MoSe2-Cu2S Vertical p-n Nanoheterostructures for High-Performance

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Functional Inorganic Materials and Devices

MoSe2-Cu2S Vertical p-n Nanoheterostructures for High-Performance Photodetector Md. Samim Hassan, Susnata Bera, Divya Gupta, Samit K. Ray, and Sameer Sapra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16205 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Materials & Interfaces

MoSe2-Cu2S Vertical p-n Nanoheterostructures for High-Performance Photodetector Md. Samim Hassan,a Susnata Bera,a Divya Gupta,a Samit K. Ray,b,c Sameer Sapraa* a Department

of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

b Department

of Physics, Indian Institute of Technology Kharagpur, West Bengal, 721302, India

c S.

N. Bose National Centre for Basic Sciences, Kolkata-700106, India

ABSTRACT: Heterostructures based on atomically thin two dimensional layered transition metal dichalcogenides are highly promising for optoelectronic device applications owing to their tunable optical and electronic properties. However, the synthesis of heterostructures with desired materials having proper interfacial contacts has been a challenging task. Here, we develop a colloidal synthetic route for the design of MoSe2–Cu2S nanoheterostructures where the Cu2S islands grow vertically on top of the defect sites present on the MoSe2 surface, thereby forming vertical p-n junction having plasmonic characteristics. These MoSe2–Cu2S nanoheterostructures have been used to fabricate photodetectors with superior photo response characteristics. The fabricated device exhibits a broadband spectral photo response over the visible to near infrared range with a peak responsivity of 410 mAW−1 at -2.0 V and over 3000-fold photo-to-dark current ratio. The superior device performance of MoSe2-Cu2S over only MoSe2 devices is due to the combined effect of the formation of p-n junction, pronounced light-mater interaction and passivation of surface defects. This study would pave the way for designing a new class of nanoheterostructured materials for their potential applications in the next generation photonic devices.

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KEYWORDS: MoSe2-Cu2S nanoheterostructures (NHSs), surface plasmon, p-n junction, passivation of defects, photodetectors.

INTRODUCTION: Two dimensional (2D) semiconductors of layered transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se) are highly promising materials for optoelectronic applications e.g. photodetectors,1,2 light emitting diodes3 and photovoltaics4,5 due to a number of factors such as their indirect-to-direct band crossover,6 high mobility of charge carriers,7 spin-orbit coupling,8 strongly bound excitons,9 and ambipolar characteristics.10 Further, these materials exhibit outstanding mechanical stability and bending durability on flexible substrates, making them suitable for applications in various device architectures.11 Due to the ultrathin nature of 2D-TMDs the optical absorption cross section is low, which in turn diminishing light-matter interactions. Therefore, in these 2D-TMDs the absorption of incident light is low and roughly limited to 510%,12,13 rendering them non-viable in high performance photonic device applications. The device performance can be increased by designing nanoheterostructures (NHSs) of TMDs with proper interfacial contacts to absorb more photons from sunlight as well as tuning their electrical properties.14–18 MoSe2 is an n-type semiconductor which is composed of vertically aligned, weakly interacting layers held together by weak van der Waals interactions.19,20 The number of layers are responsible for both the energy of the band transition21 and mobility of charge carriers.22 Though with decreasing number of layers the mobility of charge carrier increases concurrently it suffers from lower absorption of light. Further, these TMDs are known to have a number of vacant sites.23 The presence of dangling bonds and vacancy induced defects adversely affects the transport


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properties immensely influencing the performance of the optoelectronic device.24 The performance of the optoelectronic device can be improved through (a) increasing the light matter-interactions,12 (b) reducing the adverse effect of defects24 and (c) tuning the electronic properties.25 Further, an amalgamation of TMDs with different functionalities has shown to be highly promising for optoelectronic device applications4,14,16 However, the currently available literature reports p-n heterostructures of 2D materials that have been essentially limited to both components being TMDs.4,26,27 Cu2S crystallizes in a hexagonal structure28 and has great prospect for applications in solar cells,29 nanoscale switches,30 and optoelectronic devices.31 Cu2S is a p-type semiconductor which is ascribed to the presence of copper vacancies in the crystal. A carrier concentration dependent plasmonic absorption band in the near-infrared (NIR) region attests for the presence of Cu vacancies.32,33 The presence of surface plasmon consequently increases light-matter interaction by trapping photons in the NIR region.16,33 The nanocrystal morphologies of Cu2S are more sensitive to the plasmon frequency as the change in carrier density for a smaller particle is greater, which enhances the light harvesting efficiency and electro-optic sensitivity of the devices.34 The hexagonal crystal structures of the two materials should render easy deposition of Cu2S on MoSe2 nanosheets, thereby enabling formation of new p-n junction NHSs,31,35 with a high absorption cross-section. Recently, p-n junction heterostructures of atomically thin TMDs such as MoS2WSe2, WS2-MoS2, WSe2-SnS2 have been explored for fabrication of flexible electronic and optoelectronic devices.4,14,26,36,37 The synthesis of TMD based heterostructures have been largely limited to the mechanical,16 and chemical vapor deposition (CVD) synthetic route.14,16 Colloidal synthesis routes, which are substrate free and result in high yield have been much less explored. Recently, Zhang et al. have epitaxially grown MoSe2 and MoS2 along the CdS nanowires by a



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colloidal route.38 Yang et al. synthesized MoSe2−NiSe vertical NHSs via colloidal route.39 Badhulika et al. have grown MoS2/Cu2S on cellulose paper via the solvothermal route.40 In the present study, we have developed a colloidal synthetic route for the synthesis of MoSe2-Cu2S vertical p-n NHSs. Dodecanethiol (DDT), which is used as the Sulphur source, also passivates the surface and reduces the Se vacancy related defects. Raman spectra unveils that DDT treatment largely passivates the defects present in the nanosheets. The as-synthesized NHSs form p-n junction demonstrating wide spectral range right from the ultra violet (UV) to the NIR region; allowing it to interact with wider radiation range vis-à-vis the pure MoSe2 nanosheets or Cu2S structures. Presence of surface plasmons in these NHSs increases light matter interaction through light trapping phenomenon. The MoSe2-Cu2S NHSs based photodetector devices have been fabricated by very simple, reproducible and low cost solution based techniques. The fabricated devices exhibited high photoresponse properties in the visible to NIR wavelength region with a maximum responsivity and detectivity of 410 mAW-1 and 2.72 ×1012 cm Hz1/2 W-1, respectively. The MoSe2-Cu2S devices exhibit high photo-to-dark current ratio around 3500-fold as compared to MoSe2 devices where it shows only 26-fold enhancement. This study reveals the superior device performance of MoSe2-Cu2S over only MoSe2 based hybrid devices on Si platform for fabrication of next generation photodetectors and ultra-dense nanophotonic integrated circuits.41

RESULTS AND DISCUSSION: The MoSe2-Cu2S NHSs have been synthesized via colloidal route using defect-rich single layered MoSe2 NSs; the details are given in the experimental section and a schematic is shown in Figure 1a. These structures are referred to as “vertically grown MoSe2-Cu2S NHSs”. The defect-rich MoSe2 NSs were passivated with DDT, subsequently Cu2S NPs were grown vertically on single 4 ACS Paragon Plus Environment

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monolayer (ML) MoSe2 NSs. This method establishes an intimate bonding between MoSe2 and Cu2S NPs. Individually single ML MoSe2 NSs and Cu2S NPs have also been synthesized for clearly seeing the differences upon making NHSs. The MoSe2 NSs have a large number of Se vacancies as confirmed by quantitative elemental detection using ICP-MS and EDS (Table S1, Supporting Information). The signatures for the defects due to the presence of Se vacancies are also evidenced from the Raman spectra shown in Figure 1f which is discussed below. These defects have an affinity for thiol molecules as it culminates in passivating the vacancies and functionalizing the surface of MoSe2 NSs.42 Generally, defect-less MoSe2 NSs are free of dangling bonds and therefore it is difficult to functionalize the surface. Thus defect-rich NSs are advantageous for the synthesis of NHSs as molecules like thiol can be used to readily functionalize the surface due to the presence of vacancies, principally the unsaturated Mo edges.43 CuI molecules easily react with thiol passivated MoSe2 NSs enabling the vertical growth of Cu2S NPs on the surface of MoSe2 NSs. The long hydrophobic alkyl chains of DDT and OLAM provide better solubility of the NSs in organic solvents as well as prevent the aggregation and stacking of the NSs.44 Excess DDT and OLAM were removed by repeated cycles of washing with a 2:1 toluene:ethanol mixture. The structural, optical, and compositional analyses verify the formation of vertical p-n NHSs with intimate contact. As seen in Figure 1b, PXRD patterns reveal hexagonal crystal structures of the MoSe2 NSs and Cu2S NPs. In case of MoSe2 NSs, the PXRD pattern matches well with the 2H phase of MoSe2 (JCPDS #87-2419). The growth of MoSe2 NSs along (002) direction determines the number of monolayers present in MoSe2 NSs. The single ML morphology of MoSe2 NSs has been confirmed by the absence of (002) peak owing to the limited number of planes along this direction; there cannot be two reflected waves for interference if there is only one plane. An expanded view 5 ACS Paragon Plus Environment


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of the diffraction pattern is shown in Figure 1c which establishes the absence of the (002) reflection, thereby confirming the sample is very thin along this direction. The single layer formation of MoSe2 NSs has also been confirmed from AFM as shown in Figure 1d along with the height profile in 1e confirming the height of the film is 1.04 nm, confirming to approximately 1 ML thickness of MoSe2. There is coexistence of both the hexagonal MoSe2 phase and the hexagonal Cu2S phase in as-synthesized NHSs as seen from the diffraction pattern in Figure 1b for the MoSe2-Cu2S NHSs. The presence of same crystal symmetry both in MoSe2 and Cu2S and electrostatic interaction between DDT ligands and Se vacancies are reasons why epitaxial Cu2S grows on the surface of MoSe2 NSs.39,45 Also present is the defect-rich Cu1.97S orthorhombic phase whose characteristic signature (004) reflection is visible in the NHS pattern. Further evidence of this phase is also found in the HRTEM images discussed below. The presence of defect states can be evidenced from the Raman spectra as shown in Figure 1f. Three notable signatures are seen viz. the two peaks at 232.6 cm-1 and 287.4 cm-1 corresponding to the A1g and E2g transitions, respectively, along with a defect related peak at 251.1 cm-1. The intensity of the defect related feature indicates the presence of a large number of selenium (Se) vacancies in the as-synthesized MoSe2 NSs. The intensity of this peak drastically reduces after DDT treatment suggesting the removal of the defects. Eliminating or reducing such type of defects is required for the improvement of optical and transport properties of NSs.24,46 Due to the disturbance of crystal symmetry in the defect-rich NSs, other vibration modes also appear at 322.1 and 351.2 cm-1. With decreasing defects in MoSe2 NSs, there is a dampening of the signature defect peak as well as a shift towards higher wave number.47 The Raman spectra for the MoSe2 sheets after DDT treatment shows the diminished defect-related transitions suggesting passivation of the defects by the thiol chains. It is quite possible that the added DDT could lead to partial 6 ACS Paragon Plus Environment

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formation of MoS2. However, no signatures of MoS2, which are seen at 384 and 410 cm-1,48 were observed in any of the sample studied.

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Figure 1. (a) Schematic illustration of the synthesis of MoSe2-Cu2S NHSs; (b) PXRD pattern of pure MoSe2 NSs, pure Cu2S NPs, and MoSe2-Cu2S NHSs along with the JCPDS pattern of pure materials; (c) expanded view of diffraction pattern along (002) direction; (d) AFM image of MoSe2 NSs; (e) height profile showing a thickness of ∼1.04 nm, as measured along the red lines; (f) Raman spectra indicates that DDT treatment largely passivates defects.

We observe very interesting changes in the optical properties upon growth of Cu2S NPs over MoSe2 NSs. Figure 2a shows the optical absorption spectra of vertical MoSe2-Cu2S NHSs 7 ACS Paragon Plus Environment


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along with the spectra of individual MoSe2 NSs and Cu2S NPs. For MoSe2 NSs, the excitonic peaks are observed at 795 and 693 nm, similar to the previous report for single ML NSs.21 The excitonic features originate from the split-off valance band formed from the heavier Mo atom.49,50 The Cu2S NPs exhibit a short wavelength absorption edge below ~ 650 nm followed by an increase in absorption at higher wavelength. The lower wavelength absorbance is attributed to the band edge absorption32 whereas the higher wavelength i.e. NIR absorbance of the Cu2S NPs is due to the localized surface plasmons which are mainly derived from the oscillation of holes.32,51 Alivisatos and co-workers32 showed that Cu2-xS NPs plasmon characteristics depends upon the free charge carrier density of the NCs – the Cu vacancies in the material indicating the prevalence of hole doping. The absorption spectrum of the NHS shows some intriguing features that are (i) lower absorbance between 600-1000 nm compared to MoSe2 NS and (ii) more intense, red-shifted plasmonic feature beyond ~1000 nm in the NIR region. The lower absorbance could be due to etching of some MoSe2 patches which give way to formation of Cu2S NPs on the monolayer. The difference in the absorption spectra of the untreated vis-à-vis DDT treated MoSe2 NSs suggests a decrease in the absorbance value at wavelength > 500 nm as seen in Figure 2b. It is observed that 600-1000 nm is the region where Cu2S hardly has any absorption and therefore most of it is from MoSe2 only – removal due to etching should lower the absorbance in this region. The NHS spectrum is not simply an addition of the constituents which implies an interaction between MoSe2 and Cu2S. Then the NIR absorption band of as-synthesized NHSs is red shifted as compared to the Cu2S NPs as shown in the same graph. Similar shifts are also observed in MoSe2-Cu2S NHSs with different Mo:Cu ratios (Figure S1 in Supporting Information). The plasmon oscillation frequency of the material depends upon the carrier concentration of holes according to equation ωp =

Nhe2 ɛ0mh

(where ωp is plasma frequency Nhis density of free holes, mhis the effective mass of 8 ACS Paragon Plus Environment

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hole).32 So, the red-shifting of plasmon frequency in case of NHSs attributed to the delocalization of holes from Cu2S to MoSe2 in NHSs due to the favorable type-II band alignment. Further we intend to investigate whether similar changes would be seen upon mere addition of the two components i.e. the NSs and the NPs rather than chemically grafting the NPs on the NSs. Looking at the results shown in Figure 2c,d it can be concluded that a physical mixture of Cu2S and MoSe2 does not lead to such type of phenomenon. Essentially, the absorption spectra of the physical mixtures show diminished plasmonic features. We can definitely state there is no enhancement in all the different ratios that we have studied. Formation of an intimate p-n junction at the interface, as is the case with the NHSs, is therefore an essential condition for the shifting of plasmon frequency. It is well known that Cu2S is a p-type semiconductor due to the presence of Cu vacancies.31 Due to the presence of Se vacancies in MoSe2 NSs, it is predicted to be an n-type semiconductor; also confirmed by Mott-Schottky analysis (Figure S2 in Supporting Information). A careful examination of the absorption spectra of the physical mixtures in Figure 2c,d reveals a slightly diminished plasmonic feature compared to pure Cu2S NPs. The reason might be the shielding of light for Cu2S due to the presence of large size of MoSe2 NSs, leading to a less intense plasmon band. All this definitely suggests that the two materials come in intimate contact forming a p-n junction at the interface. In these NHSs, not only do we have the presence of plasmons, the absorption window is much wider compared to pure MoSe2 NSs, thereby having a potential to absorb more solar radiation.



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Figure 2. Absorption spectra of (a) pure MoSe2 NSs, pure Cu2S NPs and MoSe2-Cu2S NHSs; (b) untreated and DDT treated MoSe2 NSs; (c) and (d) physical mixture of MoSe2 NSs and Cu2S NPs in different weight ratio, where (c) Cu2S NPs were added to MoSe2 NSs solution and (d) MoSe2 NSs were added to Cu2S NPs solution. In both cases the plasmon intensity is diminished upon physical mixture, suggesting intimate contact is not formed between them through physical mixture. All spectra were recorded in tetrachloroethane.

The TEM images of the bare MoSe2 NSs are shown in Figure 3a,b. These images show the ultrathin nature of MoSe2 NSs with a large number of vertically aligned edges. Due to the presence of defects there is crack in the basal planes which causes folding in NSs.52 This type of folding is common phenomenon in 2D materials.53 Upon Cu2S growth certain changes are evidenced, as shown in Figure 3c-f. Figure 3c represents the schematic of MoSe2-Cu2S vertical NHSs where 10 ACS Paragon Plus Environment

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Cu2S NPs are grown vertically over the surface of MoSe2 NSs. MoSe2 NS consists of the hexagonally packed plane of Mo atoms sandwiched between two planes of Se atoms and Cu2S NPs also exhibit the same hexagonal crystal symmetry. The same crystal symmetry between these two materials is one of the reason of epitaxial growth of Cu2S over the surface of MoSe2 NSs. The TEM images of MoSe2-Cu2S NHSs clearly show the uniform distribution of Cu2S NPs over the surface of MoSe2 NSs with no free Cu2S NPs outside the NSs, as shown in Figure 3d. For more images the reader is referred to Figure S3 (Supporting Information). The Cu2S NPs were not removed from the NSs even upon repeated washing and sonication, implying formation of a strong junction between these two constituent materials. High-resolution TEM (HRTEM) image in Figure 3e shows that the Cu2S −MoSe2 NHSs consist of three components: Cu2S, Cu1.97S, and MoSe2. Three sets of lattice fringes are clearly visible in Figure 3e, where lattice fringe of 0.28 nm and 0.65 nm are indexed to (101) and (002) planes of MoSe2, respectively. Figure 3e also suggests the presence of two types of crystal of Cu2-xS i.e. Cu2S and Cu1.97S. The lattice fringes of 0.17 nm and 0.24 nm are indexed as (112) and (102) planes of Cu2S, respectively, whereas the fringes of 0.34 nm, 0.26 nm are indexed as (004), (10 00) planes of Cu1.97S, respectively. Both Cu1.97S and Cu2S have almost the similar lattice fringes having low lattice mismatch (2.5%) and hence, both of these phases are formed together.38 The analysis here strongly attests to our hypothesis that there are interfaces, or junctions, formed between these materials. Energy dispersive spectroscopy (EDS) mapping shown in Figure 3f represents the uniform spatial distribution of Mo, Cu, Se and S in the MoSe2−Cu2S NHSs. Overall the samples have been well-characterized and enough evidence can be seen that suggests there is epitaxial growth of Cu2S on MoSe2 and it is not the simple physical mixture. (a) Raman spectra suggests that there is a passivation of defects after DDT treatment. The adsorbed



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thiol molecule on the surface is the source of sulfur for Cu2S. Defect passivation would not happen in case of simple physical mixtures. (b) We have investigated in details the absorption spectra of physical mixtures of MoSe2 and Cu2S, where the features are completely different from the NHSs features. In NHSs the plasmon features are red-shifted as compared to the Cu2S NPs due to delocalization of holes in the larger heterostructures. However, for physical mixture such shifts will not be observed. (c) The Cu2S NPs were not removed from the NSs even upon repeated washing and sonication, implying formation of a strong bond between these two constituent materials. (d) The HRTEM images also suggest the formation of vertical NHSs through epitaxial growth.

Figure 3. TEM images of defect-rich MoSe2 NSs: (a) 20 nm and (b) 5 nm scale bar. The images show the presence of sheet like morphology with large number of vertically aligned edges; (c) Schematic illustration of the MoSe2-Cu2S vertical NHS (d) TEM image of MoSe2-Cu2S NHS


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shows Cu2S NPs are distributed over MoSe2 surface; (e) HRTEM image shows the presence of MoSe2, Cu2S and Cu1.97S in the heterostructure; (f) EDS mapping images showing the distribution of all the elements in the NHS.

Having thoroughly characterized the MoSe2-Cu2S NHSs, photodetectors were fabricated with these NHSs grown on p-Si. The back contact was Al whereas ~15 nm thick Au islands were deposited on the top, ensuring only a small fraction of light is absorbed by the Au electrode. The layered structure consisting of Al/p-Si/MoSe2/MoSe2-Cu2S/Au is depicted in Figure 4a. Here, the device area is defined as the area of the Au contacts. Also, we would like to mention that the contact formed between Al and p-Si is an Ohmic contact. A linear current-voltage relationship is observed for this contact. A thin layer of MoSe2 was deposited on Si surface to assure there is no flow of the charge carriers under dark conditions. We observe that the dark current is suppressed by almost two orders of magnitude upon insertion of a thin (~5 nm) MoSe2 layer on top of the Si substrate as shown in Figure S4 in Supporting Information. Similar thickness, ~60 nm, of the active layer is maintained in all the devices studied. Representative cross-sectional SEM images of the spin-coated samples on Si substrates are shown in Figure S5 in Supporting Information. Typical semi-log plot of the current voltage (I-V) characteristics of the only MoSe2 and MoSe2Cu2S devices under dark and illumination condition are depicted in Figure 4b. The asymmetric nature of dark I–V characteristics confirm the formation of good quality p-n junction in the fabricated devices. The MoSe2 devices display a very low dark current ~5 × 10−8 A at -2.0 V bias as shown in Figure 4b which is enhanced upon light illumination as a consequence of the generation and transportation of charge carriers. The dark current does not alter much when the defect-rich NSs are treated with DDT, but for DDT-treated MoSe2 NSs the photocurrent is



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increased as compared to the defect-rich MoSe2 NSs. The improvement of the photocurrent in DDT-treated NSs is a direct consequence of the improvement in the transport properties of the material due to the elimination of defects as observed from the Raman spectra.24 The reverse current of both the devices increases in presence of light than dark condition. Under 514 nm laser illumination, the photo to dark current ratio, i.e.

Iph ― Id Id

(Iph is photocurrent and Id is dark current)

of approximately 26 and 3,500 are achieved at -2.0 V bias, for DDT-treated MoSe2 and MoSe2Cu2S devices, respectively. Devices were fabricated without the presence of any active materials through similar route, i.e. only Si. These show hardly a factor of 2 enhancement in the presence of light attesting for the fact that the enhancements are due to the nanostructures. The remarkable improvement in the photo current in MoSe2-Cu2S device is due to the generation and collection of large number of photo excited minority carriers in comparison to the MoSe2 devices. This indicates the formation of a superior photo diode using MoSe2-Cu2S material, which can potentially be exploited in photo detector device fabrication. The pronounced light-matter interaction owing to the presence of plasmon and widening of the absorption windows is one of the prime rationale for enhanced photocurrent. Both MoSe2 and MoSe2-Cu2S have been used to demonstrate the photodetection in reverse bias condition. The mechanism behind the operation of MoSe2-Cu2S heterostructures based photo detector can be explained by the energy level diagram shown in Figure 4c.31,54,55 This energy level ordering is w.r.t. vacuum and upon intimate contact, the Fermi levels will get aligned as shown in Figure S6 in Supporting Information. Also, while Cu2S is shown to be in contact with Si, and such an arrangement is beneficial for the device, this cannot always be guaranteed as the MoSe2Cu2S NHSs are spin-coated and hence it is not possible to predict their orientation. In any case, while Cu2S is in contact with Si, the holes that naturally reside here due to the formation of a Type14 ACS Paragon Plus Environment

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II heterostructure, are therefore easily transferred to Si. Upon photoexcitation, the electron moves towards MoSe2 while the hole goes over to Cu2S which is then passed on to the circuit via Si once a negative potential is applied there. In a similar fashion, the electron, which resides on MoSe2 in the Type-II heterostructure now moves over towards Au. Again we must be cautious in assuming that this is the only orientation. A reverse orientation of the NHSs, i.e. Au in contact with Cu2S and Si adhered to MoSe2, would certainly impede this photocurrent. However, since we observe an enhancement, Cu2S pointing towards Si and MoSe2 forming contact with Au is a more likely situation here. The devices are stable over a repetitive measurements and this is shown by means of chronoamperometric studies. Figure 4d exhibits the dynamic time response of the fabricated devices upon pulsed optical excitation (λ = 514 nm) at an applied bias of -2.0 V at room temperature. Upon illumination, the device current increases sharply and stabilizes in a high conductivity state (ON state) and switches back quickly to a lower conductivity state (OFF state) in dark condition for both devices. The figure shows very high ON/OFF ratio of MoSe2-Cu2S than MoSe2 which corroborates with the I-V characteristics shown in Figure 4b. This clearly indicates efficient extraction of photo-carriers in the MoSe2-Cu2S due to a higher built-in electric field as compared to the planar heterojunction MoSe2 devices. This figure also indicates that the photoresponse of both the devices are very steady and reproducible over repeated cycles. The behavior of MoSe2 and MoSe2-Cu2S heterojunction diodes under illumination indicates the NHSs is a potentially attractive material for photodetector and optical switching device applications.



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3

Voltage(V)

(d)

MoSe2

8.3E-7

2.5E-4

MoSe2-Cu2S

2.0E-4

Current (A)

6.3E-7 ON

ON

ON

1.5E-4

4.3E-7 1.0E-4 2.3E-7

5.0E-5 OFF

OFF

OFF

OFF

3.2E-8 0

(e)

10

20

30

Normalized Scale

MoSe2

0.2

MoSe2-Cu2S 0.1

50

60

70

MoSe2-Cu2S

1.25

Absorbance Responsivity

0.4

0.3

40

7.6E-8

Time (s)

(f)

0.5

Responsivity (A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.00 0.75 0.50 0.25 0.00

500

600

700

800

900

600

Wavelength (nm)

800

1000

1200

1400

Wavelength (nm)

Figure 4. (a) Schematic diagram of Si supported MoSe2-Cu2S photodiode; (b) semi-log plots of the current–voltage characteristics of MoSe2 NSs with and without thiol, MoSe2-Cu2S NHSs in the dark and under illumination of 514 nm laser source: solid lines represents dark characteristic while spherical dots typify bright characteristic; (c) energy levels of the materials used in the


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device; (d) transient photoresponse recorded by switching the light source (514 laser light) ‘ON’ and ‘OFF’ for MoSe2 and MoSe2 - Cu2S devices at -2.0 V bias. Note the difference in the current scale bars of the two devices; (e) spectral responsivity of the devices show broadband response of MoSe2-Cu2S device from visible to NIR region over only MoSe2 device; (f) Responsivity along with the absorption spectrum of MoSe2-Cu2S NHSs.

Table 1: Parameters determining the efficiency of devices at -2.0 V bias

Photo to dark

Responsivity

EQE

Detectivity

LDR

current ratio

R(λ)

(%)

(cm Hz1/2 W-1)

(dB)

(mA/W) MoSe2

26

320

68

3.54 ×1012

35.6

MoSe2-Cu2S

3.5 × 103

410

82

2.72× 1012

51.2

We have investigated in detail the spectral characterization of the devices. These parameters are listed in Table 1. The spectral responsivity, R(λ), which is the ratio of the photocurrent density, 𝐽𝑝ℎ, to the intensity of incident light, 𝑃𝑜𝑝𝑡 of the devices are measured at -2.0 V applied bias as shown in Figure 4e. The highest peak responsivity for MoSe2-Cu2S devices is found to be ~ 410 mA/W at -2.0 V bias, which is higher than that of 320 mA/W observed for MoSe2 control device. Since the photocurrent depends upon the light absorbed, the responsivity should have some dependence on the absorption spectra. Although we cannot claim this for the lower responsivity at the extreme wavelengths due to the poor performance of the Si detector at these ends, the minimum in the responsivity at ~850 nm seems to be directly a consequence of the 17 ACS Paragon Plus Environment


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low absorption in this region in the case of the NHSs as seen in Figure 4f. Neither the absorption nor the responsivity do not show such a dip in MoSe2. Further, in case of MoSe2-Cu2S devices along with increase of overall responsivity, the spectral region is broader in NIR region as compared to the MoSe2 devices. The increase of responsivity in NIR region is attributed to the presence of surface plasmon in MoSe2-Cu2S devices which is also observed in absorbance. The enhancement of responsivity is the result of generation of more charge carriers due to increased light-mater interactions in the NHSs and efficient separation of carriers in the depletion region by a built-in field under reverse bias condition. The bias dependent peak responsivity shows in Figure S7a (Supporting Information) that it linearly increases upto -1.0 V and saturates at higher applied voltages for both the devices. So these NHS materials are very promising for the application of the low power photodetector devices. Other spectral characterization of the devices such as EQE, detectivity and linear dynamic response measurements have also been performed. The details of the measurements are provided in the SI. In all cases the MoSe2-Cu2S device performance is superior to the devices based on pristine MoSe2 NSs. Furthermore, the responsivity, detectivity and the EQE, all have an enhancement in the NIR region (Figure S7, Supporting Information) – a direct effect of the increase in the absorption spectrum in this region due to the surface plasmon resonance. A comparison of the present device characteristics with some of the recent reports on photodetectors using 2D TMDs in 2 terminal photodiode configuration is presented in Table S2. This supports the better characteristics and device quality of the present materials employed.

CONCLUSIONS: In summary, we have developed a strategy for the synthesis of vertical p-n NHSs by an efficient colloidal route. Defect-passivated synthesis using thiol is a unique protocol to design NHSs as it


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builds an intimate contact between the constituents and also improves the optoelectronic properties of the materials by reducing defects. The presence of plasmon and broad range of spectral wavelength in NHSs increase the light-matter interaction. The fabricated devices using the NHSs show better photoresponse behavior as compared to only MoSe2 devices. This improvement is due to the synergistic effect of formation of p-n junction, enhance light matter interaction and passivation of defects of NSs. We believe our fundamental studies open up the possibility for construction of new p-n junction NHSs beyond only TMDs based heterostructure for photonic device applications.

EXPERIMENTAL SECTION: Materials. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24.4H2O, 98%, SRL), acetyl acetone (C5H8O2, 98%, SDFCL), selenium powder (Se, 99.99%, Aldrich), oleylamine (C18H35NH2, Technical grade 70 %, Aldrich), 1-butanethiol (C4H10S, > 98%, Merck), 1-dodecanethiol (C12H28S, ≥ 98%, Merck), copper iodide (CuI, 99.9%, Spectrochem), gold (Au, 99.9% Aldrich), nitric acid ( (HNO3, assay 69-70%, Fischer Scientific), toluene (C6H5CH3, 99%, Fischer Scientific) ethanol (C2H5OH, absolute, Merck), were used as received. Double distilled water was employed throughout the study. Synthesis of MoSe2 nanosheets. Synthesis of Mo-precursor i.e. MoO2(acac)2 was performed via cleavage of (NH4)6Mo7O24·4H2O using acetyl acetone.56 The as-synthesized MoO2(acac)2 (0.163 g, 0.5 mmol) was dissolved in 22.5 mL oleylamine (OLAM) at 150 ºC in argon (Ar) atmosphere. Se precursor was prepared by suspending Se powder (0.240 g, 3 mmol) in 3.14 mL OLAM followed by addition of 1-butanethiol (0.430 mL) under inert condition. Freshly prepared Se precursor57 was (2.63 mL, 2 mmol) swiftly injected into the Mo precursor solution and increased



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the temperature at 270 ºC and kept for 4h. After cooling to room temperature (RT) in inert atmosphere, the product was purified with addition of toluene and ethanol mixture (2:1 by volume). Synthesis of MoSe2-Cu2S heterostructure. 40 mg of pre-synthesized MoSe2 NSs were dissolved in 15 mL OLAM which was degassed under vacuum for 30 min and then under Ar atmosphere for 15 min with stirring at 150 ºC. After that, DDT (200 µL) was added into the solution and stirring was continued for another 30 min to allow for the thiol to cap the MoSe2 NSs. The Cu-precursor was prepared by dissolving CuI (0.2 mmol, 0.04 g) in 2 mL distilled OLAM in the glove box under constant stirring at 60 ºC. Freshly prepared Cu precursor was swiftly injected into the mixture at this temperature. Finally 200 µL DDT was added and the temperature was raised to 210 ºC and the reaction mixture was heated for 60 min. The product was purified three times with addition of toluene and ethanol mixture (2:1 by volume). Synthesis of Cu2S nanoparticles. First, CuI (0.038 g, 0.2 mmol) was dissolved in 2 mL OLAM in glove box and the solution was added to 15 mL degassed OLAM in 25 mL four-neck round bottom (RB) flask under inert atmosphere. The temperature was then increased to 150 ºC. Then 200 µL 1-dodecanethiol was added and the temperature was raised to 210 ºC and heated for 60 min. Finally the reaction mixture was cooled down to room temperature. The same washing procedure that was used for MoSe2-Cu2S heterostructures was employed here as well. Device fabrication. Phosphorus doped p-type single side polished Si (100) substrates, resistivity 7–14 Ωcm, were first degreased in acetone and isopropanol by sonication, followed by rinsing with deionized (DI) water (resistivity of DI ~ 18 MΩ cm). Thereafter these were cleaned in piranha solution containing H2SO4 (97%) and H2O2 (35%) in a volume ratio 3:1 for 15 min at room temperature followed by several rinses with DI water. To remove the native oxide layer formed by


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the piranha treatment, samples were etched with 2% HF aqueous solution for 2–3 min at room temperature. The MoSe2 and MoSe2 −Cu2S films were coated on pre-cleaned p-Si substrates by spin coating technique. The coated film was dried at 70 °C for 10 min. For the fabrication of heterojunction photodiodes, Au electrodes (0.2 mm2) were deposited by thermal evaporation on top of the MoSe2 and MoSe2−Cu2S films under a base pressure of ~ 1 × 10−6 torr. Aluminum was deposited over a large area of back side of p-Si substrate for achieving an Ohmic back-contact.

Characterization techniques. Powder X-ray diffraction (PXRD) studied of dried powders were carried out on a Bruker D8 Advance diffractometer with Ni-filtered Cu−Kα radiation. UV-VisNIR absorption spectra were acquired on Perkin-Elmer Lambda 1050 spectrophotometer. Dilute solutions of samples in tetrachloroethylene (TCE) were placed in four window 1 cm quartz cuvettes. The slit width was 2 nm in visible region and 20 nm in NIR region during collection of spectra. In case of physical mixture studies, we made solution of MoSe2 and Cu2S by dissolving 1 mg/mL in toluene of each. In a cuvette we subsequently added Cu2S into MoSe2 solution by weight (1:1, 1:2, 1:3) ratio and vice versa. Transmission electron microscopy (TEM) images were obtained using JEOL JEM-1400 Plus operating at 120 KV accelerating voltage. High resolution TEM imaging and energy dispersive spectroscopic (EDS) mapping analysis were performed using JEOL 2100F operating at 200 kV. In both of the above instrument LaB6 filament was used and the images were captured in bright field mode. Cross-sectional SEM images of the spin-coated films were obtained using Zeiss EVO 18 system. Raman measurements were performed in Renishaw plc micro Raman Spectrophotometer using laser wavelength of 532 nm. For Raman measurements, the samples were prepared by drop casting diluted solution of samples onto glass slide. Elemental analyses were performed using Agilent 7900 ICP-MS instrument, the samples were digested in



Page 22 of 31

nitric acid solution. AFM imaging were performed in a Veeco Nanscope-IV system. The currentvoltage (I−V) characteristics of the heterojunctions were studied in two probe method using a Keithley semiconductor parameter analyzer (model no. 4200-SCS). For the photocurrent measurement we used the 514 nm laser with power of ~50 mW. External quantum efficiency (EQE) was measured using a Newport quantum efficiency/IPCE measurement system equipped with a lock-in amplifier, monochromator, and a broad-band light source. Responsivity was measured at a bias of -2.0 V in the range of 400-1000 nm by using a monochromator. The power of the illuminated incident light is ~ 0.1 mW. The LDR measurements were carried out using 514 nm laser having intensity of 1.13 μW.

Supporting Information: Elemental analysis of MoSe2 NSs; Mott-Schottky plot of MoSe2 NSs; absorption spectra of MoSe2-Cu2S NHSs having different Cu2S ratio; TEM and HRTEM images of MoSe2-Cu2S NHSs; external quantum efficiency, detectivity and linear dynamic range measurements of devices. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID: 0000-0002-1778-2884 ACKNOWLEDGMENTS M.S.H. acknowledges UGC for financial support. SB acknowledges the Science and Engineering Research Board (SERB), India for the financial support (Project no: PDF/2016/001182). We are


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thankful to AIRF, JNU for the HRTEM images. S.S. acknowledges Central Research Facility, IIT Delhi for instrument facilities and DST CERI grant no. DST/TMD/CERI/C166(G) for partial financial assistance.

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MoSe2-Cu2S Vertical p-n Nanoheterostructures for High-Performance

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