In my blogs of 16 Sept. 2010 I gave advice on how to begin research and then build up a research program. This is a continuation of that advice.
Some researchers are "polymathic," their research is diversified. A good example would be Leo Szilard who did work in:
X-rays in crystals
metallurgy and engineering (his patent of the nuclear reactor with Fermi and his weapons work)
There are disadvantages to this style of work. More time must be spent on "getting up to speed" in each new research area. One must learn different tools, methods, and basic knowledge and get to know and be known by different sets of colleagues. This impacts what journals you can hope to publish in and what funding you can hope to obtain.
Certainly nowadays interdisiplinary work is very important but for many researchers it is better to stick to some research theme. One theme I followed in my fusion energy work was open system end plugging. A more popular fusion energy theme is tokamak studies.
Sticking to a research theme has various advantages. It is easier to keep a steady source of funding. Once learned, you stick to a common set of tools, methods, and basic knowledge. You become known in the community and by the journals you need access to in order to publish your results.