What are simple applications of Monte Carlo simulations in chemistry, that can be done in Matlab?
I've done a little simulation as a senior year elective, though not in Matlab. Can you be a little more specific about how/what you're trying to model with nanoparticle growth? Perhaps direct us to an abstract or two.

The first Monte Carlo simulation you should attempt is the drunken sailor problem. The basic idea is that you have a drunken sailor who has equal probability of taking a step forwards or backwards, and you let him stagger around on a one dimensional line for some number of steps (1000) and repeat the experiment a large number of times. You then get some summary statistics and plots around your experiment.

After you're comfortable with that concept, you can look at increasing complexity into 2 or 3 dimensional random walks, changing the weighting function (not 50-50 for each direction), or some specific algorithm such as Metropolis-Hastings Monte Carlo.

I hope this helps. If you're looking for specific programming help, posting on stack exchange would be a good idea.