Atomic force microscopy (AFM) was introduced in 1986 and has since made its way into surface science, nanoscience, chemistry, biology, and material science as an imaging and manipulating tool with a rising number of applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM), where the tunneling tip of the STM is replaced by using a force sensor with an attached tip. Measuring the tiny chemical forces that act between the tip and the sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. Here, we focus on the core of the AFM, the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro-machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self-sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows us to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between the tip and the sample, has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub-piconewton sensitivity. FM-AFM is precise; because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain a constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors, and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces, and atomic resolution of organic molecules, graphite, graphene, and oxides.

The qPlus sensor, a powerful core for the atomic force microscope

Atomic-scale characterization and manipulation with scanning probe microscopy rely upon the use of an atomically sharp probe. Here we present automated methods based on machine learning to automatically detect and recondition the quality of the probe of a scanning tunneling microscope. As a model system, we employ these techniques on the technologically relevant hydrogen-terminated silicon surface, training the network to recognize abnormalities in the appearance of surface dangling bonds. Of the machine learning methods tested, a convolutional neural network yielded the greatest accuracy, achieving a positive identification of degraded tips in 97% of the test cases. By using multiple points of comparison and majority voting, the accuracy of the method is improved beyond 99%.

Autonomous Scanning Probe Microscopy in Situ Tip Conditioning through Machine Learning

Assembling matter atom-by-atom into functional devices is the ultimate goal of nanotechnology. The possibility of achieving this goal is intrinsically dependent on the ability to visualize matter at the atomic level, induce and control atomic-scale motion, facilitate and direct chemical reactions, and coordinate and guide fabrication processes towards desired structures atom-by-atom. In this Perspective, we summarize recent progress in chemical transformations, material alterations and atomic dynamics studies enabled by the converged, atomic-sized electron beam of an aberration-corrected scanning transmission electron microscope. We discuss how such top-down observations have led to the concept of controllable, beam-induced processes and then of bottom-up, atom-by-atom assembly via electron-beam control. The progress in this field, from electron-beam-induced material transformations to atomically precise doping and multi-atom assembly, is reviewed, as are the associated engineering, theoretical and big-data challenges. Electron beams in a transmission electron microscope can be used to manipulate matter atom-by-atom. This Review surveys the recent advances and discusses how the further development and integration of machine learning, theory, feedback and instrumentation will push the field forward.

Atom-by-atom fabrication with electron beams

It has been proposed that miniature circuitry will ultimately be crafted from single atoms. Despite many advances in the study of atoms and molecules on surfaces using scanning probe microscopes, challenges with patterning and limited thermal structural stability have remained. Here we demonstrate rudimentary circuit elements through the patterning of dangling bonds on a hydrogen-terminated silicon surface. Dangling bonds sequester electrons both spatially and energetically in the bulk bandgap, circumventing short-circuiting by the substrate. We deploy paired dangling bonds occupied by one moveable electron to form a binary electronic building block. Inspired by earlier quantum dot-based approaches, binary information is encoded in the electron position, allowing demonstration of a binary wire and an OR gate. Rudimentary circuit elements, including a binary wire and an OR gate, can be created through the patterning of dangling bonds on a hydrogen-terminated silicon surface.

Binary atomic silicon logic

At the atomic scale, there has always been a trade-off between the ease of fabrication of structures and their thermal stability. Complex structures that are created effortlessly often disorder above cryogenic conditions. Conversely, systems with high thermal stability do not generally permit the same degree of complex manipulations. Here, we report scanning tunneling microscope (STM) techniques to substantially improve automated hydrogen lithography (HL) on silicon, and to transform state-of-the-art hydrogen repassivation into an efficient, accessible error correction/editing tool relative to existing chemical and mechanical methods. These techniques are readily adapted to many STMs, together enabling fabrication of error-free, room-temperature stable structures of unprecedented size. We created two rewriteable atomic memories (1.1 petabits per in2), storing the alphabet letter-by-letter in 8 bits and a piece of music in 192 bits. With HL no longer faced with this trade-off, practical silicon-based atomic-scale devices are poised to make rapid advances towards their full potential.

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Lithography for robust and editable atomic-scale silicon devices and memories

A method for the patterning and control of single electrons on a surface is provided that includes implementing scanning tunneling microscopy hydrogen lithography with a scanning probe microscope to form charge structures with one or more confined charges; performing a series of field-free atomic force microscopy measurements on the charge structures with different tip heights, where interaction between the tip and the confined charge are elucidated; and adjusting tip heights to controllably position charges within the structures to write a given charge state. The present disclose also provides a Gibb's distribution machine formed with the method for the patterning and control of single electrons on a surface. A multi bit true random number generator and neural network learning hardware formed with the above described method are also provided.

Initiating and monitoring the evolution of single electrons within atom-defined structures

Microwave parametric amplifiers operating at the quantum noise limit have become indispensable tools for a range of cryogenic quantum technologies. These amplifiers are typically constructed from nonlinear Josephson junctions, which limit the ability to amplify high-power signals. This study reports a device based instead on the weakly nonlinear kinetic inductance intrinsic to a superconducting film of niobium titanium nitride. The amplifier offers large phase-sensitive gain and high power handling, plus a simple design and fabrication process. As it contains no junctions, it is robust to electrostatic discharge and potentially operable under high temperatures and large magnetic fields. 

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Degenerate Parametric Amplification via Three-Wave Mixing Using Kinetic Inductance

This paper introduces SiQAD, a computer-aided design tool enabling the rapid design and simulation of computational assemblies of atomic silicon quantum dots. SiQAD consists of several simulation tools: a ground state electron configuration finder, a non-equilibrium electron dynamics simulator, and an electric potential landscape solver for the exploration of field-driven modulation with electrodes. Simulations have been compared against past experimental results to inform the electron population estimation and dynamic behavior. Fundamental logic building blocks and circuits suitable for this platform have been designed and simulated, and a clocked wire has been demonstrated. This work paves the way and provides the first set of open design tools for the exploration of the emerging design space of atomic silicon quantum dot circuits.

SiQAD: A Design and Simulation Tool for Atomic Silicon Quantum Dot Circuits

The use of superconducting microresonators together with quantum-limited Josephson parametric amplifiers has enhanced the sensitivity of pulsed electron spin resonance (ESR) measurements by more than four orders of magnitude. So far, the microwave resonators and amplifiers have been designed as separate components due to the incompatibility of Josephson junction–based devices with magnetic fields. This has produced complex spectrometers and raised technical barriers toward adoption of the technique. Here, we circumvent this issue by coupling an ensemble of spins directly to a weakly nonlinear and magnetic field–resilient superconducting microwave resonator. We perform pulsed ESR measurements with a 1-pL mode volume containing 6 × 107 spins and amplify the resulting signals within the device. When considering only those spins that contribute to the detected signals, we find a sensitivity of 2.8×103spins/Hz for a Hahn echo sequence at a temperature of 400 mK. In situ amplification is demonstrated at fields up to 254 mT, highlighting the technique’s potential for application under conventional ESR operating conditions.

Wyatt Vine

Papers

Binary atomic silicon logic

Initiating and monitoring the evolution of single electrons within atom-defined structures

Degenerate Parametric Amplification via Three-Wave Mixing Using Kinetic Inductance

SiQAD: A Design and Simulation Tool for Atomic Silicon Quantum Dot Circuits

In situ amplification of spin echoes within a kinetic inductance parametric amplifier