In writting either a or an is acceptable in my opinion. The purpose of "an" is euphony in speech when the following word begins with a vowel sound. If I had to read such a title aloud I'd solve the problem by making the "a" rhyme with pay.
Are you sure? I think 'N-doping' stands for 'nitrogen doping', which is a kind of N-type (negative type) doping. (Please put me right if that's not the case.)
Since this is the title of the paper, I tend to think that it isn't an error, and that the writer assumes it to be read as the full word 'nitrogen' rather than the letter 'N'.