Chemical approaches to the improved performance of nanoelectronic devices

Chang, Noel

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Description

Title

Chemical approaches to the improved performance of nanoelectronic devices

Author(s)

Chang, Noel

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Girolami, Gregory S.

Doctoral Committee Chair(s)

Girolami, Gregory S.

Committee Member(s)

• Abelson, John R.
• Lyding, Joseph W.
• Suslick, Kenneth S.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Moore's law
• nanoelectronics
• molecular electronics
• carbon nanotubes
• chemical vapor deposition (CVD)
• lanthanide

Abstract

As the feature size of silicon-based information storage and processing components approach the atomic limit, molecule-based electronic components are being increasingly examined to extend the Moore’s law by utilizing the intrinsic characteristics of single molecules rather than the cooperative properties of bulk materials. Cobalt (II/III) tris(pyrazolyl)borate (CoTp2) species with various boron substituents have been synthesized and examined as candidate components for bistable molecular junctions. Specifically, Co[(pyrazolyl)3BC6H4R]2, where R = Br, CO2H, CH2OH, CHO, thiazolidine, and CH2NH(CH2)2SH were synthesized. Despite being high spin and paramagnetic, the CoTp2 species are robust enough to tolerate a variety of functionalization reactions, and undergo many organic transformations while the metal center and the first coordination sphere remain unaffected. These species undergo one-electron redox reactions to trigger a d-electron reorganization process. The electrochemical characterizations of all the species show slow kinetics in the oxidation process, whereas the reduction remains reversible across all the scan rates used. This result is consistent with the model that the spin reorganization from d6 high spin to low spin in the oxidized Co(III) state possesses a high kinetic barrier, which is advantageous in a bistable molecular junction to prevent unintended switching. A self-assembled monolayer of the thiol-substituted CoTp2 species was deposited on a gold surface. Diffuse-reflectance IR spectroscopy indicated that the molecules remain intact on the surface. A cyclic voltammogram of the surface-modified gold electrode shows a redox process consistent with the presence of a Co(II/III) couple. STM and AFM images of the surfaces were collected on samples prepared by depositing CoTp2 species with various substituents (R= Br, thiazolidine, -CH2NH(CH2)2SH) on gold substrates using both solution and sublimation methods. Both the thiol- and thiazolidine-substituted species showed circular features of about 1-nm in height and 10-nm in diameter. Although randomly oriented CNT networks and polycrystalline graphene are easy to make and robust towards mechanical deformation, both materials suffer from resistive: at the intertube junctions in CNT networks and at grain boundaries in polycrystalline graphene sheets. In a process we have named “nanosoldering,” a conductive material is deposited at the resistive junctions by operating the device in an atmosphere of a chemical precursor. The resistive heating that occurs at the “bad” junctions induces a thermal CVD process. The conductive material that is deposited at these junctions improves the overlap between the two nanotubes and decreases the junction resistance. Two precursors were utilized for this method: CpPd(allyl) to deposit Pd(0) and Hf(BH4)4 to deposit HfB2. In the SEM images post treatment, deposition is observed on many intertube junctions as well as along the lengths of some tubes; the latter can be eliminated by selecting the appropriate experimental conditions. For treatment with the Pd precursor, the ION/IOFF ratio improved by a factor of 6 on average. On the other hand, no improvement was observed for samples treated with the HfB2 precursor due to the large mismatch in work functions between CNTs and HfB2, which creates a Schottky barrier. A solution process was developed to improve the scalability of the nanosoldering process by eliminating the necessity of using a vacuum system and volatile chemical precursors. To do so, a nanosoldering precursor is spin-coated onto the solid substrate. The nanosoldering process is conducted in a commercial available probe station either in air or under vacuum, after which the excess material and byproduct are removed by a solvent rinse. Using this method, we show a comparable degree of device performance improvement using a nonvolatile Pd precursor, Pd2(dba)3. We also obtained improved device performance with a new carbon-based precursor, 1,3,5-tris(2-bromophenyl)benzene. Finally, preliminary experiments were conducted on graphene devices, which showed that nanosoldering occurred along the grain boundaries without significant device degradation. The third portion of this thesis examines the design and synthesis of new lanthanide CVD precursors. Our group has synthesized a series of lanthanide tris(N,N-dimethylaminodiboranate) (Ln(dmadb)3) complexes and demonstrated their utilization as Ln2O3 CVD precursors. The mechanism of the salt elimination reaction that generates Nd(dmadb)3 was examined by 11B NMR spectroscopy. We found that the treatment of NdCl3 with three equivalents of Na(dmadb) in thf does not directly afford the tri-substituted product. Instead, a partial substitution takes place to make Nd(dmadb)Cl2(thf)x in solution; the second and third substitutions do not take place until the thf solvent is removed from the reaction mixture. The mechanistic study demonstrates that, contrary to what is normally assumed, the salt metathesis reaction is not always driven by the precipitation of the byproduct salt. In the present case, the monosubstituted Nd(dmadb)Cl2(thf)x is the thermodynamic product of the initial reaction, and the lattice energy of NaCl is not large enough to drive the subsequent substitution reactions. The Lewis basicity of the solvent plays a major role in the thermodynamics: thf can coordinate to the Lewis acidic lanthanide center and negate the energetic advantage of the coordination of the aminodiboranate ligand. With the aid of this insight, we were able to synthesize the carbon-free target species Nd(adb)3 by employing the less coordinating solvent diethyl ether. Finally, new volatile tris(2,2,6,6-tetramethylpiperidide)lanthanide complexes (Ln(TMP)3, Ln = Er, Tb) were synthesized by treatment with LnCl3 with three equivalents of Li(TMP) in toluene. The anionic TMP ligand is strongly basic and reacts with diethyl ether and tetrahydrofuran, so that the syntheses of the target complexes is not successful in these solvents,. Attempts to synthesize Dy(TMP)3 by an analogous reaction does not afford a pentane soluble product. When the reaction products were extracted with diethyl ether, the dinuclear ethoxy complex [Dy(TMP)2(μ-OEt)]2 was obtained instead. Homoleptic Er(TMP)3 and Tb(TMP)3 both have trigonal pyramidal geometries, unlike previously reported Ln(TMP)3 species (Ln = Y, La, Ce), which were reported to have trigonal planar geometries. Closer inspection of the previous crystallographic refinements suggests that these structures should have been refined with pyramidal models as well. Both Er(TMP)3 and Tb(TMP)3 are volatile and sublime at 110 °C at 0.1 Torr. When the crystalline samples are heated, a “popping” motion is observed that could either be due to a thermosalience transition or to an N-dealkylation process.

Graduation Semester

2014-08

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Copyright 2014 Noel N. Chang

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