I Nitrogen-Vacancy centres in Diamond for NMR spectroscopy

[Jon.G]

Hello,
I have been looking into NV in diamond and how it can be used for nanoscale magnetometry, and was wondering if anyone could help explain how it works or link to a paper that does.
Is it just a spin 1 state (splits to 3 levels) that undergoes NMR or is there something else happening? I'm just having a bit of trouble seeing what's going on, and how it can be used to help image other 'things' (for lack of a better word - I'm a little tired :S )
I thought NMR spec. allowed us to discover the properties of a sample, and don't really understand how placing these centres close to it would help.

Thanks for your time.

 

[Charles Link]

I have studied a little bit about nuclear magnetic resonance, but am no expert, and probably don't have an answer. Perhaps I can give a little info that you might find useful, but I might be repeating what you already know. With the NMR/MRI, in a uniform magnetic field, the resonances of like atoms would all be at the same frequency without any spatial information provided. To get spatial information, I think the usual technique is to introduce well- controlled gradients in the static magnetic field so that localized resonances occur at a given frequency. Paul Lauterbur and Peter Mansfield were awarded a Nobel prize in 2003 for developing these techniques. To get down to a nanometer type resolution, it is likely additional refinements would be necessary, but that is about as much info as I have for the present.

 

[sharpstones]

The diamond NV center is a spin 1 system, where the spin state can be read out optically. More specifically, it fluoresces more when it is in the ms=0 spin ground state as opposed to the ms=-1 or ms=+1 states. The energy splitting between the different spin states is sensitive to the magnetic field via the Zeeman effect H_Z=gμ_BB where B is the magnetic field, mu_B is the bohr magneton, and g=2. So if you can measure the splitting between the different ms spin states then you can measure the magnetic field. This is accomplished by applying microwave radiation to the NV and sweeping the microwave frequency. When the microwave frequency is resonant with either the ms=0 to -1 or ms=0 to +1 transitions it will induce coherent oscillations of the spin state, which causes the fluorescence to decrease as compared to just the normal ground state occupation of the ms=0 state. So by looking for the fluorescence dip at a given frequency you know the energy splitting and hence the magnetic field. This is one technique to measure static magnetic fields - it is also possible to measure sensitively fluctuating AC magnetic fields by applying microwave pulses to the NV center.

The main difference compared to normal Nuclear Magnetic Resonance or Electron Paramangetic Reasonance measurements is that the diamond NV can be read out optically using lasers which can address single atom defects. NMR and EPR are bulk methods which rely on measuring the microwave absorption of a very large ensemble of spins.

Single NV centers can be brought within a couple of nanometers of the diamond surface, where you can place some interesting material which you want to make magnetic measurements. This can be used to actually measure the NMR or EPR spectrum of nanoscale materials, which is not possible with conventional techniques.

Here is a pretty nice talk discussing the NV center capabilities and using it make a nanoscale NMR scanner: