Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy

Abstract

We have developed a low noise cantilever deflection sensor with a deflection noise density of 17fm∕Hz by optimizing the parameters used in optical beam deflection (OBD) method. Using this sensor, we have developed a multienvironment frequency-modulation atomic force microscope (FM-AFM) that can achieve true molecular resolution in various environments such as in moderate vacuum, air, and liquid. The low noise characteristic of the deflection sensor makes it possible to obtain a maximum frequency sensitivity limited by the thermal Brownian motion of the cantilever in every environment. In this paper, the major noise sources in OBD method are discussed in both theoretical and experimental aspects. The excellent noise performance of the deflection sensor is demonstrated in deflection and frequency measurements. True molecular-resolution FM-AFM images of a polydiacetylene single crystal taken in vacuum, air, and water are presented.

Atomic-resolution imaging in liquid by frequency modulation atomic force microscopy using small cantilevers with megahertz-order resonance frequencies

“…The AFM experiments were performed by a custom-built AFM head with an ultralow noise cantilever deflection sensor [3,29] and a photothermal excitation setup [30]. The AFM head was controlled with a commercially available AFM controller (ARC2: Asylum Research).…”

“…Cantilever excitation methods To achieve the theoretically-limited performance given by equation (10), we should overcome two technical difficulties: the detection and excitation of the cantilever oscillation. So far, a number of detection techniques for a small cantilever have been reported [19,20,3,21,22,23]. In contrast, the excitation method for a small cantilever has yet to be established.…”

“…The solid lines show the experimentally measured spectra while the dotted lines show the frequency spectral density (n fB ) corresponding to the noise arising from the thermal Brownian motion of the cantilever. The values of n fB were calculated by following equation [3,38].…”

“…Several groups have presented practical ways to fabricate small cantilevers with a high resonance frequency [16,17,18]. In addition, several sophisticated designs for low noise and wideband deflection sensors for small cantilevers have been proposed [19,20,3,21,22,23].…”

“…However, recent advancements in its instrumentation [3] have made it possible to operate FM-AFM with true atomic resolution even in liquid [4], which has opened up new applications in chemistry [5,6] and biology [7,8,9,10,11,12,13,14,15]. To obtain true atomicresolution images by FM-AFM, it is required to detect short-range interaction force acting between the tip front atom and surface atom.…”