Attempts to resolve the energy-level structure of single DNA molecules by scanning tunnelling spectroscopy span over the past two decades, owing to the unique ability of this technique to probe the local density of states of objects deposited on a surface. Nevertheless, success was hindered by extreme technical difficulties in stable deposition and reproducibility. Here, by using scanning tunnelling spectroscopy at cryogenic temperature, we disclose the energy spectrum of poly(G)-poly(C) DNA molecules deposited on gold. The tunnelling current-voltage (I-V) characteristics and their derivative (dI/dV-V) curves at 78 K exhibit a clear gap and a peak structure around the gap. Limited fluctuations in the I-V curves are observed and statistically characterized. By means of abinitio density functional theory calculations, the character of the observed peaks is generally assigned to groups of orbitals originating from the different molecular components, namely the nucleobases, the backbone and the counterions.
Bibliographical noteFunding Information:
We thank I. Brodsky, A. Migliore, M. Cavallari and O. Millo for fruitful discussions, and laboratory and computational help. Funding was provided by the EC through contracts IST-2001-38951 (‘DNA-Based Nanowires’) and FP6-029192 (‘DNA-Based Nanodevices’). Computer time was provided by CINECA (Bologna) and by INFM-CNR through National Supercomputing Projects. Correspondence and requests for materials should be addressed to D.P. or R.D.F. Supplementary Information accompanies this paper on www.nature.com/naturematerials.