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Summer Research Fellowship Programme of India's Science Academies

2D TMD nanosheets as effective hole transport material for colloidal CdSe quantum dot solar cell

Pragya Bharti

Central University of Jharkhand, Brambey, Jharkhnad-835205

Dr. Santanu Das

Assistant professor, Department of Ceramic Engineering, Indian Institute of Technology(BHU), Varanasi, Uttar Pradesh-221005

Abstract

In the recent boom of solar cell technology, the property of band tunability of quantum dot has boosted the efficiency of solar cell to about 60% by allowing the infrared spectrum to get absorbed too which constitutes the maximum solar spectrum range that falls on earth surface. Two dimensional transition metal dichalcogenides (TMDs) have been extensively developed and characterized due to their potential application in optoelectronic devices. In the family of TMDs, 2D MoS2 has attracted significant attention due to some unique properties, such as, variable energy band gap, high-conductivity, good-flexibility, high-transparency etc. This project aims to use the 2D nanosheets as an efficient hole transport material for the colloidal CdSe quantum dot solar cells. The MoS2 nanosheets, synthesized by the hydrothermal method and functionalized with molecular iodine, are dispersed in DMF to allow deposition of MoS2/MoSe2 nanosheets by the spin coating process on the top of FTO/TiO2/CdSe QD layer for hole transport layer formation. The matching of the valence band position of the MoS2 with that of CdSe QDs is responsible for the effective hole transfer, which will be exploited to enhance the photovoltaic performance of solar cell devices. Having band gap position above the conduction band of the CdSe QD, the inserted 2D nanosheets of MoS2 block the transfer of electrons from CdSe to the MoS2 layer. The recombination of photogenerated carriers is lessened due to the selective extraction of holes at their interface which increases the current generation of the solar cells. Moreover, the voltage loss is lessened by the reduction of Fermi energy pinning at the interface due to the hole carrier selectivity of the MoS2 nanosheets, which helps to elevate the voltage generation and fill factor of the solar cell. This leads 2D TMDs nanosheets as a new hole transport layer for the fabrication of economical, long lasting, and efficient colloidal quantum dot solar cells.

Keywords: QD, molybdenum disulfide, transition metal dichalcogenides, Fermi energy pinning, hole carrier selectivity

Abbreviations

Abbreviations
                    TMD                             Transition Metal Dichalcogenides
                   QD                                     Quantum Dot
MEG Multiple Exciton Generation 
XRD X-ray diffraction  

INTRODUCTION

Background

The limited stock of fossil fuels and the environmental problems associated with it has encouraged people’s interest and support to opt for renewable energy, especially solar energy, as an alternative source of energy which is abundant in nature. Solar photovoltaic system has proved to be one of the most promising solar energy applications, by providing direct energy conversion of solar energy into electricity. Certain developments are going on this field to achieve reliability, cost effectiveness and high energy conversion efficiency in order to prevent the climate change due to global warming and meet the increasing energy demands of growing population. It is important to achieve cost effectiveness as well as high efficiency to allow high market penetration of solar electricity.

Among the emerging PV cells, QD’s based solar cells are considered most promising next generation solar cells due to their unique properties by which it can exceed the Shockley-Queisser power conversion efficiency limit of existing solar cell technologies. Quantum dots (QDs) are semiconductor nanocrystals with diameter less than 10nm that display remarkable optoeletronic properties like band tunability, multiple exciton generation, near infrared absorption, including bright luminescence, a broad excitation profile, high thermal and moisture stability, narrow emission peaks, and exceptional photostability. By the help of MEG process we can obtain more than one e-- hole pair by absorbing a high energy photon which will lead to high photocurrent generation. These nanostructures have been investigated for a wide variety of applications, such as optical probes for imaging, target labeling, temperature sensing, and sensitizers for solar cells. Colloidal quantum dots (CQDs), nanometer-scale semiconductor crystals capped with surfactant molecules and dispersed in solution, have provided a sturdy platform for the development of various classes of solution-processed optoelectronic devices including photodetectors, light emission devices and especially for solar cells.

Recently, 2D material such as Transition metal dichalcogenides (TMD) has become a promising materials for electronic​[1]​ and optoelectronic devices​[2]​, catalysis​[3]​, transistors​[4]​​[5]​​[6]​, photonics​[7]​, photodetectors​[8]​, sensors​[9]​, and photocatalyzed hydrogen evolution reactions​[10]​​[11]​. TMD are atomically thin semiconductors of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se or Te). One layer of M atoms is sandwiched between two layers of X atoms. TMD bulk crystals are formed of monolayers bound to each other by Vander-Waals attraction and with strong covalent bonding within each layer. Among various TMDs, MoS2 have attracted researcher’s interest due to their unique properties such as variable energy band gap, high conductivity, good flexibility, high transparency, high surface area etc. Its unique property of direct band gap in monolayer and an indirect band gap in bulk has opened up opportunities for various applications in optoelectronic devices​​​[12]​.

Statement of the Problems

Due to challenges in producing large scale defect-free 2D TMD materials, the development of TMD based solar cell devices are facing various difficulties. Till now the development of QD solar cell devices has been based on monolayer TMDs​​​[14]​, but due to high transmission loss occurring in them efficiencies of these remains low. Further, the process associated with producing monolayer presents a challenge in itself.

Objectives of the Research

Therefore, to increase the efficiency as well as to make the solar cell cost effective we are using multilayered 2D MoS2 as a hole transport layer for colloidal quantum dot solar cells. Here we are improving the PV parameters for solar cell devices by using functionalized 2D MoS2 on top of the CdSe QD layer. Through the spin coating of 2D TMD, the valence band position of the MoS2 aligns itself just above the valence band position of the QD layer, which facilitates the flow of holes and thus reduces the recombination losses.

LITERATURE REVIEW

The promising properties of 2D MoS2 nanoparticles have drawn many researchers’ interest. Over the last decade, many scientists have been working on the development of these 2D nanoparticles, as it shows great potential due to its unique electronic and optoelectronic properties. Shanmugan et al ​[14]​ have used MoS2 as a photoactive layer in his solar cell fabrication. He has prepared MoS2 via chemical vapor deposition (CVD) method and then deposited it on the indium tin oxide glass by the layer transfer process. A schottky junction was formed in between MoS2 and Au. This simple fabrication of ITO- MoS2-Au solar cell demonstrated a photo conversion efficiency of 0.7% for 110 nm MoS2 and 1.8% for 220 nm MoS2 [15]. Shanmugan et al demonstrated another MoS2 based solar cell. This time he fabricated a hybrid bulk heterojunction solar PV cell. He made a stacked structure of ITO/TiO2/MoS2/P3HT (poly 3-hexylthiophene)/Au. The bulk heterojunction between MoS2 and TiO2 nanocomposite gives a short circuit current density of 4.7 mA cm-1, an open circuit voltage of 560 MV, and lead to a photoconversion efficiency of 1.3% under standard AM 1.5 illumination​[14]​. Bernardi et al showed that a thin layer of MoS2/graphene or MoS2/WS2 layer with a thickness of about 0.9 nm and an efficiency of 0.1-1.0% shows a power density of 0.25-2.5 MW/Kg and the WS2/ MoS2 with a thickness of 1.2 nm and an efficiency of 0.4-1.5% shows a power density of 0.45-1.8 MW/Kg. Though the monolayer of MoS2 can effectively absorb 5-10% of incident solar radiations in a thickness less than a nm and thus demonstrates higher magnitude of sunlight absorption then GaAs and Si but there efficiency in solar cell remained below 2% due to transmission losses occurring in monolayer sheets​[16]​. Tsai et al fabricated a large scale MoS2 monolayer by CVD method and then transferred it to the p-Si substrate. This type 2 heterojunction between MoS2 and p-Si solar cell showed a power conversion efficiency of 5.23%​[17]​. Due to the challenges faced on uniform growth of 2D TMD material, multilayered TMD materials have not been widely used in devices. Tulsani et al for the first time demonstrated the use of MoS2 as a hole transport layer for the colloidal PbS quantum dot solar cell. They have fabricated n-p-p+ structured solar cell. The fabricated solar cell device showed an efficiency of 2.48% which was just 0.92% before introducing the MoS2 layer. This indicates the increase in the power conversion efficiency of the solar cell devices​[18]​. Senthikumar et al demonstrated a marigold flower like nanostuctured MoS2 grown on an FTO glass as a counter electrode for his dye-sensitized solar cell. The MoS2 CE based DSSCs showed a power conversion efficiency of 6.32% which was comparable to Pt CE based DSSCs with PCE 6.38%. It also had incident photon-to-charge carrier conversion efficiency 65.84% which was comparable to 68.38% charge efficiency of Pt CE based DSSCs. Thus his work proved MoS2 films as an alternative to the Pt-based CE in DSSCs, making it more cost effective​[19]​.

METHODOLOGY

TiO2 paste preparation

TiO2 nanoparticle was prepared using solution growth process and we obtained this from other research collaborator. TiO2 paste was prepared from the TiO2 nanoparticles by manual grinding method. 0.3 gm of TiO2 was taken and firstly ground. Then 350 µL of the mixture of DI water and ethanol in a 1:1 ratio was added gradually in the nanoparticles and then the mixture was ground for 1 hour. After that 100 µL of Triton X-100 was added into it and then it was again vigorously ground to obtain a smooth paste.

2D MoS2 nanosheet synthesis

MoS2 nanosheets were synthesized using hydrothermal method. 3.8 gm of thiourea and 2.067 gm of ammonium molybdate were weighed and dissolved in 60mL of DI water (deionized water). The prepared solution was stirred in magnetic stirrer for 30 minutes to get a completely dispersed solution and then the reaction mixture was transferred to a Teflon lined autoclave and was kept in a hot air oven at 200±10°C for 24 hours. After 24 hours, the autoclave was taken out and then cooled down at ambient temperature. Then, the prepared solution was taken into centrifuge tube and cleaned simultaneously with DI water and then with ethanol twice at speed greater than 5000 rpm. The obtained product was dried in vacuum oven to remove residual solvent for 6 hour. 100mg of the as-synthesized MoS2 material was dispersed in 3 mL of DMF (N, N-Dimethylformamide) solvent. 200 mg of TBAI (tetrabutylammonium iodide) was added to it and then the mixture was heated at 100±10°C under continuous stirring for 12 hours under argon atmosphere. Thus iodine functionalized MoS2 nanosheets were then added with toluene and then centrifuged at 10000 rpm to isolate the MoS2 nanosheets and was kept in a vacuum oven for 8 hours. This was followed by dispersing 60 mg of MoS2 dry powder in 30 mL of DMF at a concentration of 20 mg mL-1 for film fabrication.

Growth of MoS2 nanosheets over FTO glass

MoS2 nanosheets were grown on the conductive fluorine doped tin oxide glass by one-step hydrothermal method. The fluorine doped tin oxide glass was first cleaned by sonicating it with DI water followed by acetone cleaning. MoS2 sample was prepared by using precursors 3.8 gm of thiourea and 2.067 gm of ammonium molybdate, they were dispersed in DI water and then kept for stirring in magnetic stirrer for half an hour followed by bath sonication for half an hour until the solution was completely dispersed in DI water. FTO glass was checked for conductivity using multimeter. The FTO glass was then kept in a Teflon lined autoclave with the conductive side facing up. The prepared MoS2 sample was then poured in the Teflon tube and the autoclave was kept in a hot air oven at 180°C for 24 hours. After that, the autoclave was taken out and cooled at room temperature. At the end, the FTO glass with MoS2 substrate was taken out with the help of tweezer and washed with DI water and ethanol simultaneously. The prepared FTO glass with MoS2 coating on its surface can be used for counter electrode in the solar cell.

0001[1].jpg
    Growth of MoS2 nanosheets over the FTO glass as seen by the optical microscope

    CdSe QD synthesis by hot injection method:

    The CdSe QDs were prepared through a hot injection method. A stock solution of selenium precursor was prepared by combining 5mL of trioctylphosphine with 1mMol of selenium and then it was heated at 100±10°C. A heated growth solution was prepared by taking 10mL of octadecene and 5mMol of trioctylphosphine (TOP) in a 3 neck round bottom flask clamped in a heating mantle with a magnetic stirrer. Then the solution was heated. After the solution reached 110±5°C, 1mMol of Cadmium Stearate was injected to the solution under the argon environment. Then the solution was allowed to reach the equilibrate temperature of 220±5°C, at this temperature the prepared selenium stock solution was injected into the hot solution. After the precursor materials were injected, Se and Cd combined to form clusters of CdSe that grew continuously to form QDs with progressing reactions. The solution was heated for 15 minutes and then it was cooled down at room temperature.

    Synthesis_procedure_CdSe[1].jpg
      Schematic representation of CdSe quantum dot synthesis by hot injection method and then cleaning of QDs

      The octadecene CdSe QDs were transferred to a micro centrifuge tube. After that, ethanol and toluene were added to the centrifuge tube and then shaken to get an emulsion. A matched tube of the same weigh and the centrifuge tube of the freshly shaken sample were taken to opposite sides of a centrifuge. Then it was spun at speed greater than 8000 rpm for 15 minutes. A precipitate appeared in the pipet tip. Carefully the ethanol + toluene layer was removed. A second wash was added and shaken to mix and immediately centrifuge step was repeated and this process of wash was repeated until the solution no longer gave suspension. Then the ethanol + toluene was washed off. At last the QDs precipitate was dissolved in 3 mL toluene and kept in a container.

      IMG-20190720-WA0006.jpg
      CdSe QDs under visible light
        IMG-20190720-WA0005.jpg
        CdSe QDs under UV light
          CdSe QDs as seen under different spectrum of light

          Characterization

          XRD

          Structural and phase analysis of MoS2 and TiO2 was done in x-ray diffraction (XRD) (Rigaku, Japan) analysis instrument. The diffraction peaks were identified using standard line profile JCPDS data.

          Scanning electron microscopy

          The morphology of the as synthesized material was characterized using a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) (NOVA Nano SEM 450, FEI USA). For taking pictures in SEM, we used 15 kV accelerating voltage and pressure of the sample chamber was 10-6Torr while the working distance was kept at 4.5 mm.

          Transmission electron microscopy

          The particle size and morphology of the as synthesized materials were characterized using a TECHNAI G2 20 TWIN (FEI, USA) transmission electron microscopy (TEM).

          UV-vis Spectroscopy

          The absorption spectra of the MoS2 were measured with the help of UV-vis spectrophotometer from 200 nm to 800 nm wavelength. The samples for the UV-vis analysis were prepared by dispersing the as-prepared MoS2 into NMP (N-Methyl-2-pyrrolidoone) solution and then bath sonicating it for 10 minutes.

          DEVICE FABRICATION

          The glass was checked for conductive side using a multimeter. The fluorine doped tin oxide glass was first cleaned by sonicating it with DI water for 5 minutes.followed by cleaning with ethanol by sonicating it for another 15 minutes and third time cleaning again by sonicating with acetone for 15 minutes. The TiO2 paste obtained was spin coated on an FTO glass by the spin coater at a rotation speed of 3500 rpm for 30 seconds. Then the FTO glass coated with TiO2 was kept in a tube annealing furnace for heating at 450°C for 2 hours. CdSe QDs were deposited above the as-prepared FTO glass by layer by layer deposition (LBL) solid state ligand exchange approach under ambient conditions. The ligand used here was 1, 2-Ethanedithiol (EDT) 2% v/v in acetonitrile solution. The CdSe QDs dispersed in toluene (30 mg mL-1) were spin coated on the substrate at a rotation speed of 2500 rpm for 30 seconds. 4 drops of EDT were added followed by 8 drops of acetonitrile, to rinse off excess EDT, this was done under continuous spinning. This leads to single layer CdSe QD film deposition. This sequence was repeated for 10 layer deposition. On top of the CdSe QD layer, one layer of functionalized MoS2 was spin coated at 2500 rpm for 40 seconds.

          cell fab.png
            Different stages of cell fabrication

            RESULTS AND DISCUSSION

            The XRD of TiO2 indicates the presence of broad anatase peaks confirming the anatase phase of the as-synthesized material. The anatase peaks were found at 25.42°, 37.87°, 43.073°, 53.97°, 55.17°, 62.82°, 68.92°, 70.37°, 75.17° which corresponds to the plane (011), (004), (020), (015), (121), (123), (116), (220), (017) which are indexed to tetragonal crystal system of anatase phase of Ref. no. 98-003-7533.

            xrd tio2.jpg
              X-ray diffraction pattern of  TiO2

              From the SEM image of the as-synthesized TiO2, we can conclude that the TiO2 nanoparticle has a uniform particle size of diameter in the range of 0 nm to 90 nm with the mean particle size of 39.02 nm. The SEM image of the nanoparticle indicates the mesoporous structure of TiO2 nanoparticles. This morphology leads the mesoporous structure in obtaining a high surface area which is important for the enhancement of surface reactions. This leads to more effective photogeneration and hence higher photocurrent.

              300.jpg
              SEM image at 300 nm magnification
                200.jpg
                SEM image at 200 nm magnification
                  Scanning Electroscope Microscopy images of TiO2 at different magnification

                  The TEM images are further analyzed for microstructure analysis. TEM images confirm the mesoporous structure as obtained from the SEM images and indicate the non-uniform distribution of nanoparticles throughout the sample. The selected area electron diffraction of the TiO2 nanoparticles shows the anatase phase with crystallinity which corresponds to the results as obtained by the XRD.

                  tem saed tio2_1.png
                    TEM images of mesoporous TiO2 at different magnification (a) 0.5 µm, (b) 100 nm, (c) 50nm and (d) selected area diffraction pattern of TiO2

                    The as-synthesized MoS2 nanosheets, by the hydrothermal route, were firstly analyzed for their phase, crystallinity and crystal structure by using XRD technique. From the XRD pattern of MoS2 nanosheets we can see that the main diffraction peaks occur at 14.12°, 33.7°, 39.5°, 58.7° which corresponds to (002), (100), (013), (110) plane which are indexed to hexagonal phase of MoS2 (JCPDS no. 98-011-8125). There are no other peaks indicating there is no impurity.

                    xrd mos2.jpg
                    X-ray diffraction pattern of MoS2
                      raman mos2.jpg
                      Raman spectrum of MoS2
                        Tauc plot mos2.jpg
                        Tauc plot of MoS2
                          uv plot mos2 (1).jpg
                          Absorption spectrum of MoS2
                            Different characterization spectrum of as-synthesized MoS2

                            The Raman spectrum of MoS2 nanoflowers showed two eminent Raman bands at 375.67 cm-1 and 403.08 cm-1, which corresponds to E12g and A1g vibrational modes, respectively [20] . The symmetric vibrations of S atom along the c-axis are denoted by out of plane A1g and the opposite vibration of two S atoms relative to Mo atom is indicated by E12g mode. The Raman frequencies of E12g and A1g peaks changes with the number of layers in the MoS2 nanoflowers, and the frequency difference between E12g and A1g modes is 27.41 cm-1 which indicates the multilayer nature of the as-synthesized MoS2 nanoflowers [21] . A small peak was observed at ~625 nm which corresponds to the spin-orbit split pair originating from the band edge excitation transitions at the K point of the Brillouin zone [22] . A broad peak observed at 455 cm-1 indicates the transition between occupied dz2 orbital to unoccupied dxy, dyz, dxz and dx2-y2 orbitals Goki Eda, 2013 . Tauc plot was used to calculate the band gap of as-synthesized MoS2 nanoflowers which was found to be ~1.48 eV.

                            The scanning electron microscopy of the MoS2 sample clearly indicates nanoflower morphology of the as-synthesized MoS2.. The TEM (transmission electron microscopy) image analysis confirms the marigold type flower morphology and the formation of nanosheets as indicated by the SEM images.

                            100.png
                            SEM image at 100 nm magnification
                              500.png
                              SEM image at 500 nm magnification
                                SEM images showing 2D MoS2 nanoflowers at different magnification

                                The EDAX of the MoS2 nanoflowers indicates that there are no other elements present in the sample indicating that there are no impurities present in the as-synthesized nanoflowers

                                edax.png
                                  EDAX of 2D MoS2
                                  EDAX result of 2D MoS2
                                   ELEMENTWEIGHT%  ATOMIC%
                                   MoL93.24 82.18 
                                   S K 6.76 17.82

                                  The folds shown in the TEM images represent different layers of MoS2 nanosheets with a d-spacing of 0.63 nm. The corresponding selected area electron diffraction (SAED) pattern indicates the crystalline nature of the MoS2 nanosheets which supports the XRD and the TEM results.

                                  tem mos2_1.png
                                    TEM images of MoS2 at different magnification (a) 200 nm, (b) 100 nm, (c) HRTEM image of MoS2 indicating d spacing and (d) SAED image

                                    CONCLUSION

                                    Here we have synthesized 2D MoS2 nanoflowers by hydrothermal method. The as-synthesized MoS2 nanoflowers were functionalized with iodine and thus it can be used as a p+ layer in the QDSC. We have find out the band gap of MoS2 which is ~1.48 eV due to which, it can easily pin its conduction band just above the conduction band of CdSe QDs which has a band gap of ~1.7 eV and thus, it will block the flow of electrons from CdSe QDs to MoS2 layer and hence will lead to the selective absorption of holes at the interface. This will lead to lowering of recombination losses and an increase in the efficiency of the solar cell.

                                    We have synthesized TiO2 nanoparticles using solution growth method. XRD of the as-synthesized TiO2 nanoparticles showed anatase phase and SEM, TEM confirmed the mesoporous structure of TiO2 nanoparticles. These results indicate that the as-synthesized nanoparticles have a higher surface area which leads to higher surface reactions and thus it can generate higher photocurrent. Here, we have coated a thin film of TiO2 nanoparticles over an FTO glass. This will act as an electrode of our QDSC and will facilitate the flow of electrons and thus improving the efficiency.

                                    FUTURE WORK

                                    After the functionalized MoS2 coating we will deposit top electrodes by using a thermal evaporator.

                                    Parameters for coating top electrodes
                                     MATERIALTHICKNESS (in nm) RATE OF DEPOSITION (in  Å s-1)
                                     MoO310  0.1
                                     Au30  0.5
                                    Ag  120 1

                                    Then we will take current-voltage measurements at ambient temperature. This will help us in determining the open circuit voltage, short circuit current and fill factor of the solar cell and thus we can calculate the power conversion efficiency of the solar cell. This solar cell, with MoS2 as a hole transport layer is supposed to increase the efficiency of solar cell as it will decrease the recombination losses.

                                    ACKNOWLEDGEMENTS

                                    I would like to express my sincere gratitude to IASc-INSA-NASI for providing me with this golden opportunity to do my project in one of India’s prestigious institute and providing me with their help and support throughout the duration of my project.

                                    I would like to extend my heartiest gratitude to my guide Dr. Santanu Das, assistant professor, Department of Ceramic Engineering, IIT BHU for his sizable subvention and visionary guidance at each and every step of the project.

                                    I would like to convey my deepest thanks to Mr. Vivek Kumar Singh for taking part in useful decisions and giving necessary advices and guidance. I would also like to thank my senior (Mr. Sayon Chattopadhyay, MR. Raja Kumar) who helped me in finalizing this report within given time frame. It was an extremely enlightening experience to work under truly devoted and ingenious scholars at one of the most reputed institutes of India

                                    I would have liked to gain more from all the aforementioned delightful personnel. It would be grateful of me to lend my hand in future for any further task in the project and even experiment on it.

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                                    • M. Thripuranthaka, Ranjit V. Kashid, Chandra Sekhar Rout, Dattatray J. Late, 2014, Publisher’s Note: “Temperature dependent Raman spectroscopy of chemically derived few layer MoS2 and WS2 nanosheets” [Appl. Phys. Lett. 104, 081911 (2014)], Applied Physics Letters, vol. 104, no. 12, pp. 129901

                                    • R. Coehoorn, C. Haas, J. Dijkstra, C. J. F. Flipse, R. A. de Groot, A. Wold, 1987, Electronic structure ofMoSe2,MoS2, andWSe2. I. Band-structure calculations and photoelectron spectroscopy, Physical Review B, vol. 35, no. 12, pp. 6195-6202

                                    • Laurie A. King, Weijie Zhao, Manish Chhowalla, D. Jason Riley, Goki Eda, 2013, Photoelectrochemical properties of chemically exfoliated MoS2, Journal of Materials Chemistry A, vol. 1, no. 31, pp. 8935

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