This past week I was fortunate enough to attend this workshop at Trinity College, Dublin, all about the physics of atomic- and molecular-scale electronics. It was a great meeting, and I feel like I really learned several new things (some of which I may elaborate upon in future posts). One topic that comes up persistently when looking at this subject is the concept of the work function, defined typically as the minimum amount of energy it takes to kick an electron completely out of a material (so that it can go "all the way to infinity", rather than being bound to the material somehow). As Einstein and others pointed out when trying to understand the photoelectric effect, each material has an intrinsic work function that can be measured, in principle, using photoemission. You can hit a material surface with ultraviolet light and measure the energy of the electrons that get kicked out (for example, by slowing them down with an electric field and seeing how long it takes them to arrive at a detector). Alternately, with a fancy tunable light source like a synchrotron, you can dial around the energy of the incident light and see when electrons start getting kicked out. As you might imagine, if you are trying to understand electronic transport, where an electron has to leave one electrode, traverse through a system such as a molecule, and end up back in another electrode, the work function is important to know.
One problem with work functions is, they are extremely sensitive to the atomic-scale details of a surface. For example, different crystallographic faces of even the same material (e.g., gold) can have work functions that differ by a couple of hundred millielectronvolts (meV). Remember, the thermal energy scale at room temperature is 25 meV or so, so these are not small differences. Moreover, anything that messes with the electronic cloud that spills a little out of the surface of materials at the atomic scale can alter the work function. Adsorbed impurities on metal surfaces can change the effective work function by more than 1 eV (!). To see how tricky this gets, imagine chemically assembling a layer of covalently bound molecules on a metal surface. There is some charge transfer where the molecule chemically bonds to the metal, leading to an electric dipole moment and a corresponding change in work function. The molecule itself can also polarize or be inherently polar based on its structure. In the end, ordinary photoemission measures just the total of all of these effects. Finally, ponder what then happens if the other end of the molecules is also tethered chemically to a piece of metal. How big are all the dipole shifts? What is the actual energy landscape "seen" by an electron going from one metal to the other, and is there any way to measure it experimentally, let alone compute it reliably from quantum chemistry methods? Really understanding the details is difficult yet ultimately essential for progress here.