MATLAB: Image resolution of exported images

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Hello,
I have some MATLAB simulation results which are intended for publishing in a journal. But when I export the images (graphs) to a .png format, I see that the quality of the image suffers. The image has to fit into a two-column format prescribed by the journal, and I would like to have the best clarity and resolution possible while exporting the image to either png or pdf format from MATLAB. How can this be acheived? Any suggestions?
TIA Vinod
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