Alex Belianinov

Band Excitation in Scanning Probe Microscopy: Recognition and Functional Imaging

Field confinement at the junction between a biased scanning probe microscopes tip and solid surface enables local probing of various bias-induced transformations, such as polarization switching, ionic motion, and electrochemical reactions. The nanoscale size of the biased region, smaller or comparable to that of features such as grain boundaries and dislocations, potentially allows for the study of kinetics and thermodynamics at the level of a single defect. In contrast to classical statistically averaged approaches, this approach allows one to link structure to functionality and deterministically decipher associated mesoscopic and atomistic mechanisms. Furthermore, responses measured as a function of frequency and bias can serve as a fingerprint of local material functionality, allowing for local recognition imaging of inorganic and biological systems. This article reviews current progress in multidimensional scanning probe microscopy techniques based on band excitation time and voltage spectroscopies, including discussions on data acquisition, dimensionality reduction, and visualization, along with future challenges and opportunities for the field.

Deciphering Halogen Competition in Organometallic Halide Perovskite Growth

Organometallic halide perovskites (OHPs) hold great promise for next-generation, low-cost optoelectronic devices. During the chemical synthesis and crystallization of OHP thin films, a major unresolved question is the competition between multiple halide species (e.g., I(-), Cl(-), Br(-)) in the formation of the mixed-halide perovskite crystals. Whether Cl(-) ions are successfully incorporated into the perovskite crystal structure or, alternatively, where they are located is not yet fully understood. Here, in situ X-ray diffraction measurements of crystallization dynamics are combined with ex situ TOF-SIMS chemical analysis to reveal that Br(-) or Cl(-) ions can promote crystal growth, yet reactive I(-) ions prevent them from incorporating into the lattice of the final perovskite crystal structure. The Cl(-) ions are located in the grain boundaries of the perovskite films. These findings significantly advance our understanding of the role of halogens during synthesis of hybrid perovskites and provide an insightful guidance to the engineering of high-quality perovskite films, essential for exploring superior-performing and cost-effective optoelectronic devices.

Big data and deep data in scanning and electron microscopies: deriving functionality from multidimensional data sets

The development of electron and scanning probe microscopies in the second half of the twentieth century has produced spectacular images of the internal structure and composition of matter with nanometer, molecular, and atomic resolution. Largely, this progress was enabled by computer-assisted methods of microscope operation, data acquisition, and analysis. Advances in imaging technology in the beginning of the twenty-first century have opened the proverbial floodgates on the availability of high-veracity information on structure and functionality. From the hardware perspective, high-resolution imaging methods now routinely resolve atomic positions with approximately picometer precision, allowing for quantitative measurements of individual bond lengths and angles. Similarly, functional imaging often leads to multidimensional data sets containing partial or full information on properties of interest, acquired as a function of multiple parameters (time, temperature, or other external stimuli). Here, we review several recent applications of the big and deep data analysis methods to visualize, compress, and translate this multidimensional structural and functional data into physically and chemically relevant information.

Directing Matter: Toward Atomic-Scale 3D Nanofabrication

Enabling memristive, neuromorphic, and quantum-based computing as well as efficient mainstream energy storage and conversion technologies requires the next generation of materials customized at the atomic scale. This requires full control of atomic arrangement and bonding in three dimensions. The last two decades witnessed substantial industrial, academic, and government research efforts directed toward this goal through various lithographies and scanning-probe-based methods. These technologies emphasize 2D surface structures, with some limited 3D capability. Recently, a range of focused electron- and ion-based methods have demonstrated compelling alternative pathways to achieving atomically precise manufacturing of 3D structures in solids, liquids, and at interfaces. Electron and ion microscopies offer a platform that can simultaneously observe dynamic and static structures at the nano- and atomic scales and also induce structural rearrangements and chemical transformation. The addition of predictive modeling or rapid image analytics and feedback enables guiding these in a controlled manner. Here, we review the recent results that used focused electron and ion beams to create free-standing nanoscale 3D structures, radiolysis, and the fabrication potential with liquid precursors, epitaxial crystallization of amorphous oxides with atomic layer precision, as well as visualization and control of individual dopant motion within a 3D crystal lattice. These works lay the foundation for approaches to directing nanoscale level architectures and offer a potential roadmap to full 3D atomic control in materials. In this paper, we lay out the gaps that currently constrain the processing range of these platforms, reflect on indirect requirements, such as the integration of large-scale data analysis with theory, and discuss future prospects of these technologies.

Atomistic-Scale Simulations of Defect Formation in Graphene under Noble Gas Ion Irradiation

Despite the frequent use of noble gas ion irradiation of graphene, the atomistic-scale details, including the effects of dose, energy, and ion bombardment species on defect formation, and the associated dynamic processes involved in the irradiations and subsequent relaxation have not yet been thoroughly studied. Here, we simulated the irradiation of graphene with noble gas ions and the subsequent effects of annealing. Lattice defects, including nanopores, were generated after the annealing of the irradiated graphene, which was the result of structural relaxation that allowed the vacancy-type defects to coalesce into a larger defect. Larger nanopores were generated by irradiation with a series of heavier noble gas ions, due to a larger collision cross section that led to more detrimental effects in the graphene, and by a higher ion dose that increased the chance of displacing the carbon atoms from graphene. Overall trends in the evolution of defects with respect to a dose, as well as the defect characteristics, were in good agreement with experimental results. Additionally, the statistics in the defect types generated by different irradiating ions suggested that the most frequently observed defect types were Stone-Thrower-Wales (STW) defects for He(+) irradiation and monovacancy (MV) defects for all other ion irradiations.

Identification of phases, symmetries and defects through local crystallography

Advances in electron and probe microscopies allow 10 pm or higher precision in measurements of atomic positions. This level of fidelity is sufficient to correlate the length (and hence energy) of bonds, as well as bond angles to functional properties of materials. Traditionally, this relied on mapping locally measured parameters to macroscopic variables, for example, average unit cell. This description effectively ignores the information contained in the microscopic degrees of freedom available in a high-resolution image. Here we introduce an approach for local analysis of material structure based on statistical analysis of individual atomic neighbourhoods. Clustering and multivariate algorithms such as principal component analysis explore the connectivity of lattice and bond structure, as well as identify minute structural distortions, thus allowing for chemical description and identification of phases. This analysis lays the framework for building image genomes and structure–property libraries, based on conjoining structural and spectral realms through local atomic behaviour.

Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: Enabling nanoscale direct write homo-junctions

Atomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe2 thereby locally tuning the resistivity and transport properties of the material. Hole transport in the few layer WSe2 is degraded more severely relative to electron transport after helium ion irradiation. Furthermore, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe2 flakes, which constitutes an important advance towards two dimensional opto-electronic devices.

Big, Deep, and Smart Data in Scanning Probe Microscopy

Scanning probe microscopy (SPM) techniques have opened the door to nanoscience and nanotechnology by enabling imaging and manipulation of the structure and functionality of matter at nanometer and atomic scales. Here, we analyze the scientific discovery process in SPM by following the information flow from the tip-surface junction, to knowledge adoption by the wider scientific community. We further discuss the challenges and opportunities offered by merging SPM with advanced data mining, visual analytics, and knowledge discovery technologies.

Effect of Doping on Surface Reactivity and Conduction Mechanism in Samarium-Doped Ceria Thin Films

A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

Co-registered Topographical, Band Excitation Nanomechanical, and Mass Spectral Imaging Using a Combined Atomic Force Microscopy/Mass Spectrometry Platform

The advancement of a hybrid atomic force microscopy/mass spectrometry imaging platform demonstrating the co-registered topographical, band excitation nanomechanical, and mass spectral imaging of a surface using a single instrument is reported. The mass spectrometry-based chemical imaging component of the system utilized nanothermal analysis probes for pyrolytic surface sampling followed by atmospheric pressure chemical ionization of the gas-phase species produced with subsequent mass analysis. The basic instrumental setup and operation are discussed, and the multimodal imaging capability and utility are demonstrated using a phase-separated polystyrene/poly(2-vinylpyridine) polymer blend thin film. The topography and band excitation images showed that the valley and plateau regions of the thin film surface were comprised primarily of one of the two polymers in the blend with the mass spectral chemical image used to definitively identify the polymers at the different locations. Data point pixel size for the topography (390 nm × 390 nm), band excitation (781 nm × 781 nm), and mass spectrometry (690 nm × 500 nm) images was comparable and submicrometer in all three cases, but the data voxel size for each of the three images was dramatically different. The topography image was uniquely a surface measurement, whereas the band excitation image included information from an estimated 20 nm deep into the sample and the mass spectral image from 110 to 140 nm in depth. Because of this dramatic sampling depth variance, some differences in the band excitation and mass spectrometry chemical images were observed and were interpreted to indicate the presence of a buried interface in the sample. The spatial resolution of the chemical image was estimated to be between 1.5 and 2.6 μm, based on the ability to distinguish surface features in that image that were also observed in the other images.