My platelet count is 140. Should I be concerned? Can I raise it through diet?

Near normal. Normal platelets counts are 150000 to 250000, so 140000 is near normal. If you do not have abnormal bleeding, e.g., easy bruising, bleeding from gums, blood in stools or urine, there is no cause for concern. It would be good to know your complete blood count because if other blood components are also low, then it should be investigated.
How old are you. I'd need age and meds you are taking.
Rx you, not the # A platelet count of 100, 000 in the absence of any suggestion of illness isn't a concern. Lower levels should prompt an effort to find the cause (often a medicine) but things become dangerous if you're below 50, 000 and going to surgery or a nfl football field. Lower values become life-threatening. If your physician is not concerned, I'm not either.