Silicon is an amazing element: it’s abundant, inexpensive, relativity easy to find work with, an excellent insulator, and non-toxic in normal handling (we’re not talking about Silicon dust here). Beyond those basic attributes, it is an amazing material in what it can be used for: as a substrate for ICs (yes, there are newer competitors such as GaAs, but it’s a mostly silicon world for the foreseeable future), as a piezoelectric material usable in both stress-to-electric and electric-to-dimensional motion modes, and for micromachined electromechanical systems (MEMS). We are constantly reading about how silicon-based components and structures are leading to innovations in electronics, advanced mechanical, electrochemical and biochemical sensors, and more.
Now there’s another material which is taking on a silicon-like guise: Graphene. Like Silicon, it is based on a single abundant element – here, Carbon – and has similar material advantages (of course, unlike Silicon, it is highly conductive). Carbon and Graphite (a gray, crystalline, allotropic form of carbon) has long been used in electronic, mechanical, and chemical applications, as brushed contacts, as a bearing material, as a bio-attractant, and even as a lubricant, to cite a few of its many uses. We also know of Carbon in its form as buckminsterfullerene (a discrete soccer-ball-shaped molecule containing carbon 60 atoms), and as diamonds, of course. That’s quite a span of appearances.
So, what’s Graphene?: In brief, it’s a sheet of a single layer of Carbon atoms, tightly bound in a hexagonal honeycomb lattice. It was analyzed and created in the early 2000s by physicists Sir Konstantin Sergeevich Novoselov and Sir Andre Konstantin Geim, both at the University of Manchester UK; they received the Nobel Prize for their work in 2010. It has extraordinary mechanical properties including very high strength-to-weight ratio.
But its uses go beyond that of a structural material or building block. It’s seeing diverse applications including battery electrodes, optical sensors, and even electro-optics. A few recent examples make this dramatically clear:
As a sensor: A research team supported by the Department of Energy at the UCLA Samueli School of Engineering has now used it, along with gold and semiconductor processing techniques, to create a greatly improved photodetectors. Going beyond the tradeoff between a narrow-band/high-sensitivity or wide-band/low sensitivity detector, this one has ultrabroad operation from the visible to the infrared regime, with responsivity ranging from 0.6 A/W (at 0.8 μm wavelength) to 11.5 A/W (20 μm wavelength). (See “Gold-patched graphene nano-stripes for high-responsivity and ultrafast photodetection from the visible to infrared regime”.)
Schematic of the photodetector developed by UCLA engineers, showing gold comb-shaped nanopatterns on top of graphene nanostripes (hexagonal structures) and light source (pink cylinder). (Image source: UCLA)
As a washable circuit material: Iowa State University is leading a team which has combined Graphene and sophisticated laser-based processing, and they have developed circuits that are low-cost, flexible, highly conductive, and water-repellent. They use an inkjet printer to deposit the Graphene, then use a rapid-pulse laser process to treat the Graphene without damaging the printing surface even if it is paper or ultrathin polymers, which transforms Graphene-printed circuits from ones which hold water droplets (hydrophilic) into circuits that repel water (superhydrophobic). (See “Superhydrophobic inkjet printed flexible graphene circuits via direct-pulsed laser writing”.)
(left) The Graphene surface pattern and (right) wettability contact angle, shown before (top pair) and after (bottom pair) the direct-pulsed laser writing procedure. (Image source: Iowa State University)
As a high-speed electro-optical modulator: Using Graphene on Silicon, researchers at Columbia University are integrating an optical phase-delay component that can be embedded in a waveguide and function as an electrostatically tuned modulator with low insertion low and high refractive index. (see “Low Power Optical Phase Array Using Graphene on Silicon Photonics”.)
Graphene’s Electro-Optic Properties. (a) Theoretical absorption and refractive index as a function of Fermi level for intrinsic graphene (region I - high absorption, region II - low absorption). (b) Optical micrograph of the fabricated device (interferometer arms false colored). (c) Device cross section showing Graphene-HfO2-Graphene capacitor on Si3N4 waveguide. (Image source: Columbia University)
As a human-organ replacement: A team at Tsinghua University has developed an intelligent artificial throat based on laser-induced Graphene (LIG). The intelligent device both detects and generates sound in a single unit. The biocompatible artificial throat, which attaches to the larynx, is designed to produce recognizable and controllable sounds, while the integrated acoustic device generates sound based on the thermoacoustic effect of Graphene and detects sound based on its piezoresistive effect. (See “An intelligent artificial throat with sound-sensing ability based on laser induced graphene”.)
(a) The working procedure of the artificial throat; (b) The tester wearing the LIG artificial throat. (Scale bar, 1 cm); (c) high-volume, low-volume and elongated tone hum are detected by LIG throat and converted into high-volume 1- kHz, low-volume 10-kHz and low-volume 5-kHz sound, respectively; (d) The magnified wave of high-volume 10 kHz sound. (e) The magnified wave of low-volume 10-kHz sound; (f) The magnified wave of low-volume-5 kHz sound. (image source: Tsinghua University)
Where this all leads is a question that can’t be answered, at least not at this time. There’s no doubt that innovation is often highly dependent on new, ultrapure, well-understood materials, ranging from Silicon to magnetic disk coating, to polymers and highly refined metals. Brilliant and iconoclast physicist Richard Feynman pointed this out in his 1959 lecture for the American Physical Society meeting at Caltech, “There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics.” His perspectives on the possibilities of nanotechnology and nanomaterials has certainly been proven highly prescient.
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