Einstein’s 1905 publications were:
On the Electrodynamics of Moving Bodies, where Einstein found a way to reconcile Maxwell’s equations for electricity and magnetism with the laws of mechanics – but only by introducing some major changes to mechanics close to the speed of light, later known as Einstein’s special theory of relativity.
A followup paper 3 months later, Does the Inertia of a Body Depend upon its Energy-Content? showed that his new theory linked matter and energy, resulting in the famous equation “E = mc2“.
In On a Heuristic Point of View about the Creation and Conversion of Light?, the primary work for which Einstein later received the Nobel prize, he introduced the quantum, an explanation of the properties of light that required it to, at least in certain circumstances, behave as discrete particles, rather than the continuum that Maxwell’s equations suggested.
Einstein’s dissertation, “A New Determination of Molecular Dimensions”, was actually his second attempt at getting a PhD from the Swiss university ETH. Using the viscosity of solutions of large molecules (like sugar) in smaller ones (like water), Einstein was able to describe theoretically how to determine the actual molecular sizes involved.
Einstein’s paper on Brownian motion, Investigations on the Theory of the Brownian Movement, extending his thesis work, correctly attributed the random jostling of objects seen in microscopes to kicks from individual atoms and molecules – proof that these entities really do exist and have visible effects at a time when their existence was still being questioned. Einstein’s results also provided a way to make a new determination of Avogadro’s number.