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QuantumATK => General Questions and Answers => Topic started by: fan0221 on May 12, 2009, 11:34

Title: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: fan0221 on May 12, 2009, 11:34

Hello everyone,
       
        I calculated a little molecule with Si electrode and got the energy of HOMO and LUMO in various bias voltage. I  found that the energy of the HOMO and LUMO shifted together with different bias voltage. What's the physical insight of the variation of HOMO and LUMO with bias voltage?

Thank you if you could give me some advices.

Title: Re: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: fan0221 on May 13, 2009, 02:13

nobody ???

Title: Re: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: nori on May 13, 2009, 09:39

Dear fan0221,

One of the main reason is the potential shift due to applied bias voltage.
However, there is another factor to get MPSH spectrum changed, which is non-equilibrium density matrix.

In your case from Vb = 0V to Vb = 1V, HOMO and LUMO shift are almost the same because only the potential shift seem to affect the spectrum shift.
But above Vb = 1V, the electric structure gets drastically changed because LUMO is included in non-equilibrium density matrix,
 so that the spectrum shift get more complex.

It is the general suggestion, so the detailed analysis for your specific system is needed to understand more precisely why the spectrum get such a change under finite bias.

I'd be happy if my advice can help you.

Title: Re: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: fan0221 on May 13, 2009, 10:17

Thanks for your answer!

Title: Re: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: Anders Blom on May 13, 2009, 13:01

What complicates matter in this case is that your electrodes are semi-conducting. Thus, I suspect you have no current in the low-bias calculations. However, as the bias exceeds the band gap, you start to get tunneling from the valence band of the left electrode to the conduction band of the right one, via the molecule. I imagine this somehow pins the HOMO to the valence band top of the left electrode, perhaps.

Title: Re: What's the physical insight of the variation of HOMO and LUMO with bias voltage?
Post by: fan0221 on May 18, 2009, 07:32

Thanks for your help!