In conventional semiconductors used in integrated circuits, electrons travel as independent particles. As they travel, they are scattered by phonons (quanta of the crystal lattice vibrations), a phenomenon that produces electrical resistance in the material and subsequent loss of power in a finished device. The charge density wave (CDW) is a quantum state in which electrons and phonons become closely coupled and propagate together through the material, giving rise to a collective current and thus greatly reduced electrical resistance.

The problem is that the transition to this collective quantum state usually occurs at relatively low temperatures – of around 200K in CDW materials. Alexander Balandin's team has now shown that this can be increased by about 40K in titanium diselenide if its thickness is reduced to below about 100 nm.

Reducing energy dissipation

As integrated circuits and electronic devices made from silicon decrease in size, energy dissipation is one of the main limiting factors to continued miniaturization, explains Balandin. This energy, which is lost each time a transistor switches, for example, is proportional to the number of electrons in the device material and temperature.

"This fundamental condition arises from the laws of thermodynamics and cannot be changed," he told nanotechweb.org. "But the assumption underlying this limit is that the electrons act as an ensemble of independent particles. If the electrons were instead in a collective state, then the minimum dissipation limit for one switching cycle would be greatly reduced. We would thus like to exploit such collective states."

The researchers measured the charge-wave-density transition temperatures in samples of titanium diselenide of varying thicknesses – ranging from microns to nanometres. They obtained their results by looking at the changes in the Raman spectra of these samples. TiSe2 belongs to the family of so-called van der Waals materials that typically have layered crystalline structures. The layers are weakly bound together and so can easily be shaved off to produce films of different thicknesses from the corresponding bulk crystals.

The Riverside team now plans to start making the first room-temperature electronic circuits using its charge-density-wave materials.

The current work is detailed in Nano Letters.