Stabilizing Scanning Tunneling Microscopy with Ultra-Low Frequency Vibration

Table of Contents

Introduction
Understanding ACSTM
Vibration Isolation

Introduction

The tunable microwave-frequency alternating current scanning tunneling microscope (ACSTM) is capable of recording local spectra and local chemical information on insulator surfaces – much like the traditional STM can do for semiconductors and metals. Spectroscopy in the microwave frequency range allows previously unachievable measurements on conducting substrates, such as the rotational spectroscopy of a single adsorbed molecule.

The technology was developed in the early 1990s by Professor Paul Weiss, the nano-pioneering director of the Weiss Group, a nanotechnology research unit of UCLA’s California NanoSystems Institute. The ACSTM’s single-molecule measurement techniques have revealed unparalleled details of chemical behavior, including observations of the movement of a single molecule on a surface, and even the vibration of a single bond inside a molecule. Such measurements are crucial to learning about entities ranging from single atoms to the most complex protein assemblies.

We use molecular design, tailored syntheses, intermolecular interactions and selective chemistry to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measuring single or bundled molecules. The ACSTM enables interactions within and between molecules to be designed, directed, measured, understood, and exploited.

David McMillan, Lead Technician, The Weiss Group

The group examines how these interactions have an effect on dynamics, chemistry, electronic function, structure, and other properties. Such interactions can be used to form precise molecular assemblies nanostructures and patterns, and to control and stabilize function. By understanding interactions, function and dynamics at the smallest possible scales, the group looks forward to enhancing synthetic systems at all scales.

UCLA’s negative-stiffness vibration insulator.

UCLA’s negative-stiffness vibration insulator.

Understanding ACSTM

The scanning tunneling microscope is founded on the principle of quantum tunneling. When a conducting tip is brought very close to the surface to be investigated, a bias (voltage difference) applied between the two enables the electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of applied voltage, tip position, and the local density of states of the sample.

Information is obtained by observing the current as the tip’s position scans across the surface, and is usually displayed in image form. STM can prove to be a demanding technique, as it requires extremely stable and clean surfaces, sharp tips, sophisticated electronics, and excellent vibration control. For an STM, 0.01 nm depth resolution and 0.1 nm lateral resolution are considered to be good resolution.

ACSTMs probe the chemistry of insulator surfaces with the help of microwaves reflected from the surface, microwave throughput attenuation, and harmonics of the microwaves generated by the tunnel junction. They measure emitted photons from the tunneling junction, excited by the tunneling electrons. This offers tremendous gains in spatial resolution, since the photons only come from the molecules or atoms through which electrons are tunneling.

Testing has revealed how harmonic amplitudes in nonlinear spectroscopy with the ACSTM can be interpreted in terms of electronic structure, charging, and molecular motions. The Weiss Group has used these nonlinearities in order to examine the electronic energies of insulator surface states.

negative-stiffness vibration insulator.

Schematic of negative-stiffness vibration insulator.

Vibration Isolation

In order to attain these nanolevel chemical and spectroscopic data sets, the ACSTM has to be positioned in an ultra-stable operating environment.

The lab was using almost exclusively optical tables on pneumatic isolation. One of our big problems has been space constraint. We needed smaller pneumatic optical tables to fit. But as the air tables get smaller, their vibration isolation performance diminishes.

David McMillan

Space was not the only issue; the lab was occasionally moved to different locations at UCLA. In 2009, the lab was on the sixth floor of a steel-structure building that had considerable movement issues, which created low-frequency vibrations.

“We brought in an active vibration isolation unit to test, as well as a negative-stiffness system”, explains McMillan. “We compared the two systems, and the negative-stiffness system performed better with the frequencies of our concern, which were the lower frequencies caused by the movements of the building, between 10 and 24 Hertz.”

McMillan was at first interested in the negative-stiffness isolator because of reports that promised very good low-frequency vibration isolation in a small footprint, making it much convenient to place in a compact box for thermal stability.

I put it on a cyclic accelerometer and it performed outstandingly. Much better than any other vibration isolation we had used before.

David McMillan

In negative-stiffness vibration isolation, vertical-motion isolation is provided by a stiff spring supporting the weight, coupled with a negative-stiffness mechanism. The net vertical stiffness is made extremely low without affecting the static load-supporting capability of the spring. Horizontal-motion isolation is provided by beam-columns connected in series with the vertical-motion isolator.

A beam-column functions like a spring combined with a negative-stiffness mechanism. The result is a compact passive isolator capable of very low horizontal and vertical natural frequencies and very high internal structural frequencies. The isolator provides 0.5 Hz vertical and horizontal isolation, using a totally passive mechanical system — no electricity or air required.

In the case of an isolation system with a 0.5 Hz natural frequency, isolation begins at about 0.7 Hz and improves with increase in the vibration frequency. The natural frequency is generally used to describe the system performance.

Negative-stiffness isolators resonate at 0.5 Hz. At this frequency, the energy present is almost zero. It would be very rare to find a significant vibration at 0.5 Hz. Vibrations with frequencies above 0.7 Hz are rapidly attenuated with an increase in frequency. Air tables, as vibration isolation systems, deliver limited isolation vertically and less isolation horizontally. They can worsen the vibration isolation problems, since they have a resonant frequency that can match that of floor vibrations. Air tables will, in fact, amplify, and not reduce vibrations in a typical range of 2 to 7 Hz, due to the natural frequencies at which the tables resonate. All isolators will amplify at their resonant frequency, and then they will start isolating. With air tables, any vibrations in that resonant frequency range could not only fail to be attenuated, they could be amplified.

In addition, transmissibility with negative-stiffness isolators is considerably improved over air systems. Transmissibility is a measure of the vibrations that are transmitted through the isolator relative to the input vibrations. The negative-stiffness isolators, when adjusted to 0.5 Hz, achieve 93% isolation efficiency at 2 Hz, 99% at 5 Hz and 99.7% at 10 Hz.

“Over the past four years, we have put six negative-stiffness isolation systems into the lab,” explains McMillan. “And since, we have relocated into quarters with less movement. Although we do still have some air tables in use, our staff and graduate students prefer the negative-stiffness isolators.”

Minus K Technology

This information has been sourced, reviewed and adapted from materials provided by Minus K Technology.

For more information on this source, please visit Minus K Technology.

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