  Modern growth and fabrication techniques can produce lower dimensional
structures in the crossover regime. Such structures in the crossover regime can
provide tunability of various properties. For example, a zero-dimensional (0-D)
structure evolving towards a 2-D structure shows electronic structure which is
neither 0-D-like, nor 2-D-like. Within the crossover regime the electronic
density of states (DOS) at Fermi level (Ef) keeps on changing as the size of
the system changes. DOS at Ef determines many properties of materials, such as
electronic specific heat, spin susceptibility etc. Keeping the importance of
DOS at Ef in mind, we determine their values and other details of electronic
structure of lower dimensional structures of metals, in the 0-D to 1-D, 1-D to
2-D, 2-D to 3-D, 0-D to 2-D, 0-D to 3-Dand 1-D to 3-Dcrossover regimes, in a
simple free electron model. We compare our results with analytical theory and
experimental results, wherever available. We also present some results obtained
by scanning tunneling spectroscopy measurements on Ag islands on Si(111)
evolving from a 0-D to a 2-D structure. This simple model is quite useful in
understanding lower dimensional structures in the crossover regimes.
