Authors: Sergei V. Kalinin | Ondrej Dyck
Source: Advanced Functional Materials (article, 2019-12-01)
Field: Materials Science — Materials Chemistry

Materials and Devices with Probes and Beams: Down to the Atomic Level and Back Up Richard Feynman famously wrote “What I cannot create, I do not understand.” Areas in which this statement rings especially true are nanoscience, quantum materials, and quantum information systems – whereby control of matter at the atomic level is a necessary step to enable quantum computing devices,1-6 single spin systems,7-9 and other devices10-13 and applications we can only imagine. However, while the dream of atom-by-atom fabrication has been in the minds of the scientific community for a while, the experimental realization of this dream has been slow. This dream started to take shape with the publication in 1982 of the paper by Binnig and Rohrer14 who reported the direct atomic visualization of the Si(111) surface via scanning tunneling microscopy (STM). While the concept of a tunneling probe revealing surface topography had been explored before, the demonstration of real-space atomic resolution using a benchtop instrument galvanized the scientific world, leading to extensive instrument development efforts15 and multiple discoveries of exotic surface structures, terminations, and physical phenomena.16 Yet among all the advances brought forth by STM, one stood apart. Since the first days of STM, multiple practitioners encountered the presence of mobile surface atoms and atomic groups, sometimes visible as moving atoms, sometimes as indiscrete regions with loss of atomic resolution. Correspondingly, efforts were undertaken to minimize their effects via surface preparation or choice of optimal imaging conditions. The idea that changed the course of STM as a field was to use the tip to direct the motion of the surface atoms and assemble them into structures, transitioning STM from a purely observational tool into a tool for manipulation and assembly at the atomic level. The letters I, B, and M (I.B.M.) “written” by positioning Xe atoms on a Ni surface17 has been one of the key, and arguably the most visible, factor that launched the nanotechnology revolution. Interestingly, while STM and the related atomic force microscopy (AFM) techniques rapidly propagated in the scientific world, the vast majority of this growth was accomplished in the imaging modes. STM manipulation was perceived as more of a scientific curiosity, or at best a way to create atomically defined structures for fundamental physics studies. However, these structures had to be explored under ultra-high vacuum and liquid-He temperature conditions, enabling the power of STM as both a fabrication and imaging/spectroscopy tool but limiting its impact to practical areas such as the semiconductor industry and information technologies. The situation changed dramatically over the last decade. The work of multiple groups, most notably Michelle Simmons at the University of New South Wales, demonstrated that single atom manipulation with STM can be combined with classical surface science and semiconductor processing methods, enabling the creation of single-atom based devices.18-21 Similar demonstrations were also pioneered in the AFM community.22-27 While more than 25 years after Eigler,17 this work created the connection between basic science and cutting-edge computing and semiconductor applications, resulting in the rapid growth of related efforts at multiple research institutions worldwide. At the same time, approaching the end of the classic Moore's law roadmap when the mainstream semiconductor technologies are now at the ∼5 nm structure level and the rise of quantum computing with the need for associated technologies such as solid state qubits and single photon emitters now provides a strong driver for atomic fabrication methods. The question arises whether scanned probe methods are the only techniques that can be leveraged for direct atomic manipulation. A prerequisite for this level of control is the ability to detect subtle, sub-nm-level changes; essentially one must be able to image the events at this scale. From the perspective of imaging, the primary paradigms for atomically resolved imaging are SPM and electron microscopy, notably scanning transmission electron microscopy (STEM), where the electron beam can be focused on single atoms and atomic columns. Can STEM also be used for atomic manipulation? The extent to which the development in the electron microscopy community mirrored that in SPM is quite remarkable. While STEM and transmission electron microscopy (TEM) were invented in the 1930s, a significant breakthrough arrived with the experimental realization and commercialization of the aberration corrector.28 The impact of aberration correction (AC) on STEM imaging and spectroscopy is well known and has been reviewed in a number of publications.28-30 What is remarkable is that AC has rendered atomically resolved imaging and spectroscopy as routine in the electron microscopy community. Combined with the advent of 2D materials, atomically resolved imaging of single- and few-layer materials became common around 2010. Similar to STM studies in the late 1980s, researchers worldwide noted that the electron beam (e-beam) can introduce structural damage, which could be resolved one atom at a time!31-33 Traditionally, the vast majority of efforts were unanimously intent on preventing beam damage; however, following the analogy with STM, it was argued that damage can be harnessed to produce controlled structures.34 Recent demonstrations illustrate that such “damage” can be used to perform directed transformations at the atomic scale.35 Drndic et al. leveraged e-beam damage to perform nanosculpting of graphene to form devices far smaller than traditional fabrication techniques would allow.36-43 Dyck et al. and Susi et al. used the e-beam for targeting single atoms in graphene and demonstrated dopant insertion and controlled atomic movement in a 2D material.44-49 Hudak et al. demonstrated e-beam-controlled dopant movement in a 3D slab of crystalline Si,50, 51 and Jesse et al. used the e-beam to crystallize amorphous SrTiO3 in epitaxial registration with a SrTiO3 substrate with atomic plane precision.52 These demonstrations highlight the unique ability of the STEM to alter suspended 2D materials and to “reach into” the bulk of a 3D material to perform controlled atomic-scale modifications. Despite the relatively small overlap between STM and STEM communities in terms of instrumentation, theory, and research areas, the two communities share many intrinsic similarities. Both are instrument-oriented communities focused on the development of improved tools and understanding their performance. Both communities prize high spatial- and energy-resolutions and routinely work with atomically resolved imaging and spectroscopy data. And now, both communities have demonstrated approaches for modifying materials on the atomic level. These deep similarities between the STEM and SPM communities, along with the growing need for device applications, suggest the tremendous potential for exploiting synergies between the two, from sample preparation and surface science aspects to image analytics and manipulation workflows. To spark synergistic cross-collaboration, in 2018 the Oak Ridge National Laboratory hosted the first international workshop on atom-by-atom fabrication with e-beams and scanning probes, drawing together leading researchers in both fields as well as industry representatives and US Department of Energy and National Science Foundation program managers. This special issue of Advanced Functional Materials was derived from ideas, presentations, and collaborations that emerged during this workshop. This issue is structured to provide an overview from the perspective of atomic fabrication for quantum devices, to discuss the well-established STM-based nanofabrication, and to showcase the materials systems amenable for e-beam fabrication, associated applications, and theory. A perspective on atomic-based fabrication and impact on quantum devices is given in article number 1904557 by P. Narang and D. Englund. An overview of the potential of classical STM-based fabrication is described by A.P. Li in article number 1903770. Recent developments in atomic-scale manipulation with a particular focus on progress toward the fabrication of structures exhibiting quantum properties of technological relevance are summarized. The core of the issue is comprised of papers describing new e-beam fabrication techniques. T. Susi describes the controlled movement of dopant atoms through the walls of graphene nanotubes in article number 1901327. Expanding further on the material side, X. Sang and R. Unocic describe the effects of thermal energy and electron beam irradiation on the formation and evolution of MoS2 flakes in article number 1902149. F. Camino provides the classical lithographic counterpart in article number 1903429. M. Drndić showcases applications of single-atom fabrication for real-world medical devices in article number 1904668. R. Silver discusses STM-based quantum structure fabrication in article number 1903475. D. Kepaptsoglou describes the mechanisms for atomic-scale spectroscopic imaging of the extreme-UV optical response of B and N doped graphene in article number 1901819. The relevant theory is developed by D. Lingerfelt in article number 1901901. Finally, the role of machine learning in these endeavors is discussed by M. Ziatdinov in article number 1904480. This special issue underscores the exciting developments in the field of atomic manipulation and the profound impacts such capabilities could have, particularly in the realm of building devices that harness the quantum properties of matter. Many challenges remain to be solved, for example the complicated surface science now being faced by the STEM community and the need to connect atomic fabrication techniques to traditional semiconductor manufacturing workflows. Nevertheless, successes such as the deep learning approach to image analysis, which will enable feedback controlled and fully automated atomic fabrication, provide fresh perspectives on what is possible and where the path forward lies. It remains to be seen how STEM and SPM techniques can be combined to leverage the strengths of each; the first steps are now being taken and the ability to “arrange the atoms one-by-one, the way we want”53 has never been closer. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Division of Materials Science and Engineering, Basic Energy Sciences and was performed at the Oak Ridge National Laboratory's Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy, Office of Science User Facility. Materials science ; Nanotechnology ; Atomic beam ; Atomic physics ; Engineering physics ; Optoelectronics ; Beam (structure) ; Optics ; Physics Graphene research and applications ; Surface and Thin Film Phenomena ; Electronic and Structural Properties of Oxides