Ummm no they can't.
Another thread got me thinking that this might be a good topic for people to consider before going to school. There are still students, practicing NPs, hiring physicians and school perpetrating the fallacy that FNPs are able to practice in any arena. Not true so before you spend time in an expensive program that might not allow you to be hired in your desired specialty please do your due diligence.
Some of it varies by state depending on your BON as well as local culture hiring practices. Examples include psychiatry. In my area, although as everywhere there is an extreme shortage, you can not practice solely in psych without a psychNP designation. Another example is acute care and emergency medicine. At this time hospitals in my area will not hire FNPs in the inpatient attending role, or in the ED although a few have managed to get in the ED in the Urgent Care section. Generally if you don't have an extensive background, documented orientation or fellowship in a specialty you probably shouldn't be practicing solely in that area, definitely check with your BON to clarify.
Please don't fall prey to employers and schools who have little regard for your license and scope of practice. As they say "its all well and good until somebody loses an eye".