Email appears to be acceptable these days for this type of communication. Having said that, I would send out the email later on the same day as the interview. That type of promptness will impress the interviewer, & take advantage of the speed with which email messages can be sent.
First interview: I did not write a thank you, only because I could not find my interviewers contact information--I got the job. Second interview: I emailed my interviewers a thank you note --did not get the job. Third interview: I wrote out a thank you on a simple note card, took it to the post office to mail right after my interview--got the offer two days after the interview.
My third interview went so well. During the interview I was introduced to the staff on the floor and given a tour. I was told how nice I was by the NM and how much easier it is to teach a new nurse how to be a good nurse over teaching a new nurse how to be a nice nurse. I made sure to reiterate my interviewers inferences about me by writing and mailing that thank you note immediately.
My second interview did not go so well. It was in a field of nursing I had no strong desire to be a part of; and I'm fairly certain my answers to the interview questions reflected that attitude. So it did not matter if I sent a card that sings or dances; or if I tried to bribe them; within the first few questions, I was already on the 'do not call' list--I knew it, too.
My first interview went great. I felt like I was with friends. I really wanted to write thank you notes but I never asked for their full name or any other contact information--lesson learned. But I did try to call several of the locations where I thought they originated from.
So, I'm somewhat convinced that writing a thank you note for a bad job interview does nothing to up your chances; whereas, a thank you note for a great interview will only reiterate what they think of you and COULD up your chances if they are undecided.