What do you know about two-dimensional (2D) nanomaterials?

Dr. Omar Mhaidi Dawood Habib-Department of Physics

esp.omarm.dawood@uoanbar.edu.iq

The author's official website

Nanomaterials can be defined as materials having at least one external dimension on a scale of 1 to 100 nanometers. There are wide applications for the use of nanomaterials spanning across industries, from healthcare and cosmetics to environmental conservation and air purification.

Accordingly, if all dimensions of a material are measured within the nanoscale, then this material is classified as a zero-dimensional (0D) nanomaterials. Nanoparticles are the most common materials for 0D materials and fullerenes, a form of carbon with a large spherical molecule consisting of a hollow cage of atoms, was the first known example.

If one of the dimensions of the material is outside the nanoscale, then these nanomaterials are classified as one-dimensional (1D) materials. Carbon nanotubes (CNT) and nanowires are a common example of this category. Carbon nanotubes (CNT) are a tube made of carbon with diameters usually measured in nanometers.

If a material has two dimensions outside the nanoscale and only one dimension with one thickness or a few atomic layers, then these nanomaterials are classified as two-dimensional (2D) nanomaterials. This category includes many forms, the most famous of which are graphene, phosphorous, tungsten disulfide, molybdenum disulfide and other forms. Another classification is three-dimensional (3D) nanomaterials. They are materials that are not limited to the nanoscale in any dimension. This category contains bulk powders,  nanoparticle dispersions, nanowire bundles, nanotubes as well as multiple nano-layers.

 Each class of nanomaterials above has important applications in different areas of life, but what concerns us is the study of two-dimensional (2D) nanomaterials. We will focus on graphene. We must mention an important thing, which is that graphene is the basic building block for other graphite materials such as mentioned above, meaning: If sheets of graphene are bonded on top of each other, we can make 3D materials like graphite; If a sheet of graphene is rolled up, we can make a carbon nanotube, 1D material; If a sheet of graphene is cut and folded into a spherical shape, we can make a fullerene, 0D.

The first known 2D material is graphene, which consists of a single layer of carbon atoms arranged in a hexagonal lattice. To compare with 0D material (fullerene) and 1D material (carbon nanotubes), research on 2D materials (graphene) has grown rapidly on other carbon properties. Based on the Scopus database (search by keyword “graphene” on March 18, 2019), publications on graphene increased from 3,772 papers in 2010 to 21,439 papers in 2018. The total number of publications on graphene is 132,628 documents. However, it is not only 2D graphene that has been widely applied in a great variety of potential applications but also other 2D materials such as tungsten disulfide, molybdenum disulfide and silicon nitride open up new opportunities for future devices.

Graphene is manufactured in several ways. We will briefly address some of them

(1)- The first method is the mechanical exfoliation method, which was used by the two scientists, Novoselov and Jim, in 2004 from the University of Manchester in the United Kingdom, to obtain the first two-dimensional material, graphene. Since then this method is widely used in the manufacture of 2D materials due to its flexibility and low cost. Mechanical exfoliation is a simple material synthesis process that produces one to several layers of two-dimensional crystal flakes while preserving the crystal structure and properties. Easy adhesive tape is used to peel off a thick layer of graphite, such that the peeled layers are deposited on the substrate for study and possible application. Mechanical exfoliation is a low-cost method for producing 2D materials, which is ideal for fundamental analysis. However, this technology is not well scalable for 2D chips.

(2)-  Chemical exfoliation method, modified Hammers method, which is one of the common methods for the growth of graphene oxide based on suitable oxidizing agents of graphite oxide. This method offers a large amount of graphene products and is low in cost.

 (3)- The electrochemical exfoliation method is based on the formation of a graphene product from a graphite or highly pyrolytic graphite (HOPG) rod using electricity to peel a graphite or HOPG rod immersed in electrolyte solutions.

(4)- Chemical vapor deposition (CVD) method provides high quality graphene products with controllable graphene layers over a large area [3, 4]. Typically, methane (CH4) and acetylene (C2H2) were used as the carbon source to grow graphene on copper (Cu) or nickel (Ni) foam under high temperature around 1000 °C.

 Graphene properties

Graphene is a material with interesting properties. These properties, combined with the abundance of carbon in nature, have made graphene a highly studied material with great potential.

1- The most prominent properties of graphene are:

2- high thermal conductivity

3- high electrical conductivity

4- High flexibility and ductility

5- high hardness

6- high resistance. Graphene is about 200 times stronger than steel, similar in strength to diamond, but much lighter.

7- Not affected by ionizing radiation

8- Capable of generating electricity by exposure to sunlight

9- transparent material

10- A high density does not allow the passage of helium atoms, but allows the passage of water, which evaporates at the same speed as if it were in an open container.

11- antibacterial effect; Bacteria are unable to grow in it.

12- Low joule effect, heating when conducting electrons.

13- Low electricity consumption compared to other vehicles.

Applications of graphene

Despite its relatively recent discovery, graphene has been tested extensively in fabricating many different devices, some in the early stages of development, others already on the market. Due to its high transparency and high electrical conductivity, graphene is very attractive as a transparent conductor, which could be a less environmentally damaging replacement for indium tin oxide (ITO). Due to its flexibility, graphene can also be used in flexible displays. The electrical properties of graphene are also taken of advantage of in creating field-effect transistors (FET), photodetectors, photovoltaics, nano electromechanical systems (NEMS), flexible supercapacitors and flexible lithium-ion batteries.

In addition to its electrical properties, its chemical and thermal properties lend it for use in strain sensors, gas sensors, temperature sensors, biosensors and flow sensors. Smartphone manufacturer Huawei has begun employing graphene in certain smartphones as a cooling device for integrated circuits. The mechanical properties of graphene have also been taken advantage of to make polymer-based graphene composites and graphene fibres. It is also being incorporated into rubber to improve mechanical properties.