Highly stable atom-tracking scanning tunneling microscopy

Abstract

In this article, we propose a technique for highly stabilized atom-tracking control of a scanning tunneling microscope (STM) tip by referring to an atomic point on a regular crystalline surface. Our aim is to prevent jumping of the STM tip to neighboring atoms and to use it even in a noisy environment. Graphite crystal, whose lattice spacing is approximately 0.25 nm, was utilized as the reference. To improve the performance of the tracking controller against external disturbances, the influence of a disturbance on the STM under various environmental conditions was compared experimentally with the frequency response of the open-loop tracking system. The atom-tracking conditions required to avoid jumping of the STM tip are proposed and applied to the design of the tracking controller by referring to the results of the comparison. The new tracking controller consists of integrator, tracer, and limiter units. The integrator unit is designed to eliminate the steady-state error due to thermal drift. A phase-lag low-pass filter is utilized as the tracer unit to compensate for the dominant disturbance due to vibration/acoustic noise with a frequency lower than the cutoff frequency, fco, of the open-loop tracking system. To improve the phase margin condition of the controller at fco and to suppress the disturbance with a frequency higher than fco, the limiter is designed to include a phase-lead high-pass filter and a saturator whose output is less than one-half of the lattice spacing. The performance of the stabilizing technique, which is to combine the new tracking controller with enhanced STM stiffness, was evaluated using internal/external artificial disturbance generators. The experimental results show that the proposed method has a high capability for maintaining atom-tracking control without any jumping of the STM tip, even in a noisy environment.

Vertical and lateral drift corrections of scanning probe microscopy images

“…A very elegant experimental approach to compensate for drift during AFM a͒ measurements was recently presented by Abe et al [7][8][9] There, an atom-tracking procedure is used for determining the actual drift velocity, which is then fed into a feed-forward routine. For STM, this atom-tracking technique was introduced by Pohl and Möller 10 and was analyzed in detail by Rerkkumsup et al 11 Atom tracking has been employed for studying the diffusion of adsorbates with STM ͑Refs. 12 and 13͒ and the identification and manipulation of single atoms with AFM ͑Refs.…”

The new FAST module: A portable and transparent add-on module for time-resolved investigations with commercial scanning probe microscopes

“…This technique, proposed by Pohl and Möller [17], has been implemented for the first time by Swartzentruber for the study of adatom diffusion [16]. It is currently included in advanced scanning probe controllers for accurate drift control [27][28][29]. In this technique, circular dithering of the tip with a typical frequency below 1 kHz and a dithering radius of the order of 1 nm and below, combined with a phase sensitive detection in x and y direction, allows for determining the respective error signals that can be directly applied as input for a lateral feedback via a proportional-integral-derivative (PID) controller [30].…”

A Survey of Non-Raster Scan Methods with Application to Atomic Force Microscopy

“…Under this approach, tracking is achieved by moving the tip in a small circular motion, using the resulting data to estimate the gradient of the property of interest, and then using the gradient to drive the overall motion of the tip. This idea was later used to study surface diffusion by tracking single atoms [9]- [11] and continues to be developed and utilized in STM [12]- [15]. It has recently found application in AFM as well [16], [17].…”