A diode is a device that will let current flow in one direction only. It is at the base of many electronic devices we use every day. Scientists have been able to squeeze its dimension and increase its effectiveness (that is basically measured in the ratio between the conductivity in the two directions, the higher that ratio the better the diode).
Several groups have demonstrated a diode made by a single molecule but in these cases the effectiveness was not that good. Some current would still flow in the undesired direction making the ratio not very good (in the range of 4).
Now a team at Columbia University and at the Berkeley Lab have designed a molecule (a symmetrical one) that can work as a diode with very high efficiency (the technical term is rectification ratio), 200 ratio between the current flowing in the desired direction and the one in the opposite one; that is 50 times better than previous demonstrations.
The diode is made by a specially designed molecule (oxidized thiophene) sandwiched between two gold electrodes. The flow of electrons is explained by quantum mechanics rules and is related to the alignment of the energy levels between the molecule and the electrodes (quantum tunneling). By applying a positive voltage the alignment favors the flow of electrons, whilst applying a negative voltage the flow of electrons is blocked.
Obviously, the molecule is a crucial part in creating the diode but you also need the electrodes and the resulting package is much bigger than a single molecule. Besides, the cost for producing a single molecule diode would be very high.
The importance of this achievement is in the increased theoretical understanding of managing the flow of electrons that can be used in the manufacturing of more affordable electronic devices.