Fabrication and Investigation of the Properties of 2D Hybrid Materials
Abstract: 2D nanomaterials have the potential to redefine and enhance our lives from invention of wearable electronics to the production of pollutant free water. This literature review explores the properties of these 2D materials and its future prospective, focusing on the fabrication of hybrid materials from graphene and MoS2 and its uses in electrochemical devices. The structure of the hybrid material heterostructures and its advantage of being able to possess the combined properties of its constituent 2D materials is also a topic of discussion in this review. Furthermore, the review describes the current production methods of these 2D materials and introduces the revolutionary method used in UCL labs and it’s benefits in terms of scalability. Apart from that, this review examines the methods used to analyse the 2D materials. This includes the use of TEM, XRD and Raman spectroscopy. The review also gives a summary of the aim and outline for the 4th year MSci research project which will be undertaken by the author over the course of the next 6 months.
Table of Contents
Since the discovery of Graphene, the rise of the revolution of 2D materials in the 21st century has sparked a large interest amongst the academic community. This has led to many fundamental investigations due to its potential applications in fields such as nanoelectronics, flexible devices, sustainable energy and catalyst . 2d materials are materials with thickness of a few nanometres or less, comparable to thickness of a single layer of atom. There is a strong bonding inside the plane between the atoms and weak van der Waals bonding between the planes. The figure below shows some examples of the structures of these materials.
Figure 1: From left to right, graphene, hBN and MoS2. The figure illustrates the top and side view of these materials and the distance between its layers. Taken from ref.
2D materials are normally categorized as 2D allotropes of elements, or compounds that has 2 or more covalently bonded elements . The electrons in these types of materials are only free to move in 2 dimensions and their restricted motions are governed by quantum mechanics.
These materials normally have enhanced properties compared to their 3D counterparts such as mechanically strength, flexibility, optically transparent and a good conductor of heat and electricity as they have very high surface area ratio. This enables them to be imbedded in other materials to form functional composites and constructed into nanoscale devices.
Thanks to their minimum defects, some 2D materials exhibit large mobilities in charge carriers as scattering is minimized . Moreover, some 2D semiconducting materials such as MoS2 which are transition metal-dichalcogenides (TMDs) have interesting electronic band structures as they are thinned to monolayers which allows the absorption of photons between the infrared to the ultraviolet region  . A larger number of electronic transitions options are now allowed, and this opens the door for high performance optoelectronic devices and fields related to optics and photonics .
Figure 2: Illustrates electron mobility at room temperature against the bandgap for various materials. Experimental data for GNRs, graphene and BLG were taken from ref . III–V materials, from left to right InSb, InAs, In0.53Ga0.47As, InP, GaAs, In0.52Al0.48As, Al0.3Ga0.7As, Ga0.51In0.49P. Data for these materials were compiled from ref . Data for silicene and germanene were taken from ref   . Data for MoS2 taken from ref        . Data for WS2, MoSe2 and WSe2 were taken from ref    .
Despite the potential applications, it still is a challenge to integrate 2D functional layers with 3D systems which is one of the topics of this research.
Graphene is one of the most studied material since its isolation by Andre Geim and Konstantin Novoselov in 2004 at the University of Manchester . Graphene is an allotrope of carbon, in fact it is the basic structural element of other allotropes of carbon such as graphite, diamond and fullerenes. It has only a single layer of carbon atoms arranged in a hexagonal lattice and has a zero-bandgap due to the small overlap between its valence and conduction bands. See figure 1. Graphene is well researched for its properties such as its ultrahigh carrier mobility, exceptional electric and thermal conductivity, larger surface to volume ratio, excellent optical transparency, quantum hall effect and high Young’s modules . Other than that, graphene is known to be very strong. It has an incredible tensile strength of 130.5 GPa which is 200 times higher than steel but at the same time it is very flexible . Coupled with its ability to conduct heat and electricity very efficiently, this makes graphene very desirable to be used in electronic components.
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The bulk of MoS2 is composed of its monolayers stacked on each other and held together by weak van der Waals force and as before there is a difference in property between the bulk and the monolayer of MoS2. In particular, the bulk MoS2 has an indirect electronic band gap of 1.2 eV   while the monolayer has a direct band gap of 1.8 eV . Unlike graphene, this layer dependence of band structure property of MoS2 makes its optoelectronic properties much more interesting and useful . Other than that, MoS2 also possess high mechanical strength, electrical conductivity and emits light which opens up to more applications such as photodetectors. See figure 1 for structure of MoS2.
Hybrid materials are materials consisting of 2 or more constituents at the microscopic level, with new properties created from the formation of new electron orbitals between these materials . The new properties may be a combination of the individual properties of its constituent materials. Traditionally, the constituents are made up of an organic and inorganic material and the hybrid materials can be classified based on the interactions between its constituents. Class I hybrid materials exhibit weak interactions between the organic and inorganic constituents such as hydrogen bonding, van der Waals interaction and weak electrostatic force. Class II hybrid materials exhibit strong interactions between its constituents such as covalent bonding .
Heterostructures are simply hybrid materials arranged in such a way that the 2D monolayers are stacked in a vertical stack with van der Waals forces holding them together.  When stacking these layers together, the synergic effect becomes dominant where charge distribution might be affected between neighbouring and distant layers. This leads to numerous exciting physics phenomena and yields a range of possible applications. For examples, high electron mobility graphene transistors and LED are produced by encapsulating graphene with hexagonal boron nitride (hBN) .
Figure 3: Schematic of a traditional heterostructure LED taken from ref . The encapsulating hBN layers act as tunnelling barriers while the grapheme layers act as electrodes to which a bias voltage (Vbias) is applied. The electrodes will then inject electron and holes into the central layer of monolayer MoS2 which has a direct bandgap. The electrons and holes will combine here to emit photons.
The following are the possible applications of 2D materials in the various areas:
i) Energy: Batteries Storage and Consumption, Super Capacitors, Solar Cells
ii) Electronics: Integrated circuits, wearable electronics, optoelectronics
iii) Automotive: Thermal Management, Fuel Cells
iv) Medical: Drug Delivery, bio-sensing, anti-bacterial coating
My project focuses more on fabrication of hybrid materials to be used in electrochemical devices for energy storage based on the UCL production novel routes for 2D material. This will be discussed in the next section.
Mechanical exfoliation was performed for isolating graphene flakes for the first time, by using adhesive tapes to separate them from the bulk and transfer them onto a silicon wafer. To successfully exfoliate, a peeling force needs to be applied to a single or a few atom-thick sheets by attaching them to a scotch tape. The material can then be cleaved from the tape using a substrate. 
Chemical vapour deposition (CVD) is the method where the desired deposit is produced by exposing the wafer(substrate) to volatile precursors which then will react and decompose on the substrate’s surface .
Although monolayers produced by the mechanical exfoliation and CVD methods are of high quality, yet these methods lack scalability and are costly which proves them to be unsuitable for many applications .
My project attempts to use new method of the liquid exfoliation process developed by the UCL team instead, to overcome these issues on scalability. It is important for 2D nanomaterials to achieve liquid-phase delamination from their bulky counterparts as this makes the production of these materials scalable and easily manipulated for applications      . The current methods involve applying significant energy in the form of shear force , ultrasonication    and chemical reaction  to break apart the layers of the 2D materials which are held together by the strong van der Waals force thanks to the maximized surface area of these layers. The same can also be achieved by introducing charges onto the layers and via electrochemical intercalation    . These methods only produce metastable dispersions which can be stabilized by the addition of functional groups, but this deteriorates the property of the material.
Figure 4: Visualization from ref  on how the current liquid phase exfoliation techniques are performed.
The above methods focus on retarding reaggregation as opposed to dissolution driven by thermodynamics as seen in particles such as NaCl (table salt) which dissolve in solvents and are much more stable . The team at UCL has succeeded in doing this by forming layered 2D material salts which dissolve spontaneously without any chemical reaction in a polar solvent to form an ionic solution .
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The process begins with the intercalation of the negatively charged layered material with alkali-metal cations. This produces a salt in which there is a transfer of charge between the conduction band of the layered material and the valence electrons of the intercalants.   . The layered material salt is then carefully dissolved in a aprotic polar solvent such as tetrahydrofuran, THF or N,N-dimethylformamide, DMF . This solution is only stable in an inert environment. It precipitates fast when exposed to air. One of the biggest advantages of this process is that the morphology of the initial material is maintained, and the salt solution doesn’t reaggregate. The charge on the negatively charged nanosheet solutes are also reversible which facilitates electroplating of the 2D materials. Upon drying this will initiate novel self-assembly of the 2D material on the electroplated surface . This method has high scalability and cost friendly as well.
A few techniques will be used during the course of this project to analyze the final 2D material. Raman Spectroscopy is used to find low-frequency phonons in the material which helps to identify the particles present in the material. It relies on inelastic scattering where a monochromatic light from a laser interacts with the phonons in the system which results in the energy shift of the emitted photons. This shift in energy gives us information of the extent of intercalation . Next, x-ray diffraction helps to determine the molecular structure of the material and gives information of the effect intercalation has on the lattice spacing from Bragg’s Law. Another method that will be used is the transmission electron microscopy (TEM), where an image is formed from the interaction of the beam of electron transmitted through the material. The image produced can then be magnified to observe the structure of the 2D material.
Nanomaterials in liquids makes it easier to print, assemble and embed these 2D materials into coating and composites which is ideal in applications for energy generation and storage  .
An ideal energy storage unit should have high energy and high-power density. As such lithium ion batteries, one of the commonly used rechargeable batteries, comes quite close to this requirement but it isn’t suitable for large scale application such as in electric cars .
A typical lithium ion battery consists of a graphite electrode as the anode (negative) and a transition-metal oxide as the cathode (positive). These 2 electrodes are isolated by a porous polyethylene or a polypropylene thin film separator which are filled with lithium ion conducting organic electrolyte .
Figure 5: Schematic of a Li ion battery from ref . The battery is charging when the cathode is oxidized to produce Li+ ions and electrons. The ions travel through the electrolyte while the electrons travel through the external circuit to reach the anode. The opposite happens during discharge.
Although Lithium batteries has a high energy density, it has a relatively low self-discharge compared to nickel-based batteries. Lithium ion batteries require an additional protection circuit to maintain the voltage and current within safety limits. Furthermore, the battery is prone to aging, so most batteries are only expected to function for 3 years. The rechargeable capability falters as they age. Not to mention, they are quite expensive to manufacture as compared to nickel-based batteries.
The aim of my project is to fabricate a hybrid material using graphene and MoS2 by utilizing the new liquid exfoliation method used by the UCL team as discussed in section 5. It is predicted that this hybrid material will have a new combined property of both graphene and MoS2. Thus, it is also the aim of the project to study the properties of this hybrid material and to explore options to use sodium ions (Na+), instead of potassium (K+) and lithium (Li+) ions during intercalation to produce the layered material salt. This is because, sodium is much more abundant resource as opposed to potassium and lithium. In the event of the success of this project with sodium ions, people from around the globe would be given equal opportunity to benefit by employing this new liquid exfoliation method and to develop it further to the point we are able to produce scalable 2D materials to be used in our daily life applications such as in energy storage and generation.
The total duration of my project is 25 weeks commencing on the 1st of October 2018 and ending on the last day of term 2 which is the 22th March of 2019. No work will be done during reading week in both terms which are on week 6 and week 20. No work will also be done in the labs during winter break which is from week 12 till week 14.
- Week 1 to 3: Read research materials, familiarize with the concept of the project and work on the literature review.
- Week 4 to 5: Submit literature review on the 22nd of October 2018. Attend safety and equipment training. Practice X-ray diffraction technique on graphene and MoS2. Receive feedback for literature review.
- Week 7 to 9: Start making the layered material salt with potassium and lithium ions based on the new liquid exfoliation technique developed by the UCL team. Learn Raman Spectroscopy technique and use it to analyze the layered material.
- Week 10 to 11: Explore mixing graphene and MoS2 layered salt together to create a hybrid 2D material.
- Week 12 to 14: Work on progress report during winter break.
- Week 15 to 25: Submit progress report and receiver feedback for it. Attend progress interview with supervisors. Continue working with data collected in term 1. Analyze the hybrid material fabricated from graphene and MoS2. Attempt intercalation with sodium ions and analyze the property of this material. If time permits, experiments for battery storage and generation will be carried out with this hybrid material.
- Week 20 to 25: Work on final report alongside with the project which is due on the 22nd of March 2019.
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