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Letter

Functionalized Molybdenum Disulphide Nanosheets for 0D-2D Hybrid Nanostructure: Photoinduced Charge Transfer and Enhanced Photoresponse Razi Ahmad, Ritu Srivastava, Sushma Yadav, Dinesh Singh, Govind Gupta, Suresh Chand, and Sameer Sapra J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00243 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017

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The Journal of Physical Chemistry Letters

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Functionalized Molybdenum Disulphide Nanosheets for 0D-2D Hybrid Nanostructure: Photoinduced Charge Transfer and Enhanced Photoresponse

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Razi Ahmada,b , Ritu Srivastavaa*, Sushma Yadavb, Dinesh Singha, Govind Guptac , Suresh Chanda, and Sameer Saprab

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a

Center for Organic Electronics, Physics of Energy Harvesting Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India b

Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India-110016

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c

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Abstract

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The high concentration stable dispersion of free standing mono- or few layer transition metal

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dichalcogenide (TMDs) nanosheets (NS) remains a significant barrier for their application in

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solution processed optoelectronic devices. Here, we report oleylamine (OLA) and

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dodecanethiol (DDT) assisted exfoliation of MoS2 NS in non-polar organic solvent 1,2

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dichlorobenzene (DCB) which enable high concentration stable dispersion of free standing

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mono- or few layer NS. The functionalized MoS2 NS were further utilized for the fabrication

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of solution processed 0D-2D hybrids of CuInS2 quantum dots (CIS QDs) and MoS2 NS. The

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strong photoluminescence (PL) quenching and decreased PL lifetimes of CIS QDs attached to

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MoS2 NS indicates efficient charge transfer from photoexcited CIS to MoS2 NS. The

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photocurrent of CIS/MoS2 hybrid device is dramatically enhanced compared to pure CIS and

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pristine MoS2 based devices, confirming that efficient charge separation and transfer occur

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from CIS QDs to MoS2 NS.

Advanced Materials & Devices, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India

1 ACS Paragon Plus Environment


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Atomically thin two-dimensional transition metal dichalcogenide (2D-TMD) have

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attracted great research interest owing to their unique layer dependent optical, electronic and

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catalytic properties. The most widely investigated 2D-TMD such as monolayer MoS2 have a

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direct band gap of 1.9 eV compared to the indirect bandgap of 1.2 eV in bulk state. The

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indirect to direct band gap transition from bulk state to mono or few layer limit open up

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several areas of application such as optoelectronic1–4, sensing5 and catalysis.6

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In recent years, a tremendous effort has been made to produce high quality atomically

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thin TMD nanosheets. To date, various synthetic approach such as micromechanical

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cleavage4, lithium ion intercalation7 and liquid phase exfoliation8 has been developed.

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However, physical exfoliation method produced high-quality monolayer TMDs in limited

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quantity. Lithium ion intercalation produced large quantity monolayer TMD sheets on the

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cost of semiconducting to metallic transition due to crystal distortion. Among the above-

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mentioned technique, liquid phase exfoliation is the most promising approach towards large

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scale production. TMD can be exfoliated directly in several solvents such as N-methyl-2-

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pyrrolidone (NMP), N, N-dimethylformamide (DMF) and isopropanol (IPA) by the aid of

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ultra-sonication to overcome the van der Walls interaction between adjacent layers. The 2 ACS Paragon Plus Environment

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exfoliated nanosheets are stabilized by the solvent to overcome the restacking. The solvent

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stabilized exfoliated nanosheets is limited by low concentration yield. Moreover, surface

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functionalization of pristine TMD nanosheets is another approach to obtain a high-yield

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stable dispersion. Functionalized 2D TMDs also allow tuning of their optical and physical

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properties by combining them with nanocrystals such as CdSe9, SnSe10 and PbSe11 to the

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fabrication of hybrid nanostructures. Previous reports demonstrate that functionalization of

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reduced graphene oxide (rGO) not only enhance their solubility but also provide anchoring

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site for nanocrystal attachment.12 Surface functionalization is necessary for QDs attachment

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as it provides hydrophobic interaction between QDs and GO13 or SWCNT.14 The exfoliated

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nanosheets have several defect sites such as point defects and sulfur vacancies on the basal

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plane and edges. These defects sites act as an active centers for adsorption of molecule such

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as thiols and amines.15,16 The surface passivation at sulfur vacancy by several thiol molecules

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has been reported in recent publications.16,17 The other approach for the passivation of sulfur

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vacancy at surface or edge site can be achieved by exfoliation of TMDs in the presence of

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suitable surfactant or ligands. During sonication, ligands, or surfactants, get immediately

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adsorbed at sulfur vacancy sites of the exfoliated nanosheets. This process produces

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exfoliation of mono or few layer functionalized nanosheets in high yields. However, to date

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most of the published reports on surfactant assisted exfoliation were carried out in aqueous

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medium.18–22 To make TMDs an effective component for solution processed organic

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optoelectronic devices, it is necessary to obtain TMDs dispersions in a non-polar organic

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solvent. For this purpose 1, 2 dichlorobenzene is most suitable solvent and widely used to

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dispersed the SWCNT23 and graphene.24 Furthermore, it is reported that DCB provides a

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suitable medium for functionalization of graphene.12 To date, only a few reports are available

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for exfoliation of TMDs in DCB with less than 10 % exfoliation yield. Post-synthesis surface

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modification with alkyl-trichlorosilane (ATS) gives a high concentration of multilayer MoS2



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dispersion.25 However, post-synthesis surface modification for high concentration dispersion

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required a large volume of pristine exfoliated dispersion which is actually very laborious and

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time consuming due to the required repeated cycle of precipitation and dispersion.

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In this paper, we report oleylamine (OLA) and dodecanethiol (DDT) assisted, high

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yield exfoliation of MoS2 nanosheets in dichlorobenzene solvent. Exfoliation of MoS2 in the

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presence of DDT increases the exfoliation yield by more than four times and with OLA by

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five times compared to pristine MoS2 in dichlorobenzene. The high concentration monolayer

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rich dispersion is stabilized against aggregation or restacking by OLA and DDT, adsorbed at

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defect sites on the surface or edges.16,26 Furthermore, 0D-2D hybrid nanostructure of CuInS2

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QDs and MoS2 nanosheets was synthesized by mixing the DDT capped CIS QDs and DDT

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functionalized MoS2. The 0D-2D hybrid nanostructure of QDs decorated NS receives

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considerable attention due to their high performance optoelectronic application27. The ternary

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nontoxic CuInS2 offers high flexibility towards the facile band gap tunability. The CIS QDs

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were frequently investigated as light absorbing material in solution processed solar cell due to

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its high absorption coefficient and narrow band gap.28 The photophysics of hybrid

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nanostructure was investigated using steady-state and time-resolved fluorescence

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spectroscopy.

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The exfoliated NS have several defect sites such as sulfur vacancies at edges and

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basal planes. Previous reports16,29 demonstrated that amine and thiol containing organic

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molecules could fill these sulfur vacancies. OLA and DDT was chosen because it is

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commercially available and contains long alkyl chain with functional groups (amine or thiol)

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that would allow the passivation of sulfur vacancies. Moreover, the hydrophobic nature of

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long alkyl chain provide better solubility of functionalized NS in non-polar organic solvents.

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DDT and OLA functionalized MoS2 nanosheet was synthesized by liquid phase exfoliation of

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bulk MoS2 powder in nonpolar organic solvent dichlorobenzene in the presence of respective 4 ACS Paragon Plus Environment

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organic ligands. The detailed synthesis method is described in experimental section (see

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supporting information). Briefly, ligand induced exfoliation involves interaction of MoS2

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crystals with DDT or OLA followed by sonication process in dichlorobenzene. The

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schematic representation of synthesis process illustrated in Figure 1. During sonication

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process OLA or DDT ligand immediately adsorbed at the common defect sites of MoS2 such

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as sulfur vacancies on surface or edges. The hydrophobic non-polar long alkyl chain is

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exposed externally in DCB, thereby facilitating stable dispersion of exfoliated NS. The

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exfoliation of MoS2 in the presence of DDT or OLA molecules accelerate the exfoliation

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yield and increase the colloidal stability by preventing the restacking or aggregation of

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nanosheets, thus enabling a high concentration of the monolayer-rich dispersion. The

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exfoliation yields were calculated by directly drying the powder obtained from several

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cleaning cycles of precipitation and centrifugation and then taking a weight. The control

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sample without OLA and DDT ligands was also exfoliated in DCB under identical

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experimental conditions resulting in a light-green coloured dispersion with maximum

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concentration of 1 mg/ml. In our experimental method with the ligands, we are able to

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achieve concentrations as high as 5 mg/ml for OLA-MoS2 and 4 mg/ml for DDT-MoS2 where

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the color of the solution appears very dark-green (see supporting information Figure S1).

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Figure 1: Schematic illustration for the OLA or DDT assisted exfoliation of MoS2 nanosheets

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under sonication.



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The effect of OLA and DDT ligands on the exfoliation efficiency is initially examined by

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UV-Vis absorption spectroscopy. All the samples used for absorption measurements were

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diluted fifteen times for sake of comparison. Figure 2a shows the absorption spectrum of

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pristine MoS2, DDT-MoS2 and OLA-MoS2: all display four distinct excitonic features which

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are nearly identical except for slight variations in the peak positions. The four excitonic

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electronic transitions are commonly assigned as A, B, C and D excitons.7 The doublet peak

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for pristine MoS2 positioned at 671 and 609 nm is assigned to A and B excitonic interband

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transition at the K point of Brillouin zone of 2D MoS2. The doublet peak arises due to spin-

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orbit splitting of transition at K point. The peak position for C and D transition is located at

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454 and 400 nm and originates from the interband transition between occupied orbital

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and the unoccupied , and , orbitals.20 In comparison to the pristine MoS2, the

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peak position of A-exciton in OLA and DDT functionalized MoS2 is slightly blue shifted and

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centered at 667 and 666 nm, respectively. The blue-shift in A-exciton energy level of

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functionalized MoS2 relative to control sample, indicating an increase in the bandgap due to

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quantum confinement resulting from decrease in the average thickness30,31 of the nanosheets.

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To obtain the size-selected and monolayer enriched dispersion, centrifugation was carried out

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at 10,000 rpm for 30 min, and their UV-Vis spectra are displayed in Figure S2a (see

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supporting information). In particular, a further blue shift of 3 nm in the A-exciton peak for

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DDT-MoS2 were obtained relative to the sample centrifuged at 3000 rpm, indicating further

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decrease in average thickness of the NS. A quite similar trend is also observed for OLA-

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MoS2 and pristine MoS2 NS. The ligands assisted exfoliation of MoS2 into few or mono

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layered nanosheets were further confirmed by the emergence of photoluminescence (PL),

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which is not detected in pristine MoS2 (see supporting information, Figure S3a). The PL peak

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maxima for DDT-MoS2 and OLA-MoS2 centered at 693 and 705 nm correspond to

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photoluminescence energy of 1.79 eV and 1.76 eV. According to previously published


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reports32–36 for measurements carried out in the solid state, PL peak for monolayer MoS2 lies

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between 1.8 to 1.9 eV due to a direct interband transition from the top of the valence band to

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the bottom of the conduction band at K point in the Brillouin zone. However, our solution

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based PL is slightly red shifted to 1.79 eV for DDT-MoS2 and 1.76 eV for OLA-MoS2. We

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found similar red shift had been observed by Nguyen et al. and Voiry et al. for PL spectra of

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functionalized MoS2 and commonly attributed to the radiative recombination in shallow

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surface states which results in emission energy slightly lower than band gap.16,37 The shallow

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surface states most likely originate from adsorption of thiols or amines at the common defect

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sites of MoS2 such as sulfur vacancies on surface or edges.16 Furthermore, slight variations in

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decay kinetics were also observed between OLA and DDT functionalized MoS2 (see

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supporting information, Figure S3b).

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Further assessment of exfoliation efficiency with and without ligands was estimated by

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absorbance value of A-exciton. We use absorbance value for direct comparison between

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pristine MoS2, OLA-MoS2 and DDT-MoS2 which reflect the exfoliation yield. It can be seen

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from Figure 2b the absorbance values of A-exciton for MoS2, DDT-MoS2 and OLA-MoS2

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centrifuged at 3000 rpm was found to be 0.16, 0.38 and 0.55, respectively. Under the

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identical experimental conditions, the absorbance values are enhanced by 2.4 fold for DDT-

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MoS2 and 3.4 fold for OLA-MoS2. The optical images of MoS2, DDT-MoS2 and OLA-MoS2

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are shown in Figure S1 where color of the solution appears very dark green for OLA and

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DDT stabilized MoS2 compared to the light-green colored dispersion of pristine MoS2

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suggesting high concentration dispersion was obtained under identical conditions.

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(a)

(b)



0.8 MoS2 DDT-MoS2

Absorbance

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OLA-MoS2

0.6

0.4

0.2

0.0 600

700

800

Wavelength (nm)

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Figure: 2(a) UV-Vis absorption spectra of exfoliated P-MoS2, DDT-MoS2 and OLA-MoS2.

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All samples were sonicated for 6h and centrifuged at 3000 rpm. The as-synthesized sample

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was diluted fifteen times for recording the absorption spectra (b) Closer look of Figure 2a

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showing enhancement in absorbance of exfoliated nanosheets at a wavelength of 665 nm

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under identical experimental conditions.

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The shape and lateral size of exfoliated MoS2 nanosheets were investigated by

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transmission electron microscopy (TEM). Figure S4 (see supporting information) displays

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low and high magnification TEM images of P-MoS2 (a, b), OLA-MoS2 (c, d) and DDT-MoS2

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(e, f) centrifuged at 3000 rpm. The broad lateral size of nanosheets ranging from 50 to 250

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nm was obtained. The relative thickness of nanosheets could be visualized by their relative

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transparency. The thicker sheets appear darker while thinner nanosheet surface appears

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transparent. Compared to functionalized nanosheets, the pristine nanosheets shows some

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restacking and aggregation due to bare surface whereas functionalized nanosheets do not

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show any restacking or aggregation. The TEM image of DDT-MoS2 nanosheets centrifuged

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at 10,000 rpm shown in Figure 3a. The small and thin flakes with lateral size 50 -100 nm are

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clearly observed. Figure 3b displays high-resolution TEM image of a single layer MoS2

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nanosheet in which regular atomic arrangement is observed showing hexagonal closed pack

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symmetry throughout the sheet. The lattice spacing of 2.7 Å is assigned to (100) plane MoS2


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NS in the 2H phase.38 The selected area electron diffraction pattern (Figure 3c) of single layer

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MoS2 nanosheet show the regular hexagonal arrangement of the spot which in resemblance

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with HRTEM image, supports the single crystalline nature. The monolayer rich dispersion of

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DDT-MoS2 is further confirmed by atomic force microscopy (AFM) operated in tapping

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mode. Figure S5a, b (see supporting information) shows AFM images of DDT-MoS2 and

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OLA-MoS2 nanosheets, respectively on SiO2 coated silicon substrates. The corresponding

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average step height profile gives an average thickness of 1.2 nm for DDT-MoS2 and 2.4 nm

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for OLA-MoS2. The apparent AFM height of liquid phase exfoliated nanosheets are usually

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higher than actual monolayer thickness (0.7 nm) due to trapped residual solvent and adsorbed

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surfactant. The thickness of liquid phase exfoliated monolayer of MoS2 nanosheets ranging

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between 1-2 nm.31,39 In our experiment, the height of DDT-MoS2 was measured up to 1.2 nm

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thick corresponding to one monolayer and for OLA-MoS2 the thickness was 2.4 nm

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corresponding to two layers.



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Figure 3 (a) TEM images of DDT-MoS2 centrifuged at 10000 rpm (b) HRTEM image of

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single layer nanosheet (c) Corresponding selected area electron diffraction pattern.

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Raman spectrum of pristine MoS2, OLA-MoS2 and DDT-MoS2 are shown in Figure

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S6 (see supporting information). The pristine MoS2 exhibit strong bands at 383 and 408.1 cm-

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1

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respectively. The frequency difference (∆ k) between these two modes of vibration are very

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sensitive to the number of layers in nanosheets and frequently used to determine the number

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of layers in mechanically exfoliated nanosheets. However, liquid phase exfoliated nanosheets

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show different frequency difference and cannot be used to quantify the exact number of

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layers due to adsorbed molecules and surfactant.18 However, the decrease in frequency

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difference in exfoliated nanosheets comparison to the bulk counterpart gives the qualitative

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sign of efficient exfoliation. The frequency difference (∆ k) between E2 g and A1g peak for

which are assigned to the in-plane vibration ( E2 g ) and out- of- plane vibration ( A1g ) modes,


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pristine exfoliated nanosheets was found to be 25.1 cm-1 and decreased by 1.4 cm-1 compared

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to 26.6 cm-1 for bulk MoS2.25 The frequency difference for OLA-DDT and DDT-MoS2 was

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found to be 24.9 and 24 cm-1 respectively. The further decrease in ∆ k for functionalized

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nanosheets gives the qualitative sign of effective exfoliation in the presence of OLA and

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DDT ligand.

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The chemical composition and oxidation state of exfoliated MoS2, OLA-MoS2 and

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DDT-MoS2 were investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectra

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of Mo3d region in pristine MoS2 shown in Figure 4 consist several sets of the peak that can

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be assigned to S 2s, doublet of Mo4+ (Mo 3d5/2 and Mo 3d3/2) and Mo6+. Singlet of S 2s

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located at 226 eV, a doublet of Mo4+ at 229 (Mo 3d5/2) and 232 (Mo 3d3/2) and Mo6+ at 236

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eV consistence with previous reports.20,37 The Mo3d peak of OLA-MoS2 and DDT-MoS2

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display similar features. However, the intensity of oxidation peak for amine passivated MoS2

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is suppressed and completely vanishes for DDT-MoS2. The Mo6+ peak might originate from

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partial oxidation of Mo atoms at the edges or vacant sites of MoS2. After functionalization,

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the vacant sites is passivated by amine or thiol groups and act as a protective layer for further

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oxidation. The S 2p region of pristine MoS2 consist doublet at 161.9 and 162.9 eV were

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assigned to S 2p1/2 and S 2p3/2, respectively. Similar to the MoO3 peak in Mo3d region, a

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small peak at 168 eV are also observed in S 2p region and assign to oxidation peak of sulfur.

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The sulfur oxidation peak decreases for OLA-MoS2 and completely diminished in DDT-

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MoS2 due to surface passivation by respective ligands discussed above.



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Figure 4: Fitted XPS spectra of Mo 3d (left panel) and S 2p (right panel). The suppression of

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oxidation peak of both Mo and S atom were observed after surface functionalization by OLA

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and DDT ligand. In the S 2p spectra of DDT-MoS2 peak broadening at 164 to 166 eV and

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presence of C-S bond at 164.5 eV suggested successful functionalization.

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The functionalization of MoS2 by OLA and DDT ligands were confirmed by

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combination of FTIR (see supporting information Figure S7a, b and c) and XPS analysis.

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Figure S8 (see supporting information) takes a closer look at S 2p region. In comparison to

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pristine MoS2, S 2p peak of DDT-MoS2 is broadened between 164 to 166 eV. After careful

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deconvolution of S 2p peak, the presence of C-S bond can be ascertained. The corresponding

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peak is located at 164.5 eV which is absent in pristine MoS2 nanosheets.17 The S 2p peak

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broadening and presence of C-S bond indicates successful binding of thiols at sulfur

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vacancies. Similarly we also compared the XPS spectra of S 2p region (Figure S8) of DDT

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functionalized samples obtained by several cleaning steps from excessive DDT. The S 2p 12 ACS Paragon Plus Environment

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spectrum of DDT-MoS2 after first cleaning step resulted characteristics doublet along with

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more pronounced C-S binding peak appeared at 164.5 eV with 25 % of the S-atom on the

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surface, indicating higher degree of thiol conjugation. A gradual decrease in the intensity of

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C-S binding peak were observed after successive cleaning steps, showing removal of

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excessive DDT from the sample surface. After four repeated purification cycles the C-S

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binding peak was preserved with less intensity, indicating strong binding affinity of thiols to

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the surface of NS.

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functionalization of MoS2 by OLA and DDT ligand. The FTIR spectra of purified OLA-

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MoS2 and DDT-MoS2 along with native OLA and DDT are shown in Figure S 7a, b (see

10

supporting information). The asymmetric bands of aliphatic alkyl chain at 2852 cm-1 and

11

2924 cm-1 are attributed to the –CH stretching vibration from OLA and DDT ligand. The

12

presence of these features verifying the successful functionalization of MoS2 by OLA and

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DDT ligand. Furthermore, the closer look of FTIR spectra of DDT-MoS2 (Figure S7c) from

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2400-2700 cm-1 shows that the –SH peak are not present at 2567 cm-1, suggesting successful

15

functionalization of thiol group on the surface of MoS2 nanosheets. According to the previous

16

reports15–17,26 the absence of –SH peak in the FTIR spectra of thiol functionalized MoS2 are

17

strong indicator for binding of thiols at the NS surface. The amine functionalization is further

18

confirmed by XPS spectra of Mo 3p region (see supporting information Figure S9). The peak

19

in the spectrum located at 395.1 eV is assigned to Mo 3p3/2 and present in both pristine and

20

functionalized sample. In addition to the Mo 3p peak, the weak peak located at 400 eV in

21

OLA-MoS2 is assigned to N 1s originate from oleylamine molecules attached to NS surface.

FTIR spectroscopy was also performed to verify the surface

22

The functionalized nanosheets were utilized for the synthesis of 0D-2D hybrid of

23

photoactive CuInS2 QDs and high mobility 2D network of MoS2 nanosheets. The hybrid was

24

synthesized by simple solution mixing of DDT functionalized MoS2 nanosheets and DDT

25

capped CIS QDs (see supporting information for detailed method). The QDs was anchored at 13 ACS Paragon Plus Environment


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the surface of nanosheets by hydrophobic interaction between long alkyl chains of DDT

2

molecules present at both surfaces. The unbound QDs and excess DDT were removed by

3

repeated cycles of precipitation and dispersion of the hybrid. The morphology of CIS QDs,

4

MoS2 and CIS/MoS2 hybrid nanostructures was investigated by transmission electron

5

microscopy. Figure S10 a, b (see supporting information) shows low and high magnification

6

TEM images of CIS QDs with average size of 3 nm. The HRTEM images of CIS QDs (inset

7

of Figure S 10b) display clear lattice fringes with a lattice spacing of 3.2 Å corresponding to

8

the (112) plane of highly crystalline CIS QDs.14 The low and high magnification TEM

9

images of 0D-2D hybrid (CIS/MoS2) are shown in Figure 5a, b. The CIS QDs are uniformly

10

anchored at the surface of 2D-MoS2 nanosheets by hydrophobic interaction between alkyl

11

chains of dodecanethiol molecules present at the surface of both CIS and MoS2 NS. The

12

anchoring of QDs on the surface of SWCNT14, MWCNT13 and GO12 by hydrophobic

13

interaction between alkyl chain of various thiols and amines is well-established. The high-

14

resolution TEM images of CIS/MoS2 hybrid in Figure 5c display lattice fringes of both CIS

15

and MoS2 NS. The lattice spacing of 3.2 Å and 2.7 Å corresponding to (112) plane of CIS

16

and (100) plane of MoS2 NS, respectively. The selected area electron diffraction (SAED)

17

pattern of CIS/MoS2 shown in Figure 5d display two sets of the diffraction patterns, the six-

18

fold symmetry of hexagonal spots assigned to MoS2 and the ring pattern corresponds to CIS

19

QDs. The SAED pattern of QDs (Figure 5d) shows several distinct rings instead of spots due

20

to random orientation of the crystallites which correspond to diffraction from different planes

21

of the QDs.40In the SAED pattern of hybrid, the lattice spacing of 2.73 Å and 1.6 Å

22

correspond to the (100) and (110) planes of MoS2 NS, respectively. The continuous

23

diffraction rings with a spacing of 3.23 Å and 1.95 Å correspond to (112) and (220) planes of

24

the CIS chalcopyrite phase.41


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1 2

Figure 5: Transmission electron microscopy images of 0D-2D hybrid nanomaterial. (a) Low

3

magnification TEM image of QDs decorated surface of MoS2 nanosheets (b) High

4

magnification TEM image of CIS/QDs hybrid showing uniform decoration of QDs on the

5

surface of monolayer MoS2 nanosheets. (c) HRTEM image of CIS QDs attached on

6

monolayer MoS2 nanosheets. Lattice planes of both QDs and MoS2 nanosheets are clearly

7

observed and the lattice spacing of 3.2 Å and 2.7 Å are corresponding to (112) and (100)

8

planes of CIS QDs and MoS2 nanosheets, respectively and (d) Corresponding SAED pattern

9

of CIS/MoS2 hybrid.

10

Figure 6a presents the absorption spectra of CIS/MoS2 hybrids along with the

11

individual QDs and NS. The QDs displays broad absorption features with shoulder peak

12

centered at a wavelength of 500 nm. In comparison to the individual QDs and NS, the hybrid

13

displays combined absorption features of both QDs and NS.

14

CIS/MoS2 hybrid was performed by both steady-state and time-resolved fluorescence


Photophysical study of


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

1

spectroscopy. Steady-state PL spectra of pure CIS QDs and CIS/MoS2 hybrids at different

2

concentration of MoS2 are shown in Figure 6b. The intensity of PL emission peak in hybrid is

3

dramatically reduced and almost completely quenched at higher concentration of MoS2

4

nanosheets. The PL quenching of QDs at the surface of 2D nanosheets of MoS2 are attributed

5

to nonradiative relaxation of excitons via alternative decay pathway induced by MoS2. The

6

excited state non-radiative relaxation of excitons can occur by excited state electron transfer

7

or/and energy transfer from donor to acceptor.42–45 The staggered type II band alignment

8

between donor-acceptor systems favor electron transfer whereas the extent of overlap

9

between donor PL and acceptor absorption spectra favors energy transfer. To elucidate the

10

reason for PL quenching of QDs attached to the NS we consider energy band alignment of

11

hybrid which is shown in Figure 7b. The position of valence band maxima (VBM) and

12

conduction band maxima (CBM) of CIS QDs and MoS2 NS was calculated from ultraviolet

13

photoemission spectroscopy (UPS) measurements. The detailed method for UPS

14

measurement and extracted parameter are given in supplementary information (Figure S12,

15

S13 and Table S14). The calculated values are in good agreement with the reported values for

16

CIS QDs41,46 and MoS2 NS.47,48 In CIS/MoS2 hybrid, due to type II offset with large driving

17

force of 600 meV, the photoinduced charge transfer is the favorable pathway for excited state

18

relaxation. Furthermore, the energy transfer from 0D QDs to the 2D NS is previously

19

confirmed by enhancement in PL of 2D NS.9,49–51 In our case the PL emission peak for both

20

QDs and NS are superimposed (λ≈693 nm) and the strong PL quenching at this wavelength

21

presumably ruled out the possibility of energy transfer.

22

In order to provide the more insight to the mechanism for the non-radiative relaxation

23

process in the hybrid, we have performed the time-resolved photoluminescence (TRPL)

24

measurement. The PL decay profiles of pure CIS and CIS/MoS2 hybrid with varying

25

concentrations of MoS2 are shown in Figure 6c. The PL decay profiles were fitted with well16 ACS Paragon Plus Environment

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1

known multiexponential function describe elsewhere.14 The decay profile of CIS QDs fitted

2

with three components of lifetimes assigned as fast component ( ) medium component

3

( ) and slowest component ( ). As per previous reports14,52 ternary QDs such as CuInS2 do

4

not show band edge emission. The radiative recombination from the surface states, as well as

5

internal defects such as Cu vacancy sites, mainly contribute to the observed PL emission. The

6

multi-state PL from CIS also reflects in the decay parameter. The fast decay components are

7

attributed to the radiative recombination from the surface states. The medium and slowest

8

decay component were assigned to donor-acceptor recombination and conduction band to

9

acceptor recombination, respectively.14 The detailed parameter of TRPL data extracted from

10

triexponential fitting for pure CIS and CIS/MoS2 with different concentration of MoS2 are

11

shown in Table 1. It is seen that, compared to the pure CIS, the average life time of CIS

12

attached to the NS are dramatically reduced. The shortened PL lifetime of CIS QDs anchored

13

at the MoS2 surface compared to pristine QDs arises due to electron transfer from donor QDs

14

to acceptor MoS2. After close investigation of lifetimes and their relative contributions

15

presented in Table 1, it can be seen that the faster components is more affected than and

16

when CIS is attached to MoS2 NS. On the view of above results and values presented in

17

Table 1, we predict that the decrease in lifetime of QDs attached to MoS2 nanosheets is

18

mainly attributed to the suppression of radiative recombination from surface related states

19

due to electron transfer from surface states of CIS to the conduction band of MoS2 NS. In the

20

absence of an alternative decay path (for pure CIS), photoexcited electron decay by radiative

21

recombination through surface state sites whereas when QDs attached on the NS the

22

photoexcited electron transfer from CB and surface trap states of CIS to conduction band of

23

NS driven by favourable band alignment between CIS and MoS2 shown in Figure 7b. The

24

rate constant for non-radiative relaxation for CIS/MoS2 hybrid can be estimated by the

25

following equation.



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

=

1 ⁄

−

1

1

where is the rate of non-radiative relaxation for photoexcited CIS QDs to MoS2,

2

⁄ and are the average lifetimes of pristine CIS QDs and CIS attached to MoS2

3

NS. The calculated rate constant for non-radiative relaxation from QDs to NS was found to 1

4

× 107 s-1, consistent with previous reports for CIS based ternary QDs.14,46 In addition to the

5

0D-2D hybrid of CIS/MoS2 we also investigate charge transfer process in the blends of CIS

6

QDs and pristine MoS2 where QDs are not attached to the nanosheets surface. The steady

7

state PL spectra and corresponding decay profiles are presented in Figure S15 a, b. The

8

significant PL quenching and decrease in PL lifetimes indicates charge transfer likely to be

9

occur in the blends of CIS QDs and pristine MoS2. However, in contrast to the 0D-2D hybrid

10

where almost complete PL quenching was observed, the blends of CIS and pristine MoS2

11

only show moderate quenching efficiency of 53 %. Furthermore, with 50 wt % NS loading

12

the percentage reduction in average lifetime for 0D-2D is 59 % whereas for QDs and NS

13

blends is 16.2 % which is less compared to the hybrid. The calculated rate constant for non-

14

radiative relaxation from QDs to NS was found to 1.2 × 106 s-1, which is order of magnitude

15

lower in compared to the rate constant observed in hybrid. Therefore, these results verifying

16

the efficient exciton dissociation and charge transfer should occur when QDs are in contact

17

with NS and when QDs are not attached to the NS the charge transfer efficiency dramatically

18

reduced due to the absence of direct and interconnected charge transfer pathway.

19

(a)

(b)

(c)


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1 2

Figure 6: (a) UV-Vis absorption spectra of CIS QDs, MoS2 nanosheets and CIS/MoS2 hybrid.

3

(b) Steady-state photoluminescence spectra and (c) Time-resolved PL decay profiles of

4

CIS/MoS2 hybrids with different QDs to nanosheets ratio.

5

Table 1: TRPL fitting parameter of CIS/MoS2 hybrid with different concentration of

6

nanosheets.

7 8

The photoelectric properties of CIS/MoS2 hybrids were investigated to understand the

9

charge transfer between CIS and MoS2. The device structure and corresponding energy level

10

diagram are shown in Figure 7a,b. The detailed device fabrication method is given in

11

supplementary information. The typical I-V characteristics of photoconductive devices under

12

dark and visible irradiation of pure CIS, pristine MoS2 and hybrid CIS/MoS2 are shown in the

13

Figure 7c, d and e, respectively. Upon illumination, the CIS QDs and pristine MoS2 film

14

display significant contribution to photocurrent shown in Figure 7 c, d. In the presence of

15

light, the current is enhanced by 20% and 50 % for CIS QDs and MoS2, respectively under a

16

bias of 1 V. The low photocurrent of QDs only device is limited by exciton recombination



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

1

arises due to interparticle hopping and surface state defects.53 The QDs generally have a high

2

density of surface defects such as dangling bond and surface groups. The energy levels of

3

these surface defects lie within the band gap of QDs and act as a recombination center for

4

photogenerated charge carriers which causes a reduction in photocurrent and ultimately

5

deteriorates the device performance. In contrast to the low absorption coefficient and poor

6

light absorption, the MoS2 only devices have higher photocurrents compared to the pristine

7

QDs devices. The higher photocurrent results from high charge carrier mobility in 2D

8

MoS2.54 The high photoconductivity gain from mono or few layer pristine MoS2 nanosheet

9

was reported previously.55,56 Compared to individual 0D CIS QDs and 2D MoS2 NS, the

10

photocurrent for the 0D-2D hybrid of CIS/MoS2 is dramatically increased over 400 %

11

(Fivefold enhancement) with respect to the dark current (Figure 7e). The dramatically

12

enhanced photocurrent in the hybrid devices results from the synergic contribution of both

13

the components as individually both CIS and MoS2 show small contributions to the

14

photocurrent. The light harvesting component of CIS QDs contributes to enhanced light

15

absorption and exciton generation whereas the 2D network of MoS2 NS provides high charge

16

carrier mobility which is a key parameter for obtaining efficient optoelectronic devices. The

17

dramatically enhancement in photocurrent for CIS/MoS2 hybrid can be explained by

18

considering energy band alignment between CIS and MoS2 shown in Figure 7b. According to

19

energy level diagram, upon light illumination, both CIS QDs and MoS2 nanosheets absorb

20

light, however exciton generation is dominated in the QDs domain due to the large absorption

21

coefficient of QDs over MoS2. The photogenerated electrons in CIS are transferred from

22

CBM of CIS to MoS2 and transported through high mobility 2D network of MoS2 and

23

collected at the Al cathode. The holes from VBM of CIS are collected by ITO through hole

24

transporting layer of PEDOT:PSS. Furthermore, when CIS QDs are attached to MoS2 NS,

25

due to suitable energy levels they form type II heterojunction, the photogenerated excitons


Page 20 of 28

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1

get dissociated rapidly at the CIS/MoS2 interface as 2D MoS2 network provides a large

2

interfacial area for exciton dissociation. Due to high charge carrier mobility of 2D networks,

3

MoS2 NS serves as a transportation pathway for the direct flow of charge carriers towards

4

electrodes. To further verify the efficient exciton dissociation at the interface of 0D-2D

5

hybrid, we also fabricate photoconductive device with blends of CIS QDs and pristine MoS2

6

in which QDs are not attached to the NS surface. The typical I-V characteristics of

7

photoconductive device made with blends of QDs and NS under dark and light are shown in

8

the Figure S15c. In contrast to the device with 0D-2D hybrid of CIS/MoS2 where current in

9

the presence of light dramatically enhanced over 400 %, the device with blends of CIS and

10

pristine MoS2 only shows moderate enhancement in current of 81 % under illumination. The

11

result depict that the interface in 0D-2D hybrid provide excellent exciton dissociation centre

12

which is absent in the blend where QDs is not attached to the NS. Furthermore, the 0D-2D

13

hybrid such as QD/graphene system in which only QDs contribute to the light absorption, our

14

CIS/MoS2 hybrids have certain advantages as both the components absorb light and

15

contribute to the photocurrent. Therefore, in combination with the photophysics of CIS/MoS2

16

hybrid, we believe that the dramatically enhancement in photocurrent can be attributed to the

17

several synergistic processes such as strong light absorption and exciton generation, efficient

18

charge separation and transfer of an electron from CIS QDs to MoS2 NS.

19

(a)

(b)



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

1 2

3

4

5

6

(c)

(d)

(e)

7 8

Figure 7: (a) Typical device structure (b) Energy level diagram of photoresponse devices. The

9

position of energy levels of each component used for their fabrication are also shown. The

10

typical I-V characteristics in the absence and presence of light (c) CIS QDs (d) MoS2 and (e)

11

CIS/MoS2 hybrid.

12

In summary, we report a high concentration stable dispersion of mono or few-layer

13

MoS2 nanosheets by OLA or DDT assisted liquid phase exfoliation in non-polar organic


Page 22 of 28

Page 23 of 28

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1

solvent 1,2 dichlorobenzene. The exfoliation efficiency of OLA assisted exfoliation was

2

enhanced by more than five times in comparison to pure DCB. The DDT functionalized

3

MoS2 nanosheets exhibited PL at 1.79 eV which is red shifted by 0.1 eV with reference to

4

solid state PL and can be attributed to the formation of shallow trap state upon

5

functionalization. From XPS analysis it is interesting to find that the OLA or DDT bound

6

surface act as a protective layer from the oxidation of highly reactive edges and sulfur

7

vacancy sites. Moreover, a photoactive 0D-2D hybrid of CIS/MoS2 was synthesized by

8

simple mixing of two components. The QDs was anchored at the surface of 2D nanosheets by

9

hydrophobic interaction between alkyl chain of dodecanethiol molecules present at the

10

surface of both QDs and MoS2. The type II band alignment between QDs and nanosheets

11

favor efficient charge transfer which is further confirmed by a combination of steady-state

12

and time-resolved photoluminescence measurements. The photocurrent of the hybrid was

13

dramatically enhanced as compared to QDs or MoS2 only device. The dramatically enhanced

14

photocurrent is attributed to enhance charge separation, charge transfer and charge carrier

15

transport through a highly conductive network of 2D MoS2 nanosheets.

16

ASSOCIATED CONTENTS

17

Supporting Information

18

Synthesis method of CIS QDs, ligand exfoliated nanosheets and 0D-2D hybrid, device

19

fabrication procedure, charecterization methods, photographs of MoS2 NS dispersed in

20

dichlorobenzene with different ligands centrifuged at 3000 rpm, UV-Vis spectra and

21

photograph of NS centrifuged at 10,000 rpm, PL and TRPL spectra of OLA-MoS2 and DDT-

22

MoS2, TEM images of exfoliated NS with and without ligand centrifuged at 3000 rpm, AFM

23

image of OLA and DDT functionalized MoS2, Raman spectra, FTIR spectra , high resolution

24

XPS spectra of S 2p region of DDT-MoS2 and Mo 3p region for OLA-MoS2, TEM and

25

HRTEM images of CIS QDs, Tauc plot of CIS QDs, UPS spectra of DDT-MoS2 and CIS



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

1

QDs with experimental parameter, PL spectra and decay profile of blends of CIS QDs and

2

pristine MoS2 nanosheets.

3

Author Information

4

Corresponding Author

5

*

6

Acknowledgements

7

The authors are grateful to the Director NPL, New Delhi, India for the facility. R.A and S.Y

8

gratefully acknowledge the financial support from the Council of Scientific and Industrial

9

Research (CSIR), New Delhi, for the award of SRF. The research was partially funded from

10

Email: [email protected]

DRDO grant ERIPR/ER/1000389/M/01/1407.

11

12

References

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

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Functionalized Molybdenum Disulfide Nanosheets ... - ACS Publications

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