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Quantum Dots in the Lab

  1. Oct 29, 2013 #1

    I feel as though I should mention (before I say anything else) that this is my first year in grad school (first SEMESTER even). I haven't taken any modern physics lab courses (aside from an undergrad course years ago) that would help me with my research or my question.

    My PhD research involves the use of quantum dots to create single photons. At this point, I have read a whole bunch of papers and documentation of what quantum dots are, and how they can be used to create single photons. But now it has come time to actually purchase these things and use them as they papers describe.

    The problem I am having is that there seems to be a "disconnect." Meaning, I don't know what to expect when buying these things, or how I would use them in the lab. I googled "Purchase Quantum Dots." and got this website..


    I don't know why the dots come in those jars, or how I would use that. According to the papers I have been reading, making a single photon emitter involves connecting 4 dots via "tunneling connections" and then exciting one of them. I imagine these things are extremely small and I don't know how I am supposed to deal with them.

    I am aware that I need to speak to someone in person that deals with these types of experiments, and I will. But I was wondering if I could gain any insight from these forums.

  2. jcsd
  3. Oct 29, 2013 #2


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    Do you know what you need exactly? There are two main kinds of quantum dots. Colloidal dots and epitaxial self-assembled dots. The first kind is the one sold in the link you provided. These are simply speaking nanocrystals in a solution. To prepare samples made of those one usually plays around with the solution until the density of nanocrystals is roughly equivalent to some desired target density and then one spin coats the solution of quantum dots on a slide of glass. This gives a certain density of QDs on the glass sheet. If you now pump this sample optically and have less than one quantum dot within your spot area, this gives you a simple single photon source (see, e.g. X Brokmann et al. (2004) New J. Phys. 6 99).

    If you need real devices, the main problem is rather an engineering one. Creating reasonable electrical contacts and optimizing the system design is not trivial, but those details should be something your supervisor can provide you with. Also, see Y. Shirasaki et al., Nature Photonics 7, 13–23 (2013) for a review on colloidal QD-LEDs. In terms of electrical contacting LEDs are similar to single photon sources. You just need way lower QD density or an active design to single out a single QD for a single photon source.

    On the other hand, epitaxial quantum dots are created by starting with some material, putting few layers of another material with slightly different lattice constant on it by adding monoatomic layers using molecular beam epitaxy. Then you put the material you started with on top again. Due to strain, the thin layer in the middle will rather consist of small islands of the other material, which will form the quantum dot. For those, it is easier to create electrical contacts as we are talking about a piece of a semiconductor here. However, molecular beam epitaxy is a complicated and expensive technique and you usually cannot simply buy those samples.
  4. Oct 29, 2013 #3
    Thanks a lot! I am going to look into the things you suggested and post back at a later time if I have any more questions.
  5. Oct 30, 2013 #4


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    There are also semiconductor quantum dots that are electrically defined. In fact, the question is so broad that you can't even be sure which discipline this falls under: the work could be anything from experimental semiconductor physics (working in a cleanroom) to inorganic chemistry.
    There are also many different ways to generate photons for each system.

    I find the whole thing a bit strange. From what I can tell from the OP he/she has no idea whatsoever about what the work actually entails; which for a PhD project is a bit weird. The project must be better defined than this, if nothing else bythe field experise of the thesis advisor.

    I'd suggest you talk to you advisor, until you know what the project is actually avbout there isn't much we can help you with.
  6. Oct 30, 2013 #5
    The research is on "Quantum Radar" and it calls for the use of quantum dots for use in a transmitter that creates single photons in the microwave spectrum.

    You're right though, I don't have any idea of what this work entails. But I am willing to learn. As I said, this is my first semester of grad school. I am fresh out of undergrad and going straight for a PhD. Technically I am in the EE department, and my adviser know nothing of quantum mechanics, so it is making things a bit more challenging.
  7. Oct 30, 2013 #6


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    So what kind of resources and expertise do you have access to? Are there other people working in this area at your university?
    Do you e..g have access to a cleanroom?
    Do you have access to the neccesary optics (lasers etc)?

    I must say I find it a bit strange that you supervisor has created a project in an area he/she does not know anything about. This is quite specilised work and it is most definitly no an area where I would expect a PhD student to be able to start up work that is new to the department.

    My advice would be to talk to your supervisor, and unless he/she can give you some more definite guidance to what it is you are expected to do I would seriously consider changing to another topic.
  8. Oct 30, 2013 #7
    I am actually meeting with some experimentalists in the Physics department soon so I will find out soon enough. My advisor's plan is to work closely with the physics department in a joint effort.

    Listen, I have heard this line so many times in my life in various forms. When I first started to want to learn about Physics I heard it. When I first started to want to fix cars I heard it. I didn't listen to these people and eventually graduated with almost a 4.0 GPA in applied physics, and am now a mechanic who has done everything from changing oil, to rebuilding automatic transmissions and engines in my garage. All I can say is that I found that this "too complicated for where you are at" business is never the case. Specialists need to start somewhere, I feel like people forget this, even when they themselves are a specialist.

    What I am NOT looking for is someone to once again tell me, "this is for specialists, do something easier." I want to do this, its very interesting to me, and the fact that I will have to do a lot of challenging work is motivation for me to pursue it. Nothing in life is too complicated or specialized to learn. I am willing to put in the work.

    What I am going to do now is research all the different kinds of quantum dots mentioned here, and where their applications lie. I will then bring it up with the people in the physics department so I can get some guidance on where to go next.
  9. Oct 30, 2013 #8


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    When I wrote "specialized" I did not mean that it was difficult, I meant that it requries access to specialized equipment and resources. As as PhD student you have very limited controll over your environment, and it is up to the supervisor to make he/she does not ask you to do something that can not be done because some key piece of equiment is missing. For example, from what I understand you will probably need at least a couple of lasers, optical tables and other equipment. This type equipment is relatively rare, VERY expensive (a laser can easily cost >$100 000) and the delivery times can often be 6-12 months. But, without access to it it does not matter how good you are, you will still not be able to do any experiments. Additionally, in research you often need access to things that can't be bought commercially, and you need to rely on your network of contacts (which you won't yet have as a PhD student) to source it; this can e.g. include one-off samples or custom equipment (or even access to facilities, e.g. cleanrooms).

    I -and everyone else who supevises PhD students- spend quite a lot of time making sure that the PhD students have access to the equipment and resources that they need in order to do their research, and when I come up with new projects I always have to make sure that they are doable either using equipment and resources (this can e.g. include getting samples to measure from colleges) we already have or with equipment I can be reasonable sure we will be able buy or get access to some other way.
  10. Oct 30, 2013 #9
    Ah okay, I apologize for the misconception.

    As of now, I know that the physics department does atom-cavity excitation experiments. If all else, I can use this as a means of single photon creation. (There is promising research that suggests that a reliable photon generaton can be created by these means.)

    But from what you write, it seems as though my adviser is not doing a particularly good job in this respect.
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