ABSTRACTRaman spectroscopy is an ideal tool in the characterization of materials including PuO2. The wavelength‐dependent absorptivity of the material defines the light penetration depth and the relative Raman scattering contribution from the bulk and the surface. The surface contribution to the total Raman scattering was investigated for PuO2 calcined at various temperatures and recorded with laser wavelengths of 355, 325, and 244 nm. These experiments provided the first glimpse of the wavelength‐dependent disappearance and emergence of new phonons and electronic bands from the PuO2 surface layers. The first indication of the wavelength transition in the Raman spectra was the loss of the 2LO2 (overtone, ~1155 cm−1) band and the weakening intensity of the Г1 → Γ5 electronic band (~2135 cm−1) with the 355‐nm excitation laser. The Γ5 electronic band was barely visible with the 244‐nm excitation. The electronic band located at ~1050 cm−1, corresponding to the Г1 → Γ4 electronic transition was observed to dramatically increase in intensity while the Г1 → Γ3 electronic band (2640 cm−1) sharpened as the UV wavelength was increased in energy from the near‐ to deep‐UV (355–325–244 nm). The FWHM of the T2g band was found to vary with calcination temperature (450°C and 900°C) with the 325‐nm laser and the 244‐nm laser. The T2g band attributes, the strong emergence of the Г1 → Γ4 electronic band, and the disappearance of the 2LO2 overtone acquired with the 244‐nm excitation for the different calcination temperatures suggest a shallow penetration depth.

Anhydrous plutonium tetrafluoride is an important intermediate in the production of metallic Pu. This historically important compound is also known to exist in at least two distinct, yet understudied hydrate forms, PuF4xH2O(s) (0.5 <= x <= 2) and PuF42.5H2O(s). X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) are the most common tools used to characterize these materials, often in a context for studying structural and morphological changes that arise from aging or calcination. However, fundamental electronic and vibrational spectroscopic information is rather scarce. Here, we measured the visible and shortwave infrared (SWIR) diffuse reflectance, Fourier transform infrared (FTIR), fluorescence and Raman spectra of PuF4(s) and PuF4xH2O(s) to obtain a better electronic and vibrational fingerprint. Our work provides clear indication of the polymeric structure of anhydrous PuF4, consistent with the Raman spectrum of UF4(s) and its hydrates. This is supplemented with XRD, TGA and SEM analysis. Findings in this study indicate that the spectra are modified by particle size, which in turn is influenced by synthetic technique.

The Savannah River National Laboratory (SRNL) has been developing advanced spectroscopic tools for the characterization of plutonium-bearing compounds with the intent to develop material and process signatures for nuclear forensics. Plutonium in a production, refining, or finishing facility will exist in many forms including oxide precursors (PuF4, PuF3, Pu oxalate, etc.), oxide, and metal. The ability to identify plutonium in each of these chemical forms and determine their processing history is crucial for the development of spectroscopic signatures. This presentation will focus on our work to develop and apply spectroscopic tools at SRNL using doubled-walled cells (DWC) to characterize the thermal decomposition of oxalates, calcination chemistry, alpha-decay-induced chemistry, age dating since last calcination, and other signatures related to plutonium processing.

Spectroscopic analysis of radiological materials has been historically limited to radiological labs with older or less advanced scientific instrumentation. The development of double-walled cells (DWCs) at the Savannah River National Laboratory (SRNL) has enabled Pu-bearing compounds to be removed from radiological laboratories and studied in our radiologically clean spectroscopy laboratory with state-of-the-art instrumentation. In this manuscript, we discuss the contributions of DWCs that have allowed the application of Raman spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), diffuse reflectance spectroscopy (DRS) in the shortwave infrared, and gamma spectroscopy at SRNL. Significant advances have been made in the understanding of the thermal decomposition of Pu(III) and Pu(IV) oxalates, alpha-induced damage to the PuO2 crystal lattice, and the effect of calcination temperatures on the quality of PuO2. These techniques have enabled methods to conduct PuO2 age dating since last calcination and estimate the calcination temperature with Raman spectroscopy and DRS. Additional spectroscopic information measured with DRIFTS has enabled the observation of the evolution of carbon species with calcination temperature, while gamma spectroscopy provides information on age dating since last purification.

Plutonium research has been stifled by the significant number of administrative controls and safety procedures, space and instrumentation limitations in radiological gloveboxes, and the potential for personnel and equipment contamination. To address the limited number of spectroscopic studies in Pu-bearing compounds in the current scientific literature, this work presents the use of double-walled cells (DWCs) in "clean" buildings/laboratories as an alternative to research in radiological gloveboxes. This study reports the first laser-induced breakdown spectroscopy (LIBS) experiments of a PuO2 pellet contained within a DWC, where the formation of elemental (atomic and ionic) species as well as the evolution from elemental to molecular products (PuxOy) was measured. Raman spectroscopy was also used to characterize the surface of the ablated pellet and the particulates deposited on the window of the inner cell. The full width half-maximum of the T2g band enabled us to obtain an estimate of the temperature at the pellet surface after the ablation pulse and the particulates based on the crystal lattice disorder. Particulates deposited on the window of the DWC during laser ablation were characterized using scanning electron microscopy, where molten irregular particulates and spheroids were observed. This exciting research conducted in a DWC describes our initial attempts to incorporate LIBS in the arsenal of spectroscopic tools for nuclear forensics applications.

The Raman spectrum of PuO2 was measured up to 13,000 cm(-1) with three different laser excitation wavelengths spanning the resonance (405 nm), near-resonance (457 nm), and preresonance (514 nm) energy range. Approximately 26 never-before-seen bands were observed between 3500 and 13,000 cm(-1). Given the very high energy of the Raman shifts of these bands and the relative insensitivity of their spectral position to the interrogating laser wavelength, they are believed to arise from an electronic origin. These bands are present in both freshly calcined and radiolytically aged PuO2, although a broad luminescence is observed in the aged material, which obscures many of the high frequency features. In situ laser annealing of the material attenuated this luminescence and allowed for clear observation of these never-before-seen spectral features. Discovery of these high-energy bands presents a new way of identifying PuO2 for nuclear nonproliferation and forensics purposes.

Rapid detection of nerve agents from complex matrices with minimal sample preparation is essential due to their high toxicity and bioavailability. In this work, quantum dots (QDs) were functionalized with oligonucleotide aptamers that specifically targeted a nerve agent metabolite, methylphosphonic acid (MePA). These QD-DNA bioconjugates were covalently linked to quencher molecules to form Förster resonance energy transfer (FRET) donor-acceptor pairs that quantitatively measure the presence of MePA. Using the FRET biosensor, the MePA limit of detection was 743 nM in artificial urine. A decrease in the QD lifetime was measured upon DNA binding and was recovered with MePA. The biosensor's flexible design makes it a strong candidate for the rapid detection of chemical and biological agents for deployable, in-field detectors.

Diffuse reflectance spectroscopy measurements in the shortwave infrared (930-1600 nm) spectral region were acquired for Pu-2(C2O4)(3)center dot 9H(2)O and its thermal decomposition product, PuO2. We analyzed a total of eight PuO2 samples that were produced at different calcination temperatures (300, 350, 450, 525, 600, 675, 750, and 900 degrees C). Our goal was to identify spectroscopic fingerprints that could be used to gain retrospective information regarding the production parameters of these important nuclear compounds. The diffuse reflectance spectrum of Pu-2(C2O4)(3)center dot 9H(2)O features several broad bands that currently preclude detailed analysis. However, all PuO2 samples produced relatively sharp spectral features that got sharper and more intense for samples that were produced at higher calcination temperatures. The electronic band observed at 1433 nm in the diffuse reflectance spectra of PuO2 was found to be a sensitive indicator of crystallinity; a result that is corroborated by ancillary Raman spectroscopy measurements. Principal component analysis of diffuse reflectance spectra was able to clearly rank and categorize PuO2 samples based on the calcination temperature that was employed during their production. Thus, we show herein that important retrospective information pertaining to the process history of PuO2 can be gained through the relatively simplistic combination of diffuse reflectance spectroscopy and principal component analysis. This discovery presents a new method for determining the provenance and process history of PuO2 and should have an impact in the fields of nuclear forensics and nuclear nonproliferation.

The thermal decomposition of Pu(III) oxalate was analyzed by Raman microspectroscopy, infrared spectroscopy, scanning electron microscopy, and powder X-ray diffraction. These data show that crystalline Pu2(C2O4)3•9H2O progressively loses water and oxalate ligands as it is heated, which leads to a decrease in long-range lattice ordering, though minimal changes are observed in gross crystalline morphology. The onset of PuO2 formation was observed between 200 - 250 ℃. Thermal decomposition of oxalate ligands leads to the formation of CO2 and plutonium oxalate-carbonate moieties, which had not been observed in previously published thermogravimetric measurements of Pu(III) oxalate. Formation of plutonium oxalate-carbonate moieties is believed to be associated with a change in the plutonium oxidation state from 3+ to 4+, which occurs prior to PuO2 formation. The data provided herein demonstrate the rich spectroscopic nature of a rather underexplored, and technologically relevant, plutonium system. Ideally these results will further future investigations into the Pu(III) oxalate system both experimentally and computationally.

For over 80 years, plutonium dioxide has been routinely produced via thermal decomposition of hydrated plutonium(IV) oxalate. Despite the longstanding utility of this process, the chemical structures of starting materials and intermediates produced during this thermal conversion remain ill-defined. To help resolve this uncertainty, we measured high-resolution Raman and infrared spectra of Pu(C2O4)(2)center dot 6H(2)O that was heated to 25, 100, 220, 250, 350, and 450 degrees C in air. Our measurements show that Pu(C2O4)(2)center dot 6H(2)O has a rich vibrational spectrum with at least 15 Raman bands between 180 cm(-1) and 1900 cm(-1) and 9 infrared bands between 800 cm(-1) and 4000 cm(-1). As Pu(C2O4)(2)center dot 6H(2)O is heated, water is liberated, and the oxalate ligand decomposes to produce plutonium oxycarbide species. When heated to 350 degrees C or higher, vibrational spectra are consistent with PuO2 with some residual carbon-containing species. Full vibrational spectra, powder X-ray diffraction, and scanning electron microscopy measurements of Pu(C2O4)(2)center dot 6H(2)O and its thermal degradation products are presented herein along with approximate assignments for observed spectral bands. These data can be used to validate and potentially improve existing computational models that describe the chemical structure of compounds produced during thermal degradation of plutonium (IV) oxalate. Given the utility of plutonium (IV) oxalate in synthesizing plutonium dioxide, these results are expected to provide value in the fields of nuclear fuel processing, nuclear nonproliferation, and nuclear forensics. (C) 2022 Elsevier B.V. All rights reserved.

Vibrational and luminescence spectroscopy were used in the investigation of U and Pu-bearing compounds and soot from high explosives testing. Key spectroscopic signatures of UF4 and its hydrates, PuO 2 , and soot from different explosives will be presented. The United States Government retains and the publisher, by accepting this article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

Plutonium dioxide (PuO2) is an important compound used in nuclear fuel, irradiation targets, and heat sources. As such, improved understanding of its structural and spectroscopic properties has numerous applications. Alpha particle-induced damage of a PuO2 crystal lattice modifies several properties of its Raman spectrum, including band intensities, positions, and widths. The decay also induces growth of new bands and creates electron-trapped defects with luminescent properties. Herein, we show for the first time that laser-induced heating can reverse damage to an aged and damaged PuO2 lattice. Using automated instrumentation to heat a single 6-10 mu m spot to temperatures above 1,300 degrees C, we show that laser-induced annealing of aged PuO2 results in restoration of T-2g band intensity with a concomitant decrease in defect band intensity, a result that indicates laser annealing can be used to reverse age-damage in PuO2. This laser annealing approach permitted in situ observation of temperature-dependent Raman spectral changes, thereby providing insight into the thermodynamics of structural alterations in a radiolytically damaged PuO2.

Cerium dioxide (CeO2) belongs to the fluorite structure group (O-h(5) point group) resulting in one Raman active T-2g mode and two IR active T-1u TO and LO modes. The T-2g Raman band in CeO2 has been used to probe the symmetric Ce-O vibrational mode of different sized nanoparticulates. Far infrared studies of the Ce-O interactions are difficult to conduct because of low optical transmission, efficiency of the detector, and water absorption bands interfering with the measurements. The selection rules of hyper-Raman spectroscopy (HRS) allow the study of IR bands in the far infrared spectral region, such as the motion of the Ce atoms. For the first time, HRS was used to investigate the T-1u TO and LO IR modes in CeO2. The position and full-width half-maximum (FWHM) of the TO and LO bands in the HRS spectrum were used to investigate the spectral response in relation to particle size (i.e., phonon confinement). The investigation of CeO2 with HRS provides a research platform for future work in the study of ThO2, UO2, and PuO2 IR modes.

In this paper, we use density functional theory (DFT) calculations to investigate the initial surface reactions involved in the atomic layer deposition (ALD) of Al2O3 from H2O and Al(CH3)(3) (trimethylaluminum, TMA) molecular precursors on the Si(001)-(2X1) reconstructed surface with different chemical terminations. Our results for the kinetic barriers and adsorption energies of both Al and oxygen precursors along different reaction pathways show the dependence of the ALD nucleation rate on the surface defects (Si dangling bonds or dimer trench) and how it can be modified with suitable p-doping. Finally, our ab initio thermodynamics study clearly determines the relation between typical ALD working conditions and the different chemical functionalizations of the Si(001) surface with the growth properties of Al2O3 nanofilms.

The selectivity of clean Si(100)-(2 x 1) surfaces fully reacted with H2O- and hydrogen-passivated Si(100)-(2 x 1) surfaces is investigated for atomic layer deposition (ALD) of TiO2, Al2O3, and HfO2 using TiCl4, TMA, or TDMA-Hf precursors with H2O, respectively, in an ultra-high-vacuum (UHV) environment. The initial reaction probability is estimated by determining the minimum exposure necessary for complete reaction of the metal precursors on both H2O-reacted and H-passivated Si(100)-(2 x 1) surfaces and examining the first full cycle of the ALD process for each oxide. Under these UHV conditions, the first cycle selectivity is 17:1 for TiO2, 37:1 for Al2O3, and only 4:3 for HfO2. Additionally, TMA is found to react with approximately half of the Si-H sites in addition to all the Si-OH sites, while TiCl4 and TDMA-Hf gases are found to react principally with the surface -OH on H2O-reacted Si(100) surfaces with no reaction with the -H sites.

The ability to selectively chemically functionalize silicon nitride (Si3N4) or silicon dioxide (SiO2) surfaces after cleaning would open interesting technological applications. In order to achieve this goal, the chemical composition of surfaces needs to be carefully characterized so that target chemical reactions can proceed on only one surface at a time. While wet-chemically cleaned silicon dioxide surfaces have been shown to be terminated with surficial Si-OH sites, chemical composition of the HF-etched silicon nitride surfaces is more controversial. In this work, we removed the native oxide under various aqueous HF-etching conditions and studied the chemical nature of the resulting Si3N4 surfaces using infrared absorption spectroscopy (IRAS), x-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and contact angle measurements. We find that HF-etched silicon nitride surfaces are terminated by surficial Si-F and Si-OH bonds, with slightly subsurface Si-OH, Si-O-Si, and Si-NH2 groups. The concentration of surficial Si-F sites is not dependent on HF concentration, but the distribution of oxygen and Si-NH2 displays a weak dependence. The Si-OH groups of the etched nitride surface are shown to react in a similar manner to the Si-OH sites on SiO2, and therefore no selectivity was found. Chemical selectivity was, however, demonstrated by first reacting the -NH2 groups on the etched nitride surface with aldehyde molecules, which do not react with the Si-OH sites on a SiO2 surface, and then using trichloro-organosilanes for selective reaction only on the SiO2 surface (no reactivity on the aldehyde-terminated Si3N4 surface).

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

Controlled fabrication of nanometer-scale devices such as quantum dots and nanowires requires an understanding of the initial chemisorption mechanisms involved in epitaxial growth. Vapor phase epitaxy can provide controlled deposition when using precursors that are not reactive with the H-terminated surfaces at ambient temperatures. For instance, digermane (Ge2H6) has potential as such a precursor for Ge ALE on Si(100) surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. In situ Fourier transform infrared spectroscopy and first principles calculations reveal that Ge2H6 chemisorbs through a beta-hydride elimination mechanism, forming Ge2H5 and H on both Si(100)-(2 X 1) and Ge(100)-(2 X 1) surfaces, instead of the previously proposed Ge-Ge bond breaking mechanism, and subsequently decomposes into an ad-dimer. The resulting coverage of Ge after a saturation exposure is estimated to be about 0.3 monolayers. Interestingly, the decomposition of adsorbed Ge2H5 on Si(100) is faster than Si2H5 on Ge(100) at 173 K. The desorption temperature of hydrogen on Si(100) is shown to depend on the Ge coverage, falling from 698 K for similar to 1/4 ML Ge on Si(100) to 573 K for a nearly full Ge coverage, consistent with H desorption on Ge(100). Furthermore, hydrogen is observed to migrate from Ge to Si, prior to desorption. This property opens the door for selective growth of Ge on patterned H-terminated Si surfaces.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the masks being used in order to maintain exquisite control over both feature size and feature density. Here, the authors demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top–down atomic control over nanofabrication. First, hydrogen depassivation lithography is performed on hydrogen terminated Si(100) using a scanning tunneling microscope, which spatially defined chemically reactive regions. Next, atomic layer deposition of titanium dioxide produces an etch-resistant hard mask pattern on these regions. Reactive ion etching then transfers the mask pattern onto Si with pattern height of 17 nm, critical dimension of approximately 6 nm, and full-pitch down to 13 nm. The effects of linewidth, template atomic defect density, and line-edge roughness are examined in the context of controlling fabrication with arbitrary feature control, suggesting a possible critical dimension down to 2 nm on 10 nm tall features. A metrology standard is demonstrated, where the atomically resolved mask template is used to determine the size of a nanofabricated sample showing a route to image correction.