Engineering Multilayered Nanocrystal Solids with Enhanced Optical

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Engineering Multilayered Nanocrystal Solids with Enhanced Optical Properties Using Metal Oxides for Photonic Applications Riya Bose, Aaron Dangerfield, Sara M. Rupich, Tianle Guo, Yangzi Zheng, Sunah Kwon, Moon J. Kim, Yuri N Gartstein, Alain Esteve, Yves J. Chabal, and Anton V. Malko ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01577 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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ACS Applied Nano Materials

Engineering Multilayered Nanocrystal Solids with Enhanced Optical Properties Using Metal Oxides for Photonic Applications Riya Bose1†, Aaron Dangerfield2†, Sara M. Rupich2, Tianle Guo1, Yangzi Zheng1, Sunah Kwon2, Moon J. Kim2, Yuri N. Gartstein1, Alain Esteve3, Yves J. Chabal2, and Anton V. Malko1* 1

Department of Physics, 2Department of Materials Science, The University of Texas at Dallas,

Richardson, Texas 75080, USA, and 3University of Toulouse, LAAS-CNRS, 7 avenue du colonel Roche, 31031 Toulouse, France

KEYWORDS. Nanocrystal quantum dots, defect states, surface passivation, atomic layer deposition, pulsed chemical vapor deposition, Al2O3, photoluminescence, energy transfer

ACS Paragon Plus Environment

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ABSTRACT. Managing deposition of multilayered nanocrystal quantum dot (NQD) thin films is crucial for future photonic devices to maximize solar energy extraction efficiency. Solution-based NQD deposition methods require additional protection to achieve a discrete layered structure and to prevent optical degradation during processing. An attractive method to passivate and protect NQD films is overcoating with metal oxides, usually grown using atomic layer deposition (ALD). However, a significant quenching of NQD photoluminescence (PL) is typically observed after encapsulation, hindering performance and applicability. Here, we demonstrate a modified gasphase deposition technique that fully passivates NQD assemblies and, in contrast to standard ALD, maintains PL properties. Combined in-situ FTIR and ex-situ XPS measurements reveal that, upon Al2O3 deposition by ALD, the metal precursor trimethylaluminum (TMA) interacts with oleic acid capped CdSe-CdS-ZnS core-shell NQDs by reorganizing the ligands and replacing Zn atoms with Al. This modification leads to PL quenching, particularly severe at elevated temperatures (~100°C). In contrast, simultaneous exposures of both precursors (TMA and water) lead to metal oxide deposition from gas-phase reactions taking place in the immediate vicinity of the NQD surface, without affecting the chemical nature of the NQD. Contrary to ALD, this technique retains and even improves NQDs’ photoluminescence, observed as increased PL intensity and longer lifetimes.


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INTRODUCTION Semiconductor nanocrystal quantum dots (NQDs) constitute a versatile platform for harvesting solar energy with their size/shape/composition tunable properties and inexpensive solution-based synthesis techniques.1-8 Extensive research has been carried out to design new materials systems as well as improved device architectures for optimal extraction of solar energy, such as assembly of multilayered bandgap graded NQD films to ensure maximum light absorption and optimum efficiency of the devices.2, 9-11 However, progress in NQD solid state devices has been limited by severe issues, e.g., a) poor charge transport properties of NQDs because of the presence of insulating long chain ligands on the surface, and b) the use of solution-phase deposition methods where solvent in the subsequent deposition step dissolves the initial layer, constituting a substantial challenge for fabricating multilayer NQD films. Exchange of the ligands with short chain ones has been attempted to solve both the issues, however, they are found to promote the formation of defect states and to decrease the PLQY.12-13 An alternative approach to avoid ligand exchange is to replace charge transfer based devices with energy transfer ones that rely on dipole-dipole interactions and are not affected by the presence of long chain ligands.4-5, 14-15 However, there is still need for a surface encapsulation method to stabilize each NQD layer prior to the next deposition step,16-18 as well as for precise positioning of the NQD layers for multilayer NQD energy transfer based photonic devices. Furthermore, surface passivation and encapsulation are also important to protect the deposited NQD layers from oxidation and deterioration during longterm use.19-21 A relatively new and attractive method to protect NQD films during subsequent depositions as well as from environmental exposure is to overcoat them with various metal oxides (e.g., Al2O3, ZnO) grown using atomic layer deposition (ALD), in which self-limiting surface reactions of the


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precursors allows for conformal growth of the metal oxide layer with precise thickness control.2231

There are few reports of ALD encapsulation of NQD films, though they mostly attend to charge

transfer-based devices and aim to improve carrier mobilities.22-26 The concern for energy transferbased devices2-4 and photonic applications in general, however, is to maintain a high PLQY that would enable the efficient interconversion between photons and NQD excitons. Although some protection of the NQD layer from the environment has been achieved after encapsulation, all of the studies report a significant quenching of the PL intensity.27-31 A few studies have suggested that the reason behind this quenching is the loss of surface ligands27 or the replacement of NQD cations by metal precursor (Al);29 however, the precise origin of the atomic mechanisms that lead to deterioration of PL emission remains unclear, and requires an extensive investigation of the metal oxide and NQD interfaces. In this study, we combine in-situ FTIR with ex-situ XPS and time-resolved PL measurements to uncover the mechanism for the observed PL reduction of oleic-acid-capped CdSe-CdS-ZnS coreshell nanocrystals during ALD of alumina (AlOx). We find that the interaction of the metal precursor trimethylaluminium (TMA) with the surface of the NQDs leads to the reorganization of ligand binding from Zn to Al and also significant loss of surface Zn atoms, both leading to PL intensity quenching. Lowering the temperature of the metal oxide deposition decreases the extent Zn loss from surface, although a ~ 44% decrease in PL lifetime is still observed for room temperature deposition. The understanding derived from this study of the ALD process made it possible, nonetheless, to devise a deposition method that minimizes change in ligand bonding configuration as well as Zn loss, and therefore PL degradation. Specifically, we demonstrate that a pulsed co-deposition of both metal and oxidant precursors at room temperature (RT) (reminiscent of chemical vapor deposition, CVD) is able to deposit AlOx films, originating from gas-phase


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reactions in the immediate vicinity of the NQD layer. This method is shown to greatly minimize TMA interaction with the NQD film surface and even with the ligands; the net result is a slight increase in PL emission intensity and carrier PL lifetime. These findings indicate that alumina encapsulation is a suitable technique for controlled deposition of multilayered NQD structures with the use of this CVD-like method (henceforth mentioned as CVD in the manuscript) to preserve the optoelectronic properties of the NQD film, i.e. to minimize defect formation, and can serve as the perfect gateway for fabrication of highly efficient energy transfer based NQD devices. RESULTS AND DISCUSSION

100 oC

(a)

CH3D

D2O

(b)

TMA

RT

(c)

(d)


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Figure 1. PL spectra and lifetimes for NQD thin films after alumina deposition by ALD (a-b) at 100o C and (c-d) at RT, respectively, at indicated ALD cycles (cycs). Inset in (b) shows the schematic of alumina deposition by sequential exposure of TMA and D2O on the NQD thin film. Figure 1(a-d) shows the PL spectra and lifetime of the CdSe-CdS-ZnS NQD films before and after alumina deposition at 100o C (top) and at RT (bottom). The NQDs have been synthesized following modified literature methods32-34 and a home built reactor is used for alumina deposition.35 Details of the synthesis, characterization of the NQDs and ALD technique are provided in the Supporting Information (Figure S1). Typically, a thin layer of NQDs is spin-coated on OH-terminated SiO2 followed by ALD deposition of alumina by pulsing TMA and D2O as Al and oxygen precursors, respectively. Note that a 30 nm-thick thermal SiO2 layer is grown on Si wafers prior to substrate cleaning and NQD film deposition to prevent energy transfer from the photoexcited NQDs to the underlying Si substrate.5 A purging time of 20 min between each TMA and D2O pulse ensures complete removal of all unreacted precursors after complete self-limiting reactions of TMA and D2O on the NQD surface. It is observed that only 10 cycles of ALD at 100o C almost completely quench the PL intensity and decrease the PL lifetime by ~70%. ALD performed under identical conditions, but with the substrate at RT affects both the PL intensity and lifetime to a lesser extent; nevertheless after 40 cycles of alumina deposition, the PL intensity at RT is quenched by ~74% and lifetime decreased by ~ 44%, in accordance with several previous reports.30-31


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(a)

(b)

Figure 2. (a) Differential FTIR absorbance spectra comparing the COO- modes for NQD thin film (referenced to SiO2), and after 1 pulse of TMA exposure at RT and at 100o C, respectively (both referenced to NQD thin film). (b-e) XPS spectra of b) Zn in NQD thin film, before and after 1 pulse of TMA at 100° C, (c) Al in NQD thin film, after 1 pulse of TMA at 100o C, (d) Zn in NQD thin film, before and after 1 pulse of TMA at RT, and, (e) Al in NQD thin film, after 1 pulse of TMA at RT. In order to determine if the PL properties of the NQDs are controlled by surface chemical reactions, which are typically temperature dependent (i.e., activated), we combined in-situ FTIR


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and ex-situ XPS measurements during and after alumina deposition as a function of temperature using TMA and water as precursors and focused on the COO- stretch modes of the oleate ligands. Figure 2a shows the differential IR spectra of this COO- region after 1 pulse of TMA on the NQD film at RT and at 100° C. In the 1400-1600 cm-1 region, there is a loss in COO- mode intensity associated with the Zn-bridged oleate ligands at 1579 and 1480 cm–1 and a gain in the oleate ligands coordinated to Al at 1634 and 1509 cm-1. These assignment are based on variations of the peak intensities, with features at 1634 and 1509 cm-1 (Δν = 125 cm-1) being the asymmetric and symmetric COO- stretches, respectively, for oleate ligands bridged between two aluminum atoms, or between one aluminum and one zinc atom (Scheme 1(a-b)).36 The features at 1579 and 1480 cm-1 (Δν = 99 cm-1) are therefore assigned to the asymmetric and symmetric COO- stretch modes, respectively, for bidentate oleate coordinated to aluminum (Scheme 1c).36 R

R

(a) a)

O

O

Al

Al

R

TMA

O

O

Zn

Zn

+TMA

R O

O

b) (b)

O

O

Zn

Al

(c) c)

Al

Scheme 1.

The initial bridging oleate configuration on surface Zn atoms. Oleate ligand

configurations after the 1st pulse of TMA where a) corresponds to bridging coordination between two surface aluminum atoms, b) corresponds to bridging coordination between Zn and Al, and c) corresponds to bidentate coordination to Al.


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Losses in the vibrations associated with Al-CH3 groups and a gain at ~2750 cm-1 [ν(O-D)] after the first exposure of D2O indicate that there is ligand exchange between chemisorbed TMA and D2O without incubation (Figure S2). The slight perturbation of the COO- modes in the region between 1400-1600 cm-1 is likely due to the change in chemical environment followed by exchange of Al-CH3 bonds for Al-OD bonds.37 Similar features with almost identical intensities are observed upon exposure of the NQD film to TMA at RT, which suggests that the change in the ligand bonding from Zn to Al is plausibly one of the reasons for PL quenching; this is however not the only one, since it cannot account for the difference in PL quenching at 100o C and at RT. To characterize chemical modifications of the NQD surface itself, XPS measurements of the NQDs were performed after 1 pulse of TMA at 100o C and RT, respectively (Figure 2(b-e)). A decrease in the Zn to Cd ratio (Cd3d XPS spectra provided in Supporting Information, Figure S3) is observed after ALD, revealing a loss of surface Zn atoms from the ZnS shell upon TMA exposure (presumably removed as a volatile species such as dimethylzinc). The extent of the Zn loss is also less at RT than at 100o C (27% vs 40%), in agreement with the PL temperature dependence, underscoring that this process is also responsible for PL quenching. The replacement of Zn from previously deposited films by ALD precursors is not unprecedented.29, 38-39 We have recently shown that metallic Al deposition on ZnO surfaces leads to reduction of this oxide with formation of AlOx and release of Zn.40 However, the chemistry of TMA with surfaces is less obvious due to the –CH3 ligands. For instance, on CuO surfaces, TMA is responsible for the extraction of oxygen from CuO to form an amorphous interfacial layer that stabilizes the surface, while the–CH3 ligand migrates onto surface Cu atoms but are readily desorbed at moderate temperatures.41


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Figure 3. Main energetic steps along the Zn/Al exchange reaction pathways. Two scenario are proposed. In red, a clean ZnS(110) surface (shown in red square) is exposed to a TMA molecule. In blue, an acetate-modified ZnS surface (in blue square) is exposed to a TMA molecule. The intermediate step is a non-dissociative adsorption of the TMA molecule on both surfaces. In the final step (on the right), Al substitutes a surface Zn atom, as Zn is assumed to be released as dimethylzinc (DMZ). On the clean surface, the remaining methyl group remains bonded to the Al atom (see top right image) while on the functionalized ZnS surface, full dissociation of TMA is observed with CH3 attached to a surface S atom (bottom right images). Zinc atoms are depicted by grey spheres, S atom by yellow spheres, O atoms by red spheres, Al atom by big blue spheres, C by small blue spheres, and H atoms by white spheres.


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We therefore investigate the thermodynamics of Zn/Al exchange reaction potentially occurring at the ZnS surface using DFT calculations. We compare two scenarios for the exchange reaction: (i) TMA on a perfect ZnS(110) surface (i.e. the most stable surface of ZnS; shown in Figure 3, top left box), and (ii) TMA on a ZnS surface functionalized with an acetate ligand (i.e. a model for an oleate ligand; schematized in Figure 3, bottom left box). In both cases, we assume that the exchange reaction is accompanied by the release of dimethylzinc (Zn(CH3)2), leaving a substitutional Al atom and an one adsorbed methyl group on the surface. In scenario (i) for a pure ZnS(110) surface (i.e. devoid of ligands), the exchange reaction is not the dominant process because the most stable configuration is the non-dissociated adsorbed state of TMA, with -0.59 eV energy gain. While the exchange process is slightly exothermic (-0.51 eV), the issue is with the remaining methyl ligand. Initially, it is bonded to the substitutional surface Al atom; thereafter, its migration to S is strongly endothermic (by +2.20 eV), which makes it unfavorable; and migration to Zn is even more endothermic (by 0.2 eV). In scenario (ii) in contrast, we find that acetate ligand acts as a catalyst for the exchange reaction. The overall exchange reaction becomes exothermic (by -1.11 eV). In this specific exchange configuration, the methyl is adsorbed on a surface S atom, away from the Al-acetate ligand location. Note also that adsorption of non-dissociated TMA on the functionalized surface is more favorable (-0.70 eV) than on the ligand-free ZnS surface (-0.59 eV), indicating that the residence time of the TMA precursor is higher nearby surface ligand sites, fostering the exchange reaction, thus supporting Al exchanges with Zn upon TMA exposure of NQDs. Based on these observations, it is clear that minimizing the interaction of TMA with the surface of the NQDs may lead to better preservation of optical properties. To this end, we devised a pulsed vapor-phase deposition method, resembling chemical vapor deposition (CVD), to deposit the metal


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oxide using simultaneous and controlled exposures of both the metal and oxygen precursors, at pressures compatible with long diffusion mean free paths (~10 cm). Under such conditions, there is a high probability to deposit oxide from gas phase reactions taking place in the immediate vicinity of the NQD surface (Details provided in Supporting Information, Figure S4).42 Figure 4(ab) shows the PL spectra and lifetimes of the NQDs before and after alumina encapsulation following this process. In sharp contrast to the ALD process, we now observe a notable enhancement of the PL emission intensity and increase of the PL lifetimes, both indicating much better surface passivation of the NQD films. This enhancement is attributed to the nature of the alumina film, as was noted in the context of Si solar cells. Namely, deposition of Al2O3 increases the efficiency of Si solar cells by lowering the interface trap density and reducing one carrier type at the interface by field effect passivation.43-48 It is worth mentioning that depositing alumina by conventional ALD method on top of the CVD alumina film does not affect the PL intensity or lifetime of the NQDs anymore (Figure S5).


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(a)

(c)

(b)

(d)

TMA + D2O

(e)

Figure 4. (a) PL spectra and (b) PL lifetimes for NQD thin films after 20 cycles of alumina deposition by CVD at RT. Inset in (b) shows the schematic of alumina deposition by simultaneous exposure of TMA and D2O on the NQD thin film. (c) Comparison of COO- modes in FTIR absorbance spectra for NQD thin film (referenced to SiO2), after 1 cycle of CVD and 1 pulse of TMA exposure at RT, respectively, both referenced to NQD thin film. (d-e) XPS spectra of (d) Zn for NQD thin film before and after 1 cycle of CVD and (e) Al after 1 cycle of CVD. Comparison of the FTIR and XPS spectra of this CVD-like gas phase process with the RT ALD process (Figure 4c) clearly reveals significantly less change in ligand coordination (i.e., nearly flat differential FTIR spectrum in the COO- region) and nearly absence of Al to Zn replacement (~2% change). Note that the absolute Zn intensity is lower after this process than with ALD only due to attenuation by the alumina overlayer, as the Zn:Cd ratio changes only negligibly. Furthermore, TEM images and elemental mapping of encapsulated NQD thin films confirm that there is a


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conformal overcoating of alumina layer on the surface of the NQDs for both the processes (Figure 5), with respective thicknesses of ALD alumina and CVD alumina layers of 3-4 nm and 5.5-7.5 nm. This indicates that the pulsed gas-phase technique is a much better choice than standard ALD for surface passivation of NQD thin films for preservation and enhancement of optoelectronic properties. (a)

(b)

(c)

AlOx NQDs Si Si – blue color

5 nm

(d)

5.5~7.5 nm

O (e)

(g)

(h)

Al (f)

Cd

(i)

AlOx NQDs Si 5 nm

(j)

Si – blue color

2 nm

O

(k)

Al

(l)

Cd

Figure 5. (a-f) TEM analysis of NQD thin film encapsulated by ALD of alumina. (a) Crosssectional TEM image, (b) STEM-HAADF image, and (c-f) EDX map of the marked region in b.


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(g-l) TEM analysis of NQD thin film encapsulated by CVD of alumina. (g) Cross-sectional TEM image, h) STEM-HAADF image, and (i-1) EDX map of the marked region in (h). This novel CVD-like method can be further employed in a sequential manner to fabricate multilayered NQD structures of variable compositions and interlayer spacing. As an illustration of such an approach, Figure 6(a-b) displays the PL intensity and lifetimes of a bilayer NQD structure fabricated with passivation of each layer by CVD of alumina. It clearly shows that encapsulation improves the PL properties. For multilayer systems, the application of CVD-grown alumina enhances PL intensity beyond the simple addition of NQD layers (of same NQD concentration as the initial layer), and PL lifetimes continue to increase, suggesting improved surface passivation. This finding is in stark contrast with previous observations, for which the PL properties of multilayer films were inferior to those of individual layers.12,

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The variable and tailorable

interlayer distance provided by alumina can further be used to control the rate of energy transfer between multilayers of different sized NQDs.4 Figure 6(c-d) show the PL spectra and lifetimes of a bilayer sample comprised of two different sized NQDs (emitting at 585 nm and 545 nm, respectively), and the NQD layers being separated by different intermediate alumina layer thickness grown by varying the number of CVD cycles. It is indeed observed that with sequential decrease in the alumina layer thickness, more efficient energy transfer from 545 nm NQDs to 585 nm NQDs is observed, implementing quenching of the emission intensity and decrease in the lifetime of the donor 545 nm NQDs. Nanostructured energy transfer hybrids involving NQDs in conjunction with Si substrate may provide an attractive alternative for charge transfer based p-n junction photovoltaic devices, as excitonic energy transfer and sensitization of Si layer by spectrally tunable quantum dots with high absorption coefficient eliminates the weak absorption factor in indirect bandgap Si, whereas the high carrier mobility Si component can be used for


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charge separation and transport. Also, energy transfer instead of charge transfer in quantum dot media revokes the necessity of high charge carrier transport in NQDs. We have also shown earlier2 that precise placement of each layer of a multilayered NQD structure allows one to quantify the radiative coupling of NQD excitons with the substrate upon addition of discrete layers. We now have an approach that would enable another degree of freedom in positioning individual NQD layers within various hybrid nanostructures and making these assemblies an essential part of the overall photonic environment.

(a)

(b)

(c)

(d)

Figure 6. Sequential (a) PL spectra and (b) PL excited state decay lifetimes for double layered NQD thin films encapsulated by alumina (CVD). Inset in (b) shows the schematic of the double layered NQD thin film encapsulated by alumina (c) PL spectra and (d) PL excited state decay lifetimes for double layered NQD thin films comprised of NQDs emitting at 585 nm and 545 nm, ACS Paragon Plus Environment

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respectively, with different intermediate alumina layer thickness grown between them by varying number of CVD cycles. Inset in (d) shows the schematic of the double layered NQD thin films separated by alumina. In summary, we have demonstrated a novel, CVD-like method for depositing passivation metal oxide layers on optically active NQD thin films, necessary for the fabrication of multilayered structures as well as protection from environmental degradation. Benchmarking this process with ALD of alumina films on CdSe-CdS-ZnS core-shell NQDs, we have shown, with in-situ FTIR and ex-situ XPS measurements, that the new method does not significantly alter the NQD surface ligand bonding configuration or the concentration of Zn atoms in the NQD shell. We have demonstrated that traditional ALD leads to the notable replacement of Zn by Al as well as loss of surface Zn atoms upon the metal precursor exposure, which we correlate to the decrease in PL intensity and reduction of PL lifetime observed in ALD encapsulation of NQDs. Although lowering the temperature of the ALD process reduces the Zn loss to a certain extent, the loss of PL intensity and the decrease of PL lifetime are still impractical for multilayer films. In contrast, the new CVD-like gas-phase process completely preserves and even enhances both the PL intensity and lifetime. Thus, this study serves as an important milestone in understanding and controlling the chemistry at the interface of thin film NQDs and metal oxide layer deposited by ALD/CVD; it also paves the way for controlled assembly of optically active multilayered NQDs for efficient future generation photonic devices.

ASSOCIATED CONTENT


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Supporting Information. Experimental methods, UV-vis absorbance, PL, FTIR spectra of NQDs, additional FTIR spectra for sequential alumina deposition on NQDs, XPS spectra of Cd3d before and after TMA exposure to NQDs thin film, PL and lifetime of NQDs after seqyential deposition of alumina by CVD and ALD, lifetimes of NQDs after various processes of alumina deposition. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions † These authors contributed equally to the work, listed alphabetically. Notes The authors declare no competing financial interest

ACKNOWLEDGMENT The work has been supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0010697. CALMIP is acknowledged for supercomputer resources. REFERENCES (1)

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