Thursday, May 30, 2013

Singapore University Presents Graphene Image Sensor

IEEE Spectrum: Assistant Professor Wang Qijie from Singapore’s Nanyang Technological University (NTU) presents a graphene image sensor that is said to be 1,000 times more sensitive to light than current imaging sensors found in today’s cameras. Apparently, this claim comes from a photoconductive gain of 1,000 achieved by the device. The new sensor is said to use 10 times less energy as it operates at lower voltages. When mass produced, graphene sensors are estimated to cost at least five times cheaper, not clear why.

Prof. Wang Qijie believes this to be the first time that a broad-spectrum, high photosensitive sensor has been developed using pure graphene. Prof Wang came up with an idea to create nanostructures on graphene which will “trap” light-generated electron particles for a much longer time, resulting in a much stronger electric signal.

The “trapped electrons” is the key to achieving high photoresponse in graphene, which makes it far more effective than the normal CMOS or CCD image sensors, said Asst Prof Wang. Essentially, the stronger the electric signals generated, the clearer and sharper the photos (the above description strongly points to photoconductive gain).

"The performance of our graphene sensor can be further improved, such as the response speed, through nanostructure engineering of graphene, and preliminary results already verified the feasibility of our concept," Asst Prof Wang added.

This research, costing about $200,000, is funded by the Nanyang Assistant Professorship start-up grant and supported partially by the Ministry of Education research grants. Development of this sensor took Asst Prof Wang a total of 2 years to complete. His team consisted of two research fellows, Dr Zhang Yongzhe and Dr Li Xiaohui, and four doctoral students Liu Tao, Meng Bo, Liang Guozhen and Hu Xiaonan, from EEE, NTU. Two undergraduate students were also involved.

The new graphene-based sensor is described in this month’s Nature Communications ("Broadband high photoresponse from pure monolayer graphene photodetector"). The next step is to work with industry collaborators to develop the graphene sensor into a commercial product.

From the paper's figures, it appears that the new sensor has a FET-like pixel structure and works best at cryogenic temperatures, such as 12K:

Fabrication process of the device. (a) A monolayer graphene
was mechanically exfoliated onto a 285nm SiO2/Si substrate.
(b) The graphene photodetector was processed into a FET structure.
Two electrodes (that is, the source and the drain terminals) of
Ti/Au (20 nm/80 nm) were fabricated on the graphene by
photolithography and lift-off processes. The gate terminal
was fabricated on the bottom of the Si substrate. (c) A thin
nm-scale Ti sacrificial layer was deposited onto the graphene
by electron-beam evaporation. (d) The Ti sacrificial layer was
removed via wet etching, and then GQD array structure with
various quantum dot (QD) sizes can be formed on the Si
substrate depending on the thickness of the Ti layer.

9 comments:

  1. It looks like a nice pioneering work, but I think that the title is misleading, as well as some of the claims.
    What was presented is not an image sensor, but a single photodetector.
    1000 photoconductive gain does not necessarily imply an increase of sensitivity. This also depends on pixel readout electronic noise, multiplication noise, that is not discussed in their publication, and dark current noise, which looks very high. And, most importantly, the physical limit of photon shot noise is always there.
    I feel that the strength of this device is mainly in the very wide spectral response. While I don't see how it could become competitive with silicon image sensors for visible light, there could probably be some opportunities in the mid-IR spectral range.

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    Replies
    1. On the sensitivity side, I have not seen any claim on the QE. I hope they are not compensating a low QE by a high photoconductive gain. Regarding a potential use, the 30s time constant that they report might limit the scope of possible applications too. That large time constant hints to a huge photoconductive gain, BTW.

      Regarding image sensor rather vs photodetector, both NTU and Spectrum articles talk about the image sensor. Since I do not have an access to the original Nature paper, I've just copied the name.

      Delete
  2. The original nature paper is just about photodetectors, while the image sensor is only present in the title of the IEEE spectrum article. I guess it is simply a journalist's fault.
    Regarding QE, I fear it is as you mention, that they just compensate a low QE with a high gain. But at first sight it is not very clear from the paper, I think I should read it more carefully.

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  3. The photoresponsivity aka photo conductive gain depends on QE and amplification. The reported grapheen sensor must be amplified due to the three orders of magnitude improvement in photoresponsivity and due to the fact that a QE of around 3 % is reported in the literature for a grapheen monolayer. In case the QE of the reported grapheen sensor is not improved silicon sensors would beet it in QE by 20 or 30 fold. Besides, the amplification results in amplification noise which affects the SNR.

    In the abstract it is stated that a bandgap is created in grapheen. So far the largest bandgap that I've seen in the literature has been around 130 mV corresponding to a cutoff wave length of around 10 um. Such a small bandgap would naturally generate a huge amount of dark noise unless the sensor is excessively cooled which would explain the extremely low temperature of 12 K in Fig S3.

    Taking these facts and the extremely long settling time into account it is difficult to imagine of any real life applications for the reported grapheen sensor.

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  4. someone on slashdot read the original article: http://tech.slashdot.org/comments.pl?sid=3809257&cid=43890487
    "The Nature Communications article is very clear, right from the abstract, that this sensor is 1000x more sensitive than previous *graphene* sensors"

    I'm sad that even the related press (like this blog) missed this important fact :-(

    ReplyDelete
    Replies
    1. Nature Comm abstract is not clear on that, please re-read it. More than that, the official PR coming Singapore Nanyang Technological University and mentioned in my post says:

      "Not only is the graphene sensor 1,000 times more sensitive to light than current low-cost imaging sensors found in today’s compact cameras, it also uses 10 times less energy as it operates at lower voltages. When mass produced, graphene sensors are estimated to cost at least five times cheaper."

      I believe this PR was quoted by most of the other articles. By the time of posting, I had no access to the original paper.

      Delete
    2. Here is the link to the original release - http://media.ntu.edu.sg/NewsReleases/Pages/newsdetail.aspx?news=863947f9-972d-42d2-947f-3f437f6c3877

      Delete
    3. Thanks, it was in my post too.

      Delete
  5. 12 Kelvin is a Problem in most non scientific cameras.
    Dirk

    ReplyDelete

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