop-casting method for the deposition of dye molecules on the surface of MoS2 photodetectors, and corresponding device configuration of a typical photodetector (Vg = 0 V) or phototransistor.


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Figure 2. (a, c, and e) Photocurrent of pristine MoS2 (black curve) and MO, R6G, and MB modified MoS2 photodetector (red curve) under different illumination wavelength (0.29 mW), respectively. Photocurrent (∆I) is defined as the difference between ION and IOFF. (b, d, and f) Corresponding responsitivity or detectivity of MoS2 (black or blue square) and MO, R6G, and MB modified MoS2 photodetector (black or blue triangle) under different illumination wavelength (0.29 mW), respectively.


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Figure 3. Black and red curves in (a, b, and c) are typical transfer curves (Ids vs. Vg) of pristine MoS2 and MO，R6G，or MB sensitized MoS2 transistor in dark, respectively. Blue curves in (a, b, and c) are corresponding transfer curves of MO，R6G，or MB sensitized MoS2 phototransistor at 490, 550, or 610 nm, respectively.


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Figure 4. (a, c, and e) Gating response (Ids vs. Vg) of the MoS2 phototransistor and corresponding MO，R6G，MB sensitized MoS2 phototransistor in the dark and under the illuminations of 490 nm, 550 nm,610 nm and 700 nm lights of identical intensity of 0.29 mW/cm2 (Vds = 1 V). (b, d, and f) Photocurrents of corresponding MoS2 phototransistor and corresponding MO，R6G，MB sensitized MoS2 phototransistor as a function of different wavelength (Vg = 0 V, Vds = 1 V).


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Scheme 2. Molecular structures of (a) MO, (b) R6G, and (c) MB, and their minimized energy structures in ball-and-stick models on MoS2 surface, respectively. The red circle (dotted line) in b indicates the steric hindrance effect of benzoic acid ethyl ester.


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Table 1. Carrier density of pristine MoS2 and Dye Sensitized MoS2 under light illumination at different wavelength (Light power density was fixed at 0.29 mW/cm2.).

Carrier density Dye

Devices I: MO

Device II: R6G

Device III: MB

Wavelength (nm)

Dark 700 610 550 490 Dark 700 610 550 490 Dark 700 610 550 490

MoS2 (1012 cm-2)

Dye sensitized MoS2 (1012 cm-2)

0.642 － 0.820 0.806 0.853 0.898 － 1.098 1.125 1.221 0.751 － 1.16 0.994 1.12

0.909 0.994 0.959 1.03 1.00 0.997 1.47 1.61 1.69 1.69 1.59 1.68 1.95 2.30 2.42


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TOC


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Figure 1 85x50mm (300 x 300 DPI)



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Scheme 1 85x34mm (300 x 300 DPI)


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Scheme 2. Molecular structures of (a) MO, (b) R6G, and (c) MB, and their minimized energy structures in ball-and-stick models on MoS2 surface, respectively. The red circle (dotted line) in b indicates the steric hindrance effect of benzoic acid ethyl ester. 165x153mm (300 x 300 DPI)


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Effects of Organic Molecules with Different Structures and Absorption

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