Publications by authors named "Geoff L Brennecka"

4 Publications

  • Page 1 of 1

Synthesis and Surface Chemistry of 2D TiVC Solid-Solution MXenes.

ACS Appl Mater Interfaces 2020 Apr 15;12(17):20129-20137. Epub 2020 Apr 15.

Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401, United States.

MXenes are emerging two-dimensional (2D) materials for energy-storage applications and supercapacitors. Their surface chemistry, which determines critical properties, varies due to different synthesis conditions. In this work, we synthesized TiVC solid-solution MXenes by two different synthesis methods and investigated their surface functional groups. We performed etching of the TiVAlC MAX phase using two different solutions, a highly concentrated HF (50 wt % ≈ 29 M) and a mixture of LiF and HCl (1.9 M LiF/12 M HCl). Large-scale delamination of TiVCT to produce single-flake suspension was achieved by further intercalation of the resultant MXene from LiF/HCl with tetrabutylammonium hydroxide (TBAOH). X-ray diffraction indicates a large interlayer spacing of 2.18 nm for TiVCT MXene flakes. To investigate the structural stability and adsorption energy of different functional groups on TiVC MXenes, density functional theory (DFT) calculations were performed and supported with X-ray photoelectron spectroscopy (XPS) measurements. A higher concentration of ═O and a lower concentration of -F were achieved on the TiVC synthesized by LiF/HCl, both of which provide a more favorable surface chemistry for energy-storage applications. Our results provide the first systematic study on the effect of synthesis conditions on the surface chemistry of solid-solution TiVC MXenes.
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http://dx.doi.org/10.1021/acsami.0c03181DOI Listing
April 2020

In Situ TEM Study of the Amorphous-to-Crystalline Transition during Dielectric Breakdown in TiO Film.

ACS Appl Mater Interfaces 2019 Oct 15;11(43):40726-40733. Epub 2019 Oct 15.

Department of Materials Science and Engineering , Iowa State University , Ames , Iowa 50011 , United States.

Dielectric breakdown of oxides is a main limiting factor for improvement of the performance of electronic devices. Present understanding suggests that defects produced by intense voltage accumulate in the oxide to form a percolation path connecting the two electrodes and trigger the dielectric breakdown. However, reports on directly visualizing the process at nanoscale are very limited. Here, we apply in situ transmission electron microscopy to characterize the structural and compositional changes of amorphous TiO under extreme electric field (∼100 kV/mm) in a Si/TiO/W system. Upon applying voltage pulses, the amorphous TiO gradually transformed into crystalline substoichiometric rutile TiO and the Magnéli phase TiO. The transitions started from the anode/oxide interface under both field polarities. Preferred growth orientation of rutile TiO with respect to the Si substrate was observed when Si was the anode, while oxidation and melting of the W probe occurred when W was the anode. We associate the TiO crystallization process with the electrochemical reduction of TiO, polarity-dependent oxygen migration, and Joule heating. The experimental results are supported by our phase-field modeling. These findings provide direct details of the defect formation process during dielectric breakdown in amorphous oxides and will help the design of electronic devices with higher efficiency and reliability.
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http://dx.doi.org/10.1021/acsami.9b08146DOI Listing
October 2019

COMBIgor: Data-Analysis Package for Combinatorial Materials Science.

ACS Comb Sci 2019 07 7;21(7):537-547. Epub 2019 Jun 7.

National Renewable Energy Laboratory , 15013 Denver West Parkway , Golden , Colorado 80401 , United States.

Combinatorial experiments involve synthesis of sample libraries with lateral composition gradients requiring spatially resolved characterization of structure and properties. Because of the maturation of combinatorial methods and their successful application in many fields, the modern combinatorial laboratory produces diverse and complex data sets requiring advanced analysis and visualization techniques. In order to utilize these large data sets to uncover new knowledge, the combinatorial scientist must engage in data science. For data science tasks, most laboratories adopt common-purpose data management and visualization software. However, processing and cross-correlating data from various measurement tools is no small task for such generic programs. Here we describe COMBIgor, a purpose-built open-source software package written in the commercial Igor Pro environment and designed to offer a systematic approach to loading, storing, processing, and visualizing combinatorial data. It includes (1) methods for loading and storing data sets from combinatorial libraries, (2) routines for streamlined data processing, and (3) data-analysis and -visualization features to construct figures. Most importantly, COMBIgor is designed to be easily customized by a laboratory, group, or individual in order to integrate additional instruments and data-processing algorithms. Utilizing the capabilities of COMBIgor can significantly reduce the burden of data management on the combinatorial scientist.
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http://dx.doi.org/10.1021/acscombsci.9b00077DOI Listing
July 2019

Characterization of Elastic Modulus Across the (AlSc)N System Using DFT and Substrate-Effect-Corrected Nanoindentation.

IEEE Trans Ultrason Ferroelectr Freq Control 2018 11 16;65(11):2167-2175. Epub 2018 Aug 16.

Knowledge of accurate values of elastic modulus of (AlSc)N is required for design of piezoelectric resonators and related devices. Thin films of (AlSc)N across the entire composition space are deposited and characterized. Accuracy of modulus measurements is improved and quantified by removing the influence of substrate effects and by direct comparison of experimental results with density functional theory calculations. The 5%-30% Sc compositional range is of particular interest for piezoelectric applications and is covered at higher compositional resolution here than in previous work. The reduced elastic modulus is found to decrease by as much as 40% with increasing Sc concentration in the wurtzite phase according to both experimental and computational techniques, whereas Sc-rich rocksalt-structured films exhibit little variation in modulus with composition.
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http://dx.doi.org/10.1109/TUFFC.2018.2862240DOI Listing
November 2018
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