Which one of these is more appropriate for academic email
In the US, everthing said in the comments applies (in short: everything goes, it's up to you).

In Europe (and especially in German speaking countries), it is very unsusual to call other people by ther first name, therefore serim@ would be considered inappropriate and people might be confused and think of it as your last name. 

Therefore, if you think international, I'd propose s.doe@... or, serim.doe@... . Most companies and universities in Germany use the last version since it helps in avoiding name collisions (of course it's no guarantee).