To investigate the optoelectrical properties of WS2 (Tungsten disulfide) nanosheet films
Keywords: WS2 nanosheets, WS2 film, optoelectronic properties
Transitional Metal Dichalcogenides encompasses those 2D materials, the structures of which are similar to graphene but have energy band gap, unlike graphene. TMDs are post-graphene materials and existence of direct band gap in the monolayers and co-existing with the very large exciton energies and strong spin-valley coupling offer exciting opportunities for novel thin new-generation devices. Layered TMDCs have the generic formula MX2, where M stands for a metal and X represents a chalcogen. The interatomic interaction within layer is covalent in nature, while the layers are held together by weak van der Waals forces. The presence of the latter allows the crystal to cleave easily similar to graphene. Despite rather similar structure, TMDCs cover a wide spectrum of properties ranging from insulators to semiconductors to metals . Among these materials, semiconducting TMDCs like MoS2, Ws2 are of special interest since the possibilities of gap engineering and other change of electrical and optical properties by changing external conditions and varying the number of layers make them possible candidates for device application. Now-a-days the research on TMDCs is expanding with unprecedented pace unveiling not only the fundamental properties but also they have been proposed for a host of application ranging from optical devices, energy generation and storage devices, chemical sensors etc. Though these materials have been mostly used in energy generation and storage devices  and also for gas sensing application . And for various electrochemical as well optoelectronic applications, thin films of these materials are used. Though different deposition techniques like Chemical Vapor Deposition , magnetron sputtering and layer transfer techniques  etc are there for deposition of WS2 nanosheets, these are complicated process and not even cost effective for application purposes. Therefore in this work new approach for thin film deposition has been introduced and IV characteristics of the film have been carried out in order to compare it with the existing literature.
WS2, n-methyl-2-pyrrolidone, Copper Clad from Amazon were purchased. All reagents used here are of analytical grade and used as received without further purification.
I-V characteristics were done with the help of Ketheley IV characterizer. Raman Spectra were obtained using a Raman Spectrometer (RENISHAW-inVia Raman Microscope) in the range of 100cm-1 to 3000cm-1.
Synthesis of Ws2 Nanosheets
Bulk 3D WS2 can be fabricated to 2D with the help of top-down and bottom-up approaches and layer transfer. Some of the top-down techniques are
i) Mechanical Expoliation
ii) Liquid Expoliation and
iii) Electrochemical Expoliation
Again some of the bottom-up techniques are
i) Synthesis via metal chalcogenisation
ii) Thermolysis of Thiosalts
iii) Vapor pressure reaction of transition metal and chalcogen precursors
iv) Growth of TMDC alloys and
v) Van Der Waals Epitaxy
Among these detection techniques liquid expoliation is comparatively easy and effective.
Earlier it was used for dispersing Graphene. As TMDCs are alike graphene, therefore this technique is also applied for TMDCs. The good solvents for TMDCs are thosewhich have dispersive and polar characteristic. Along with these, the H-bonding components of the of the cohesive energy density should be within certain well defined range. In this regard, some of the promising solvents are N-methyl -pyrrolidone and isopropanol. In this study we have used N-methyl-pyrrolidone fot the synthesis the nanosheets ,.
In order to do the synthesis, .05 g of bulk WS2 is taken in 40ml of n-methyl-2-pyrrolidone mixed well. The mixture has been placed in a low power bath sonicator and sonicated for every 10 minutes and shaken well at a stretch of 3 hours. After this the solutions are centrifuged for 15 minutes in the centrifuger and the solutions finally settle down as WS sheets at the bottom and on the sides of the beaker. Then the NMP was poured out of the tubes and they are filled with distilled water after which they are again centrifuged for another 15 minutes. After this the sheets finally settles down in the beaker and the water is poured off. The system is then dried for 12 hours. Finally the WS2 nanosheets are deposited on copper electrodes for further characterization.
Preparation of the Ws2 Nanosheet Film
PCB board has been purchased from Amazon. An electrode pattern shown in the figure has been made by itching the PCB by FeCl3 solution. Now a solution of nanosheets has been made and drop-casted on the electrodes to get the thin film.
The IV characteristics of the prepared sample were done with the help of IV analyzer. The optical characterization was done with the help of Raman Spectroscopy.
This is a device with the help of which the current and voltage of a sample can be measured. This will give the conductivity of a sample. In this work, the I-V characteristics of the nanosheets films were measured in order to investigate the electrical properties of the sample.
Raman spectroscopy is a spectroscopic technique which is usually used to determine vibrational modes of molecules along with the rotational and the other low frequency mode.
Principle of Raman spectroscopy
The principle of Raman Spectroscopy is the Raman Effect. Raman effect is defined as the variation occurred in the wavelength of light when a beam of light is deflected by the molecules. When a monochromatic light source, usually laser in the visible, near infrared, or near ultraviolet range is focused on the sample, the light interacts with the sample and molecular vibrations of the sample occur. For a small interval of time, the energy state of the sample changes to some virtual state and the finally photons are emitted changing the state of the sample to original one. In this process, inelastic scattering takes place. Therefore, photons coming out of the sample gets shifted either upwards or downwards compared to that of the incident photons. Due to the excitations caused by the photons of the laser light the molecules move to a new rotational-vibrational-electronic state because of which the total energy of the system changes. To conserve energy the scattered photons shifts to a different energy and hence a new frequency. This shift in energy of the photons is equal to the energy difference between the initial and final state of the molecules. If the final state is higher in energy then the initial, then the scattered photon will shift to a lower frequency, so the total energy is conserved. This shift is called Stokes shift. Again if the final state is lower in energy then the initial, the photon will shift to a higher frequency, and this shift is called anti-Stokes shift .
Fig 1. Stokes and antistokes lines in Raman Spectra, (ref: https://en.wikipedia.org/wiki/Raman_spectroscopy)
Information from Raman spectroscopy
Raman spectroscopy is a chemical analysis technique which is non-destructive and provides detailed information of chemical structures, phase and polymorphy, crystallinity ad molecular interactions. It is mainly focused upon the interaction of light with the chemical bonds within a material. The shift in energy gives information about the vibrational modes of a sample. The spectra profile of Raman scattering provides unique chemical fingerprint which is used to identify a material and differentiate it from others. Usually the spectrum is quite complex and so comprehensive Raman spectra libraries are searched to find a match and provide a chemical identification. The intensity of the Raman spectrum is directly proportional to concentration. A calibration procedure will be used to determine the relationship between peak intensity ad concentration, and measurements can be made to analyze for concentration. The relative peaks intensities in case of mixtures provide information regarding the relative concentration of the components whereas absolute peak intensities can be used to gather information regarding absolute concentration .
Types of Raman spectroscopy
1. Raman Spectroscopy for microscopic analysis: Raman spectroscopy is used to analyse in the microscopic level with a spatial resolution in the order of 0.5-1 micro meter. This type of analysis is possible with the help of a Raman microscope.
A Raman microscope consists of a Raman spectrometer and a standard optical microscope which allows high magnification visualization of a sample and Raman analysis with a microscopic laser spot. It is done by simply placing the sample under the microscope, focusing, and making a measurement.
2. Confocal Raman microscope: This can be used to analyze micron size particles or volumes. This can also be used to analyze the different layers in a multilayered sample and of contaminants and features beneath the surface of a transparent sample.
Motorized mapping stage generates Raman spectral images containing many Raman spectra from different positions on the sample. False color images can also be created on the basis of Raman spectrum which shows the distribution of individual chemical components, variation in phase polymorphism, stress/strain, and crystallinity .
Analysis of solids, liquids and gases
Raman spectra can be obtained from all samples which has true chemical bonding. This implies solids, powders, slurries, liquids, gels and gases can be analyzed using Raman spectroscopy. The concentration of gas molecules are usually very low, so specialized equipments like high power laser and long path length sample cells are used. However if the gas pressure is high Raman spectroscopy can be used easily .
RESULTS AND DISCUSSION
Raman Spectra is an important characterization technique for material analysis because it gives the information about the lattice vibrations of the material which is unique for each material. From the literature it is known that the most interesting scenario occurs for the excitation wavelength of 514nm. In this case the strongest raman peak is associated with the 2LA (M) and A1g (Γ) phonon modes. These modes also indicate the formation of few layers and layers can be calculated out by analyzing the shifting of the modes . For laser excitation of 514nm, the A1g (Γ) phonon mode appear to at 421.2 cm-1 and the intensity is higher than 2LA (M) which appear at 352.4cm-1 for the WS2 films deposited on quartz by CVD process .
In the present work for the laser used was of 50mW. For laser power (L.P) of 1%, peaks for WS2 nanosheets (powder) are achieved at 421.8cm-1 and 355cm-1 as shown in Fig 3a. So, in the present work, the effect of 2LA (M) is diminished but the A1g(Γ) and E12g(Γ ) modes are prevalent for L.P of 1% . Raman Spectra of WS2 nanosheets powder with laser power 5%, WS2 nanosheets films on copper electrode and Copper electrode less deposited by WS2 nanosheets were compared to see the effect of PCB. It can be seen from the Fig 3b, that the spectra of all three of them are different at the higher energies of the spectrum. A slanting behavior of the nanosheet films on copper electrode may be due to the effect of copper as the lightly deposited Cu electrode shows a distinct slanting behavior.
Fig 2. a) Raman Spectra of WS2 nanosheets (powder) at laser power 1% b) Raman Spectra of WS2 nanosheets powder with laser power 5%(WS2_NS(L.P-5%)), WS2 nanosheets films on copper electrode (WS2_NSs film) and Copper electrode less deposited by WS2 nanosheets (Cu prevalent WS2 film)
From the literature , it can be seen that the IV characteristics of Ws2 film deposited on quartz film shows a linear characteristics.
From the present work it can be seen the IV characteristics of films are linear and of the order of micro amp which implies that the film is acting as a semiconducting material. The results were ensured by performing the analysis for several films. The obtained nature of the film is a consequence of the interaction of the WS2 films on Copper electrode as well as the insulating portion of PCB as a whole. This effect can be confirmed by changing the IV characteristic Ws2 nanosheets films on electrodes with different configuration.
Fig 3 . The IV characteristics of WS2 films, before heating and heating at 55.30 C for two times.
Different configuration of electrodes show different current behavior.
From the present work it can be seen that electrical characteristics of the films are linear. Therefore the response of these films to particular external perturbation can be detect easily. So, this can be effectively used for the certain application. To increase the efficiency, the films can be further modified by several agents and can be doped. In the present work several aspects of the films has not been discussed like the absorbance of light and again real samples are not taken into account. So, in future, study of these properties can help to know more about such types of film and their applicability in areas like optoelectronics and sensing etc.
1. Alexander V. Kolonov, Junji Tominaga, Two Dimensional Transition-Metal Dichalcogenides, Springer International Publishing Switzerland, 2016
2. Martin Pumera, Adeline Huiling Loo, Layered transition-metal dichalcogenides (MoS2 and WS2) for sensing and biosensing, Trends in Analytical Chemistry, 61 (2014) 49–53
3. 2D Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS2, WS2 and Phosphorene Maurizio Donarelli and Luca Ottaviano, Sensors 2018, 18, 3638
4. Lopez et. al., Photosensor Device Based on Few-Layered WS 2 Films, Adv. Funct. Mater. 2013, 23, 5511–5517)
5. Longhui Zeng, Lili Tao, Chunyin Tang, Bo Zhou, Hui Long, Yang Chai, Shu Ping Lau & Yuen Hong Tsang, High-responsivity UV-V is Photodetector Based on Transferable WS2 Film Deposited by Magnetron Sputtering, Scientific Reports,6,20343
6. Alexander V. Kolonov, Junji Tominaga, Two Dimensional Transition-Metal Dichalcogenides, Springer International Publishing Switzerland, 2016
7. Ravindra Kumar Jha and Prasanta Kumar Guha, Liquid exfoliated pristine WS2 nanosheets for ultrasensitive and highly stable chemiresistive humidity sensors, Nanotechnology 27 (2016) 475503 (11pp)
8. Modern Raman Spectroscopy-A Practical Approach, Ewen Smith and Geoffrey Dent, John Wiley & Sons Ltd, Hoboken, USA, 2005
9. Raman Microscopy: Developments and Applications, G.Turrell, J.Corset, Elsevier Academic Press, California, USA,1996
10. Ayse Berkdemir, Humberto R. Gutie´rrez, Andre´s R. Botello-Me´ndez, Ne´stor Perea-Lo´pez, Ana Laura Elı´as, Chen-Ing Chia, Bei Wang, Vincent H. Crespi, Florentino Lo´pez-Uri´as, Jean-Christophe Charlier, Humberto Terrones & Mauricio Terrones, Identification of individual and few layers of WS2 using Raman Spectroscopy,Scientific reports,3,1755,2013
11. Néstor Perea-López, Ana Laura Elías, Ayse Berkdemir, Andres Castro-Beltran ,Humberto R. Gutiérrez, Simin Feng, Ruitao Lv, Takuya Hayashi, Florentino López-Urías ,Sujoy Ghosh, Baleeswaraiah Muchharla, Saikat Talapatra, Humberto Terrones, and Mauricio Terrones, Photosensor Device Based on Few-Layered WS 2 Films, Adv. Funct. Mater. 2013, 23, 5511–551
12. Néstor Perea-López, Ana Laura Elías, Ayse Berkdemir, Andres Castro-Beltran, Humberto R. Gutiérrez, Simin Feng, Ruitao Lv, Takuya Hayashi, Florentino López-Urías, Sujoy Ghosh, Baleeswaraiah Muchharla, Saikat Talapatra, Humberto Terrones, and Mauricio Terrones, Photosensor Device Based on Few-Layered WS 2 Films, Adv. Funct. Mater. 2013, 23, 5511–5517
It is my pleasure to be indebted to various people, who directly or indirectly contributed in the development of this work and who influenced my thinking, behavior, and acts during the course of study. I express my sincere gratitude to INDIAN ACADEMY OF SCIENCES for providing me an opportunity to undergo SUMMER RESEARCH FELLOWSHIP at TEZPUR UNIVERSITY. I am thankful to DR. RAJIB BISWAS for his support, cooperation, and motivation provided to me during the training for constant inspiration, presence and blessings. I also extend my sincere appreciation to Ms. Ashamoni Neog who provided her valuable suggestions and precious time in accomplishing my project report. It is my privilege to express my profound regards and deep sense of gratitude to Dr. Ranjit Sharma, former Head of the Dept., Department of Physics, Jagannath Barooah College, Assam, India for providing my letter of recommendation to Indian Academy of Sciences. I would like to thank the almighty and my parents for their moral support and my friends with whom I shared my day-to-day experience and received lots of suggestions that improved my quality of work.