Although the Chicago Tribune recommends that a thank you note be sent immediately after an interview, sending it a week after is not too late. The sooner the note is sent, the more memorable a candidate is to the recruiter.
Per the Chicago Tribune, get the note in the mail the same day as the interview to ensure you are still fresh in the interviewer's mind when they receive the note. The note should reflect the tone of the interview, emphasize your skills and traits that would be beneficial in the job and end with a personal touch. Either way, the old phrase stands true for post-interview thank you notes: better late than never.