This is Science: News from Graphene Fields



    The discovery of graphene and a description of its properties in 2004 brought its creators Game and Novoselov the Nobel Prize in 2010, but already a decade after that discovery, their followers intensively implement and offer various applications of such a unique material: from lubricant to vacuum transistors .

    At the beginning of this year, an article “Graphene - life or death?” Was published , in which we understood the prospects and, in part, the consequences of the widespread introduction of graphene, as the basis of microelectronics of the future. Well, let's see what scientists can offer us today as part of the introduction of graphene to replace materials that have already become traditional.

    Graphene quick reference


    Graphene is a two-dimensional material consisting of sp 2 -hybridized carbon atoms, which has a number of properties interesting from the point of view of physics.

    The most important and unique of them is the electrical characteristics. Graphene, on the one hand, has virtually zero band gap and very light electrons and holes, which makes it an ideal conductor, able to conduct signals faster than any other material on the planet. However, the sp 2 hybridization of carbon atoms also allows its modification to be carried out, for example, to obtain an insulator or semiconductor. Plus, the transition between the conducting and semiconducting states depends on the width of the graphene ribbon .

    image

    On the other hand, it is elastic, that is, it bends, while demonstrating a unique and unattainable tensile strength - up to ~ 1000 GPa, which is almost 100 times higher than that of steel. A rolled up graphene is a carbon nanotube, which can also be used in electronic devices; its diameter can vary from 1.5 nm to hundreds of nm.

    And finally, it, graphene, is transparent, that is, it is simply ideal as a substitute for the expensive one - mainly because of India - ITO in modern displays and partly LEDs. However, two-dimensional systems themselves are not stable. Thus, the problem of creating perfectly even graphene coatings on any surface is a difficult scientific and technical task.

    Perhaps, we will begin with the last property.

    LED or graphene-based LED



    Remarochka. If we consider an LED as such (in bulbs, for example), then it potentially does not need an ITO substrate as an electrode, and the thinnest metal contacts can do this relatively well (I wrote about this in a separate article ). However, if you want to create a display on an array of LEDs, then in this case, the replacement of ITO is extremely desirable and useful, including to improve the performance of the display.

    So, one of the industrial methods of coating graphene on various substrates is PECVD (or plasma-chemical vapor deposition) This technology consists in the “injection” of methane carrier gas with its subsequent heating under the influence of radio frequency radiation and the deposition of carbon on a cold substrate.

    Here is a group of scientists from Seoul National University and armed with this method of coating from graphene, offering a direct way to create bright blue diodes bypassing the stage of transfer of graphene from substrate to substrate. In such LEDs, the expensive ITO is replaced with a cheaper graphene substrate, and gallium nitride is used as the light-emitting layer, which is some standard for the industry.


    From left to right: PECVD installation diagram; diode layout and basic materials; current-voltage characteristic of the diode

    Of course, by varying the duration of the PECVD process, it is possible to obtain a coating of graphene of different thicknesses and, accordingly, with different light transmission. However, the minimum number of layers, as shown in the figure below, allows you to get almost 100% transmittance of the light emitted by the LED, and, as a result, a larger external current output.


    A section of a layered LED cake: from a sapphire substrate to 5-6 layers of graphene on the surface of the LED.

    The authors also tested for reproducibility and comparison with the traditional technology of transferring graphene from one substrate, usually used for growth, to another, which will already stand in the final device:


    ac) Test for reproducibility of results in a single cycle. de) Comparison of graphene-based LEDs obtained by direct deposition (DG) and transfer, transfer of graphene from substrate to substrate (TG)

    The results speak for themselves: with a relatively small spread in output power, the resulting LEDs confidently outperform conventional, “standard” technology at maximum power at a given current (comparison in Figure e).

    Original article at ACSNano (DOI: 10.1021 / nn405477f)

    Graphene RF-FET for Wearable Electronics


    The main problem of truly wearable electronics, which is built into clothes and in no way attracts attention - the combination of flexibility with certain characteristics. Scientists of the following two articles devoted their work to finding a solution to this problem.

    In the first of these, Taiwanese authors proposed an interesting way to create a field effect transistor on a flexible substrate, which could potentially become the basis for communication between the individual elements of wearable electronics.

    So, we need: graphene transferred onto a flexible PET substrate, a little aluminum to make a shutter and a drop of life-giving oxygen. Using lithography, we apply a shutter of aluminum to a strip of graphene, and then leave the device in a chamber with a couple of additional atmospheres of pure O2 . We don’t even have to do anything, chemistry and diffusion will do all the work for us, forming a blocking layer of dielectric between aluminum and graphene. After that, it remains only to "dust", that is, to apply the contacts themselves.


    Quite a simple and interesting scheme for the manufacture of a locking dielectric layer in a graphene transistor

    And voila, the assembly of the transistors is ready. This is how it looks through the eye of an electron microscope:


    An optical photograph of a substrate with field-effect transistors (a) and images of the transistors themselves obtained by scanning electron microscopy

    In order not to bore readers with the rather boring technical details of testing this sample of a graphene-based field effect transistor, let me immediately turn to potential development applications. The authors of the work assembled a frequency mixer based on the obtained field effect transistor and tested it, including during mechanical deformations.

    Read more about the principle of operation of the frequency mixer here . For a brief explanation of the picture: LO is the known unmodulated frequency with respect to which the conversion is performed, RF is the frequency that is converted / modulated, IF are used to supply and receive low and high frequency signals.


    a) Schematic diagram of a frequency mixer based on a field effect transistor. b) Radio frequency spectrum. cd) Radio frequency characteristics

    What does this give us ?! And it gives us a completely tiny transforming element of radio-frequency technology, which can be used, for example, for NFC-communication between, for example, individual devices inside smart clothes.

    Original article in ACSNano (DOI: 10.1021 / nn5036087)

    And since we touched on the topic of wearable electronics, let's turn to the example of creating field effect transistors on graphene-based fabrics.

    Bend me completely graphene


    Another group of South Korean scientists has proposed a method for creating graphene ultra-thin transistors on an ultra-thin polymer substrate, again consisting of a special epoxy resin.

    First, the thinnest polymer layer (SU-8) is applied to a conventional silicon substrate coated with a layer of silicon dioxide, on which graphene transistors are already “printed”, and then the SiO 2 layer is simply dissolved, thereby separating the thinnest film from the substrate. In this case, the film can be transferred to virtually any surface, including fabric or skin. The total film thickness is less than 100 nm!


    The process of creating a thin film with ultra-thin field effect transistors

    However, what problems await us with such a transfer? That's right, this is not surface uniformity: bends, cracks, creases - everything that can only be present on fabric surfaces. After all, changing the geometry of the transistor, we thereby change its transport properties, including the mobility of the charges or the distribution of the electric field on the gate, that is, it turns out that at the same nominal voltage, the transistor in a bent state will suddenly begin to pass current, whereas, in undeformed will lock it.

    Fortunately, all these problems were avoided; as a result, it turned out that the position of the transistor (in a bend or on a flat surface of the fabric) does not significantly affect the electrical behavior of the transistor itself.


    Testing transistors at different locations

    As well as bending, stretching, folding the fabric in half:


    Testing transistors during bending, torsion and stretching

    And without thinking twice, the scientists decided using the developed technology to create a tactile sensor capable of recognizing a touch of 9 kPa, which is equivalent to a pressure of 0.1 atmosphere or 100 grams forces per cm 2 : A


    tactile sensor capable of recognizing 0.1 kgf / cm 2

    This technology can find applications in smart clothes, as it does not require special equipment for application (as they say, pasted and forgot), including in the part of the nose My heart rate sensors, which do not cause discomfort, oxygen levels, and so on. But maybe - what the hellKorean scientists are not joking - it will help biorobots and bioprostheses acquire tactile sensations.

    Original article in ACSNano (DOI: 10.1021 / nn503446f)

    UPD: Thanks to koreec for providing a link to an article by Nobel laureates on graphene in Russian.



    A complete list of This is Science's published articles on GeekTimes:
    This is Science: Simple and Cheap Solar Energy
    This is Science: Graphene - Life or Death?
    This is Science: Blow and Get Electricity
    This is Science: Silicon Electronics: Bend Me All the Way!
    This is Science: An elastic display on quantum dots.
    This is Science: Putting triboelectricity at the service of humanity.
    This is Science: 3D optical printing moves to the micro level.
    This is Science: What's inside a neuromorphic chip?
    This is Science: News from graphene fields
    This is Science: 3D electronic lithography to the masses
    This is Science: Alkaline battery discharge or why the battery bounces
    This is Science: micro-guns and nanocores.
    This is Science: wearable electronics and triboelectricity. Part 1
    This is Science: Wearable Electronics and Triboelectricity. Part 2


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    When to wait for the first graphene-based industrial devices?

    • 10.8% for 2 years, we can handle 85
    • 44.1% for at least 3-5 years 346
    • 41% over 5 years 322
    • 3.9% Never Graphene Overcome Silicon Lobby 31

    Will graphene-based LEDs be promising without using ITO (in displays, for example)?

    • 66.4% Yes, if their price is lower than traditional with comparable characteristics 393
    • 25.5% Yes, even if their price is above 151
    • 3.3% No, due to low reliability 20
    • 4.5% No, due to poor, "inadequate" characteristics 27

    How do you feel about flexible and wearable electronics?

    • 55.2% I think this is good. I look forward to implementation and I will use 404 intensively
    • 15.3% I think this is good, but I won’t use it - I don’t want Google to know my pulse rate also 112
    • 20.2% It is unlikely that in the foreseeable future we will see progress in this area 148
    • 9.1% Absolutely useless waste of resources 67

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