Hardware for digitally controlled scanned probe microscopes

The design and implementation of a flexible and modular digital control and data acquisition system for scanned probe microscopes (SPMs) is presented. The measured performance of the system shows it to be capable of 14-bit data acquisition at a 100-kHz rate and a full 18-bit output resolution resulting in less than 0.02-Å rms position noise while maintaining a scan range in excess of 1 µm in both the X and Y dimensions. This level of performance achieves the goal of making the noise of the microscope control system an insignificant factor for most experiments. The adaptation of the system to various types of SPM experiments is discussed. Advances in audio electronics and digital signal processors have made the construction of such high performance systems possible at low cost

Observation of negative differential resistance in tunneling spectroscopy of MoS2 with a scanning tunneling microscope

A scanning tunneling microscope has been used for imaging and tunneling spectroscopy of 2Hb–MoS2 in ultrahigh vacuum. Atom-resolved images obtained in three distinct imaging modes–measuring z at constant current, barrier height at constant current, and current at constant z–are presented. Current–voltage (I–V) tunneling spectra reveal the occasional presence of negative differential resistance. Possible origins of the effect are discussed. Convolution of the sample energy density of states (DOS) with a contamination-induced peak in the tip DOS is the probable cause. Other mechanisms that may be active include charging of electron traps in the barrier or on the tip, and resonant tunneling in a double-barrier quantum well structure resulting from layer separation in the MoS2 crystal

Scanning tunneling microscopy of DNA: Atom-resolved imaging, general observations and possible contrast mechanism

We have shown that it is possible to image DNA with atomic resolution using scanning tunneling microscopy (STM), [R. J. Driscoll, M. G. Youngquist, and J. D. Baldeschwieler, Nature 346, 294 (1990)]. Here we describe that data together with our general observations on STM of DNA in ultrahigh vacuum. We also suggest a possible contrast mechanism for DNA imaging by STM based on wave function orthogonality requirements between a molecule and its substrate. Topographic images are presented which resolve atomic features in addition to the double helical structure and nucleotide pairs of the DNA molecule. Comparisons of experimental STM profiles and modeled contours of the van der Waals surface of A-DNA show excellent correlation. Successive scans show that the imaging is nondestructive and reproducible. For this study, double-stranded DNA was deposited on highly oriented pyrolitic graphite without coating, shadowing, or chemical modification