Browsing by Author "Barrera, Diego"
Now showing 1 - 4 of 4
- Results Per Page
- Sort Options
Item In Situ Chemical Oxidation of Ultrasmall MoOx Nanoparticles in Suspensions(2012-07-23) Lee, Yun-Ju; Barrera, Diego; Luo, Kaiyuan; Hsu, Julia W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)Nanoparticle suspensions represent a promising route toward low cost, large area solution deposition of functional thin films for applications in energy conversion, flexible electronics, and sensors. However, parameters such size, stoichiometry, and electronic properties must be controlled to achieve best results for the target application. In this report, we demonstrate that such control can be achieved via in situ chemical oxidation of M o O 𝑥 nanoparticles in suspensions. Starting from a microwave-synthesized suspension of ultrasmall ( 𝑑 ∼ 2 nm) M o O 𝑥 nanoparticles in n-butanol, we added H2O2 at room temperature to chemically oxidize the nanoparticles. We systematically varied H2O2 concentration and reaction time and found that they significantly affected oxidation state and work function of MoO𝑥 nanoparticle films. In particular, we achieved a continuous tuning of MoO𝑥 work function from 4.4 to 5.0 eV, corresponding to oxidation of as-synthesized MoO𝑥 nanoparticle (20% Mo6+) to essentially pure MoO3. This was achieved without significantly modifying nanoparticle size or stability. Such precise control of MoO𝑥 stoichiometry and work function is critical for the optimization of MoO𝑥 nanoparticles for applications in organic optoelectronics. Moreover, the simplicity of the chemical oxidation procedure should be applicable for the development of other transition oxide nanomaterials with tunable composition and properties.Item Nucleation and Growth of WSe₂: Enabling Large Grain Transition Metal Dichalcogenides(IOP Publishing Ltd, 2017-09-22) Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Cha, Pil-Ryung; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.; 0000-0002-2910-2938 (Liang, C); Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.The limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).Item Probing Defect States in Organic Polymers and Bulk Heterojunctions Using Surface Photovoltage Spectroscopy(American Chemical Society, 2019-04-10) Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Cao, F. -Y; Tseng, C. -C; Cheng, Y. -J; Hsu, Julia W. P.; 0000-0002-7821-3001 (Hsu, JWP); Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Hsu, Julia W. P.We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC₆₁ BM, or PFBT₂Se₂Th:PC₇₁ BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC₆₁ BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT₂Se₂Th:PC₇₁ BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems. ©2019 American Chemical Society.Item Solution-Processed Oxide Thin Film Transistors on Shape Memory Polymer Enabled by Photochemical Self-Patterning(Cambridge University Press) Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.; Hsu, Julia W. P.; 0000-0003-0135-0531 (Voit, WE); 0000-0002-7821-3001 (Hsu, JWP); Hsu, Julia W. P.; Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.Solution-processed metal oxide electronics on flexible substrates can enable applications from military to health care. Due to limited thermal budgets and mismatched coefficients of thermal expansion between oxides and substrates, achieving good performance in solution-processed oxide films remains a challenge. Additionally, the use of traditional photolithographic processes is incompatible with low-cost, high-throughput roll-to-roll processing. Here, we demonstrate solution-deposited oxide thin film transistors (TFTs) on a shape memory polymer substrate, which offers unique control of final device shape and modulus. The key enabling step is the exposure of the precursor film to UV-ozone through a shadow mask to perform patterning and photochemical conversion simultaneously. These TFTs exhibit mobility up to 160 cm2/(V s), subthreshold swing as low as 110 mV/dec, and threshold voltage between -2 and 0 V, while maintaining compatibility with a flexible form factor at processing temperatures below 250 °C. ©2018 Materials Research Society.