Structural disorder causes materials’ surface electronic properties, e.g., work function ([Formula: see text]), to vary spatially, yet it is challenging to prove exact causal relationships to underlying ensemble disorder, e.g., roughness or granularity. For polycrystalline Pt, nanoscale resolution photoemission threshold mapping reveals a spatially varying [Formula: see text] eV over a distribution of (111) vicinal grain surfaces prepared by sputter deposition and annealing. With regard to field emission and related phenomena, e.g., vacuum arc initiation, a salient feature of the [Formula: see text] distribution is that it is skewed with a long tail to values down to 5.4 eV, i.e., far below the mean, which is exponentially impactful to field emission via the Fowler–Nordheim relation. We show that the [Formula: see text] spatial variation and distribution can be explained by ensemble variations of granular tilts and surface slopes via a Smoluchowski smoothing model wherein local [Formula: see text] variations result from spatially varying densities of electric dipole moments, intrinsic to atomic steps, that locally modify [Formula: see text]. Atomic step-terrace structure is confirmed with scanning tunneling microscopy (STM) at several locations on our surfaces, and prior works showed STM evidence for atomic step dipoles at various metal surfaces. From our model, we find an atomic step edge dipole [Formula: see text] D/edge atom, which is comparable to values reported in studies that utilized other methods and materials. Our results elucidate a connection between macroscopic [Formula: see text] and the nanostructure that may contribute to the spread of reported [Formula: see text] for Pt and other surfaces and may be useful toward more complete descriptions of polycrystalline metals in the models of field emission and other related vacuum electronics phenomena, e.g., arc initiation.
While it is likely practically a bad idea to shrink a transistor to the size of an atom, there is no arguing that it would be fantastic to have atomic-scale control over every aspect of a transistor – a kind of crystal ball to understand and evaluate new ideas. This project showed that it was possible to take a niche technique used to place dopants in silicon with atomic precision and apply it broadly to study opportunities and limitations in microelectronics. In addition, it laid the foundation to attaining atomic-scale control in semiconductor manufacturing more broadly.
The stability of low-index platinum surfaces and their electronic properties is investigated with density functional theory, toward the goal of understanding the surface structure and electron emission, and identifying precursors to electrical breakdown, on nonideal platinum surfaces. Propensity for electron emission can be related to a local work function, which, in turn, is intimately dependent on the local surface structure. The (1×N) missing row reconstruction of the Pt(110) surface is systematically examined. The (1×3) missing row reconstruction is found to be the lowest in energy, with the (1×2) and (1×4) slightly less stable. In the limit of large (1×N) with wider (111) nanoterraces, the energy accurately approaches the asymptotic limit of the infinite Pt(111) surface. This suggests a local energetic stability of narrow (111) nanoterraces on free Pt surfaces that could be a common structural feature in the complex surface morphologies, leading to work functions consistent with those on thermally grown Pt substrates.
The attachment of dopant precursor molecules to depassivated areas of hydrogen-terminated silicon templated with a scanning tunneling microscope (STM) has been used to create electronic devices with subnanometer precision, typically for quantum physics experiments. This process, which we call atomic precision advanced manufacturing (APAM), dopes silicon beyond the solid-solubility limit and produces electrical and optical characteristics that may also be useful for microelectronic and plasmonic applications. However, scanned probe lithography lacks the throughput required to develop more sophisticated applications. Here, we demonstrate and characterize an APAM device workflow where scanned probe lithography of the atomic layer resist has been replaced by photolithography. An ultraviolet laser is shown to locally and controllably heat silicon above the temperature required for hydrogen depassivation on a nanosecond timescale, a process resistant to under- and overexposure. STM images indicate a narrow range of energy density where the surface is both depassivated and undamaged. Modeling that accounts for photothermal heating and the subsequent hydrogen desorption kinetics suggests that the silicon surface temperatures reached in our patterning process exceed those required for hydrogen removal in temperature-programmed desorption experiments. A phosphorus-doped van der Pauw structure made by sequentially photodepassivating a predefined area and then exposing it to phosphine is found to have a similar mobility and higher carrier density compared with devices patterned by STM. Lastly, it is also demonstrated that photodepassivation and precursor exposure steps may be performed concomitantly, a potential route to enabling APAM outside of ultrahigh vacuum.
Scanning Microwave Impedance Microscopy (sMIM) is a sensitive electrical measurement technique which can characterize local static and temporal variations of electrical permittivity, and conductivity of materials and devices as well as for failure analysis. It is being used to characterize dielectrics, semiconductors and their doping response, and metals. Measurements can be made at room temperature down to cryogenic temperatures where quantum effects become important. Leveraging near-field electrical interactions between a probe and the sample, sMIM can measure and image electrical properties and operation at the nanoscale to micron scale by incorporation into an atomic force microscope. sMIM is being applied to a wide range of industrial and scientific applications to improve fundamental and functional understanding and operational performance of advanced, exploratory and quantum electronic devices and materials and their fabrication.
In most models of vacuum breakdown, there is some initial emission of electrons from the cathodic surface, usually employing some form of Fowler-Nordheim emission. While this may be correct for 'textbook' surfaces, it is generally unreliable for real surfaces and fitted parameters are often used. For example, the beta employed is generally unphysical based on usual definitions (e.g., it incorporates more, but unexplained, physics than just a geometry-based field concentration effect). In this work, we describe experimental efforts to better characterize which surface structure parameters influence the vacuum field emission current.
We report on the fabrication and characterization of nanocrystalline ZnO films for use as a random laser physical unclonable function (PUF). Correlation between processing conditions and film microstructure will be made to optimize the lasing properties and random response. We will specifically examine the repeatability and security of PUFs demonstrated in this novel 3 system. This demonstration has promise to impact many of Sandia's core missions including counterfeit detection. 4 4
We present simulation results quantitatively showing that circularly polarized light persists in transmission through several real-world and model fog environments better than linearly polarized light over broad wavelength ranges from the visible through the infrared. We present results for polydisperse particle distributions from realistic and measured fog environments, comparing the polarization persistence of linear and circular polarization. Using a polarization-tracking Monte Carlo program, we simulate polarized light propagation through four MODTRAN fog models (moderate and heavy radiation fog and moderate and heavy advection fog) and four real-world measured fog particle distributions (Garland measured radiation and advection fogs, Kunkel measured advection fog, and Sandia National Laboratories’ Fog Facility’s fog). Simulations were performed for each fog environment with wavelengths ranging from 0.4 to 12 μm for increasing optical thicknesses of 5, 10, and 15 (increasing fog density or sensing range). Circular polarization persists superiorly for all optical wavelength bands from the visible to the long-wave infrared in nearly all fog types for all optical thicknesses. Throughout our analysis, we show that if even a small percentage of a fog’s particle size distribution is made up of large particles, those particles dominate the scattering process. In nearly all real-world fog situations, these large particles and their dominant scattering characteristics are present. Larger particles are predominantly forward-scattering and contribute to circular polarization’s persistence superiority over broad wavelength ranges and optical thicknesses/ range. Circularly polarized light can transmit over 30% more signal in its intended state compared to linearly polarized light through real-world fog environments. This work broadens the understanding of how circular polarization persists through natural fog particle distributions with natural variations in mode particle radius and single or bimodal characteristics.
Charge-transfer materials based on the self-assembly of aromatic donor–acceptor complexes enable a modular organic-synthetic approach to develop and fine-tune electronic and optical properties, and thus these material systems stand to impact a wide range of technologies. Through laser-induction of temperature gradients, in this study, user-defined patterning of strongly dichroic and piezoelectric organic thin films composed of donor–acceptor columnar liquid crystals is shown. Fine, reversible control over isotropic versus anisotropic regions in thin films is demonstrated, enabling noncontact writing/rewriting of micropolarizers, bar codes, and charge-transfer based devices.
Materials aging is a high-consequence failure mode in electronic systems. Such mechanisms can degrade the electrical properties of connectors, relays, wire bonds, and other interconnections. Lost performance will impact, not only that of the device, but also the function and reliability of next-level assemblies and the weapons system as a whole. The detections of changes to materials surfaces at the nanometer-scale resolution, provides a means to identify aging processes at their early stages before they manifest into latent failures that affect system-level performance and reliability. Diffusion will be studied on thin films that undergo accelerated aging using the nanometer scale characterization technique of Frequency Modulated Kelvin Probe Force Microscopy (FM-KPFM). The KPFM provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer spatial resolution of surface chemistry changes. The KPFM method can provide the means to measure surface, and near-surface, elemental concentrations that allow the determination of diffusion rate kinetics. These attributes will be illustrated by assessing diffusion in a thin film couple. Validation data will obtained from traditional techniques: (a) Auger electron spectroscopy (AES), x-ray fluorescence (XRF), and xray photoelectron spectroscopy (XPS).
Scattering environment conditions, such as fog, pose a challenge for many detection and surveillance active sensing operations in both ground and air platforms. For example, current autonomous vehicles rely on a range of optical sensors that are affected by degraded visual environments. Real-world fog conditions can vary widely depending on the location and environmental conditions during its creation. In our previous work we have shown benefits for increasing signal and range through scattering environments such as fog utilizing polarized light, specifically circular polarization. In this work we investigate the effect of changing fog particle sizes and distributions on polarization persistence for both circularly and linearly polarized light via simulation. We present polarization tracking Monte Carlo results for a range of realistic monodisperse particle sizes as well as varying particle size distributions as a model of scattering environments. We systematically vary the monodisperse particle size, mean particle size of a distribution, particle size distribution width, and number of distribution lobes (bi-modal), as they affect polarized light transmission through a scattering environment. We show that circular polarization signal persists better than linear polarization signal for most variations of the particle distribution parameters.
Degraded visual environments are a serious concern for modern sensing and surveillance systems. Fog is of interest due to the frequency of its formation along our coastlines disrupting border security and surveillance. Fog presents hurdles in intelligence and reconnaissance by preventing data collection with optical systems for extended periods. We will present recent results from our work in operating optical systems in our controlled fog experimental chamber. This facility is a 180-foot-long, 10-foot-wide, and 10-foot-Tall structure that has over 60 spray nozzles to achieve uniform aerosol coverage with various particle size, distributions, and densities. We will discuss the physical formation of fog in nature and how our generated fog compares. In addition, we will discuss fog distributions and characterization techniques. We will investigate the biases of different methods and discuss the different techniques that are appropriate for realistic environments. Finally, we will compare the data obtained from our characterization studies against accepted models (e.g., MODTRAN) and validate the usage of this unique capability as a controlled experimental realization of natural fog formations. By proving the capability, we will enable the testing and validation of future fog penetrating optical systems and providing a platform for performing optical propagation experimentation in a known, stable, and controlled environment.
Ferroelastic domain walls provide opportunities for deterministically controlling mechanical, optical, electrical, and thermal energy. Domain wall characterization in micro- and nanoscale systems, where their spacing may be of the order of 100 nm or less is presently limited to only a few techniques, such as piezoresponse force microscopy and transmission electron microscopy. These respective techniques cannot, however, independently characterize domain polarization orientation and domain wall motion in technologically relevant capacitor structures or in a non-destructive manner, thus presenting a limitation of their utility. In this work, we show how backscatter scanning electron microscopy utilizing channeling contrast yield can image the ferroelastic domain structure of ferroelectric films with domain wall spacing as narrow as 10 nm. Combined with electron backscatter diffraction to identify grain orientations, this technique provides information on domain orientation and domain wall type that cannot be readily measured using conventional non-destructive methods. In addition to grain orientation identification, this technique enables dynamic domain structure changes to be observed in functioning capacitors utilizing electrodes that are transparent to the high-energy backscattered electrons. This non-destructive, high-resolution domain imaging technique is applicable to a wide variety of ferroelectric thin films and a multitude of material systems where nanometer-scale crystallographic twin characterization is required.
We present simulation and experimental results showing circular polarization is more tolerant of optical collection geometry (field of view and collection area) variations than linear polarization for forward-scattering environments. Circular polarization also persists superiorly in the forward-scattering environment compared to linear polarization by maintaining its degree of polarization better through increasing optical thicknesses. In contrast, both linear and circular polarizations are susceptible to collection geometry variations for isotropic-scattering (Rayleigh regime) environments, and linear polarization maintains a small advantage in polarization persistence. Simulations and measurements are presented for laboratory-based environments of polystyrene microspheres in water. Particle diameters were 0.0824 μm (for isotropic-scattering) and 1.925 μm (for forward-scattering) with an illumination wavelength of 543.5 nm.
The ability to create optical materials with arbitrary index distributions would prove transformative for optics design and applications. However, current fabrication techniques for graded index (GRIN) materials rely on diffusion profiles and therefore are unable to realize arbitrary distribution GRIN design. Here, we demonstrate the laser direct writing of graded index structures in protein-based hydrogels using multiphoton lithography. We show index changes spanning a range of 10-2, which is comparable with laser densified glass and polymer systems. Further, we demonstrate the conversion of these written density variation structures into SiO2, opening up the possibility of transforming GRIN hydrogels to a wide range of material systems.
We present experimental and simulation results for a laboratory-based forward-scattering environment, where 1 μm diameter polystyrene spheres are suspended in water to model the optical scattering properties of fog. Circular polarization maintains its degree of polarization better than linear polarization as the optical thickness of the scattering environment increases. Both simulation and experiment quantify circular polarization's superior persistence, compared to that of linear polarization, and show that it is much less affected by variations in the field of view and collection area of the optical system. Our experimental environment's lateral extent was physically finite, causing a significant difference between measured and simulated degree of polarization values for incident linearly polarized light, but not for circularly polarized light. Through simulation we demonstrate that circular polarization is less susceptible to the finite environmental extent as well as the collection optic's limiting configuration.
This work quantifies the polarization persistence and memory of circularly polarized light in forward-scattering and isotropic (Rayleigh regime) environments; and for the first time, details the evolution of both circularly and linearly polarized states through scattering environments. Circularly polarized light persists through a larger number of scattering events longer than linearly polarized light for all forward-scattering environments; but not for scattering in the Rayleigh regime. Circular polarization's increased persistence occurs for both forward and backscattered light. The simulated environments model polystyrene microspheres in water with particle diameters of 0.1 μm, 2.0 μm, and 3.0 μm. The evolution of the polarization states as they scatter throughout the various environments are illustrated on the Poincaré sphere after one, two, and ten scattering events.
Image processing offers a potential to simplify an optical system by shifting some of the imaging burden from lenses to the more cost effective electronics. Wavefront coding using a cubic phase plate combined with image processing can extend the system's depth of focus, reducing many of the focus-related aberrations as well as material related chromatic aberrations. However, the optimal design process and physical limitations of wavefront coding systems with respect to first-order optical parameters and noise are not well documented. We examined image quality of simulated and experimental wavefront coded images before and after reconstruction in the presence of noise. Challenges in the implementation of cubic phase in an optical system are discussed. In particular, we found that limitations must be placed on system noise, aperture, field of view and bandwidth to develop a robust wavefront coded system.
Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. A solid-state solution to rapidly manipulate phonons has escaped the scientific community. We demonstrate active and reversible tuning of thermal conductivity by manipulating the nanoscale ferroelastic domain structure of a Pb(Zr0.3Ti0.7)O3 film with applied electric fields. With subsecond response times, the room-temperature thermal conductivity was modulated by 11%.
We present simulation results that show circularly polarized light persists through scattering environments better than linearly polarized light. Specifically, we show persistence is enhanced through many scattering events in an environment with a size parameter representative of advection fog at infrared wavelengths. Utilizing polarization tracking Monte Carlo simulations we show a larger persistence benefit for circular polarization versus linear polarization for both forward and backscattered photons. We show the evolution of the incident polarization states after various scattering events which highlight the mechanism leading to circular polarization's superior persistence.
We find for infrared wavelengths that there are broad ranges of particle sizes and refractive indices that represent fog and rain, where circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index, we show that, for specific scene parameters, circular polarization outperforms linear polarization in maintaining the illuminating polarization state for large optical depths. This enhancement with circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Initially, researchers employed polarizationdiscriminating schemes, often using linearly polarized active illumination, to further distinguish target signals from the background noise. More recently, researchers have investigated circular polarization as a means to separate signal from noise even more. Specifically, we quantify both linearly and circularly polarized active illumination and show here that circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave and long-wave infrared, and large particle sizes of Sahara dust around the 4 μmwavelength. Conversely, we quantify where linear polarization persists better than circular polarization for some limited particle sizes of radiation fog in the long-wave infrared, small particle sizes of Sahara dust for wavelengths of 9-10.5 μm, and large particle sizes of Sahara dust through the 8-11 μm wavelength range in the long-wave infrared.
We find for infrared wavelengths there are broad ranges of particle sizes and refractive indices that represent fog and rain where the use of circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index systematically, we show that for specific scene parameters circular polarization outperforms linear polarization in maintaining the intended polarization state for large optical depths. This enhancement in circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Specifically, circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave infrared and the long-wave infrared, and large particle sizes of Sahara dust around the 4 micron wavelength.
Radiation hard nonvolatile random access memory (NVRAM) is a crucial component for DOE and DOD surveillance and defense applications. NVRAMs based upon ferroelectric materials (also known as FERAMs) are proven to work in radiation-rich environments and inherently require less power than many other NVRAM technologies. However, fabrication and integration challenges have led to state-of-the-art FERAMs still being fabricated using a 130nm process while competing phase-change memory (PRAM) has been demonstrated with a 20nm process. Use of block copolymer lithography is a promising approach to patterning at the sub-32nm scale, but is currently limited to self-assembly directly on Si or SiO{sub 2} layers. Successful integration of ferroelectrics with discrete and addressable features of {approx}15-20nm would represent a 100-fold improvement in areal memory density and would enable more highly integrated electronic devices required for systems advances. Towards this end, we have developed a technique that allows us to carry out block copolymer self-assembly directly on a huge variety of different materials and have investigated the fabrication, integration, and characterization of electroceramic materials - primarily focused on solution-derived ferroelectrics - with discrete features of {approx}20nm and below. Significant challenges remain before such techniques will be capable of fabricating fully integrated NVRAM devices, but the tools developed for this effort are already finding broader use. This report introduces the nanopatterned NVRAM device concept as a mechanism for motivating the subsequent studies, but the bulk of the document will focus on the platform and technology development.
LDRD Project 105876 was a research project whose primary goal was to discover the currently unknown science underlying the basic linear and nonlinear electrodynamic response of nanotubes and nanowires in a manner that will support future efforts aimed at converting forefront nanoscience into innovative new high-frequency nanodevices. The project involved experimental and theoretical efforts to discover and understand high frequency (MHz through tens of GHz) electrodynamic response properties of nanomaterials, emphasizing nanowires of silicon, zinc oxide, and carbon nanotubes. While there is much research on DC electrical properties of nanowires, electrodynamic characteristics still represent a major new frontier in nanotechnology. We generated world-leading insight into how the low dimensionality of these nanomaterials yields sometimes desirable and sometimes problematic high-frequency properties that are outside standard model electron dynamics. In the cases of silicon nanowires and carbon nanotubes, evidence of strong disorder or glass-like charge dynamics was measured, indicating that these materials still suffer from serious inhomogeneities that limit there high frequency performance. Zinc oxide nanowires were found to obey conventional Drude dynamics. In all cases, a significant practical problem involving large impedance mismatch between the high intrinsic impedance of all nanowires and nanotubes and high-frequency test equipment had to be overcome.