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December, 2017


Ultra-Low Frequency Vibration Isolation Stabilizes Scanning Tunneling Microscopy


By Patrick Roberts


chemical information on insulator surfaces, much like the conventional STM can do for metals and semiconductors. Spectroscopy in the microwave fre- quency range enables previously unattainable meas- urements on conducting substrates, such as the rota- tional spectroscopy of a single adsorbed molecule. The technology was developed in the early


T


1990s by Professor Paul Weiss, the nano-pio- neering director of the Weiss Group, a nan- otechnology research unit of UCLA’s California NanoSystems Institute. The ACSTM’s single-molecule measurement tech- niques have illuminated unprecedented details of chemical behavior, including obser- vations of the motion of a single molecule on a surface, and even the vibration of a single bond within a molecule. Such measurements are critical to understanding entities ranging from single atoms to the most complex protein assemblies. “We use molecular design, tailored syntheses,


intermolecular interactions and selective chem- istry to direct molecules into desired positions to create nanostructures, to connect functional mole- cules to the outside world, and to serve as test structures for measuring single or bundled mole- cules,” says David McMillan, lead technician at the Weiss Group. “The ACSTM enables interactions within and between molecules to be designed, directed, measured, understood, and exploited.” The group examines how these interactions


influence chemistry, dynamics, structure, electron- ic function, and other properties. Such interactions


he tunable microwave-frequency alternating current scanning tunneling microscope (ACSTM) can record local spectra and local


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 hopes to improve synthetic sys- tems at all scales.


Understanding ACSTM The scanning tunneling microscope is based


on quantum tunneling. When a conducting tip is


eral resolution and 0.01 nm depth resolution. ACSTMs probe the chemistry of insulator sur-


faces using microwaves reflected from the surface, microwave throughput attenuation and harmonics of the microwaves generated by the tunnel junction. They measure photons emitted from the tunneling junction, excited by the tunneling electrons. This gives tremendous gains in spatial resolution, since the photons only come from the atoms or molecules through which electrons are tunneling. Testing has shown how harmonic ampli-


tudes in nonlinear spectroscopy with the ACSTM can be interpreted in terms of molec- ular motions, charging and electronic struc- ture. The Weiss Group has used these nonlin- earities to study the electronic energies of insulator surface states.


Vibration Isolation Achieving these nano-level chemical


UCLA’s negative-stiffness vibration isolator.


brought very near to the surface to be examined, a bias (voltage difference) applied between the two allows electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states of the sample. Information is acquired by monitoring the cur-


rent as the tip’s position scans across the surface, and is usually displayed in image form. STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibra- tion control and sophisticated electronics. For an STM, good resolution is considered to be 0.1 nm lat-


and spectroscopic data sets requires that the ACSTM be positioned in an ultra-stable oper- ating environment. “The lab was using almost


exclusively optical tables on pneumatic isolation,” says McMillan. “One of our big problems has been space constraint. We needed smaller pneumatic optical tables to fit. But as the air tables get small- er, their vibration isolation performance diminish- es.”


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 significant movement issues, which created low-frequency vibrations. “We brought in an active vibration isolation


Continued on page 58


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