Graphene Properties and Potential Applications
Graphene is a 2-dimensional material composed of a hexagonal array of carbon atoms wherein each atom is bonded to its three nearest neighbors in the plane of the sheet. This sheet has a thickness equivalent to the atomic diameter of a carbon atom, roughly 0.34 nm. Some basic math and chemical properties can be used to estimate that an entire football field could be covered with a single graphene sheet that weighed less than a one gram! Indeed, the theoretical specific surface area of graphene is approximately 2,630 m2/g. Although its existence had been hypothesized decades before and it had been studied in various layered forms on substrates for more than 50 years, graphene did not become a popular sensation until it was mechanically isolated and electronically characterized in 2004. The results were published in the famous article by Novoselov et al. [Science, vol. 306, p. 666, 2004], which ignited a firestorm of research into the electronic, optical, and mechanical physicochemical properties of this amazing material.
Even though various forms of graphene layers had been studied theoretically and experimentally before, it was widely considered to be impossible to isolate a single graphitic sheet because of thermally-induced fluctuations of atomic motions perpendicular to the atomic plane; i.e., thermal energy imparts kinetic energy to the constituent atoms of the graphene sheet which can induce large-amplitude fluctuations in the bond lengths normal to the 2-d plane. If the fluctuations are large enough, the carbon-carbon bonds can break, releasing the atoms and ultimately destroying the entire sheet. After the isolation of single-layer graphene, however, it became obvious that this was not the case, and at first researchers thought that the rippling patterns apparent in single sheets were responsible for stabilizing the hexagonal network of carbon atoms by imparting 3-dimensional matrix characteristics to the 2-d sheet; i.e., the slight local out-of-plane distortion of the carbon network effectively mitigated large-scale fluctuations in bond lengths. Later research, however, revealed that the primary reason for the chemical stabilization of graphene was simply that the in-plane carbon-carbon bonds (about 0.14 nm in length) in the hexagonal 2-d form are strong enough to suppress large-amplitude, thermally-induced atomic displacements perpendicular to the material plane.
So what is it about the carbon-carbon bonds that makes graphene so stable and gives it such incredible physicochemical properties? Besides being a 2-d material, what makes graphene so versatile is its unique bonding structure, which gives rise to a remarkable array of outstanding thermophysical properties; specifically, the carbon network in graphene is sp2 hybridized, which refers to the orbital combination of the four outer shell (i.e., the 2-shell) electrons of each carbon atom. In this case, the 2-sigma (2s) orbital combines with two of the three available pi orbitals (2p) to form the hexagonal packing arrangement, leaving the third available pi orbital perpendicular to the plane with a free electron that provides graphene with its amazing electrical and thermal conductivity.
Electronic properties With the network of pi orbitals oriented perpendicular to the sheet plane, graphene possesses almost magical properties of electrical conduction, potentially changing the game in the semiconductor industry. The electron mobility at room temperature has been reported in excess of 15,000 cm2/V s and it can operate at terahertz frequencies (trillions of operations per second). It is more electrically conductive than copper, can function as a nanocapacitor, and acts as a low temperature zero-gap semiconductor. Its ultrahigh specific surface area makes it an ideal coating for electrodes in batteries and fuel cells since maximal coverage can be achieved with low graphene mass. Furthermore, the high electron mobility translates into an in-plane thermal conductivity of up to 5,000 W/m K, which is over 10 times higher than that of highly conductive silver; this opens up many opportunities for increasing the rate of heat transport and dissipation in many high temperature applications, such as thermal pastes, extreme-environment lubricants, and heat transfer equipment such as radiators, refrigerators, and HVAC units.
When it comes to toughness, graphene has no peers. It is 250 times stronger than diamond and steel with a tensile strength of over 125 GPa. It is also highly elastic, deforming under strain with an unbelievable value of Young’s modulus of about 1,000 GPa, over twice the value of tungsten. Nevertheless, graphene is flexible in spite of its inherent crystallinity and mechanical strength, making it a potentially revolutionary material for flexible displays and sensors. Small amounts of graphene have been used to reinforce thermosetting plastics, rubbers, and cements, which have shown remarkable enhancement in the resulting nanocomposite’s overall mechanical properties, such as increasing tensile strength, preventing crack formation, increasing lubricity, and extending the product lifetime. Many new applications in a wide range of industries are currently being studied as potential nanocomposites systems ranging from mundane structural and pavement applications to 3-d printing materials.
Optical properties Graphene’s prospects for revolutionizing the optical electronics industry are equally startling as its mechanical properties. Its high opacity for an atomic monolayer material caught everyone by surprise and sparked a thousand ideas of possible applications in relevant industries. Graphene absorbs only 2.3% of incident white light and can be readily saturated under excitation throughout the visible wavelengths and near-infrared region. What this translates to in actual real-world terms is that graphene might be an appropriate material for applications ranging from light-weight, scratch-resistant screens for digital imaging to high intensity, fast saturation absorption for phase-locking of lasers that is relatively insensitive to the wavelength.
The potential applications of graphene are virtually mindboggling. In the past ten years, an explosion of research has appeared as researchers worldwide have looked at this material from every conceivable angle, hoping to turn its amazing physical properties into new inventions to better the human condition. Several potential applications of graphene technology have already been mentioned above; below is a list of some of the more conservative potential applications of graphene. Many more fantastical (one might even say science-fiction) applications have been proposed at various levels of probability (or even sensibility). Nevertheless, the future of graphene is very promising, even if only the most mundane of the potential applications actually become reality.
High performance films
Improved coatings and paints
3D printing technology
Thermally conductive plastics
High/low temperature greases
Automotive fluid additives
Thermally conductive foams
High performance polymer composites
Barrier coatings and films
Electrode materials for batteries
Electrically conductive films and coatings
Heat exchange transfer media
Structural materials applications
Touchscreens and LCDs
High temperature lubricants
Gas separation membranes
High frequency transistors
Electrode materials for batteries
Electrically conductive polymers
Electrically conductive inks
Optical displays and sensors
Fuel cell energy storage