Stable Subloop Behavior in Ferroelectric Si-Doped HfO2.

The recent demand for analogue devices for neuromorphic applications requires modulation of multiple nonvolatile states. Ferroelectricity with multiple polarization states enables neuromorphic applications with various architectures. However, deterministic control of ferroelectric polarization states with conventional ferroelectric materials has been met with accessibility issues. Here, we report unprecedented stable accessibility with robust stability of multiple polarization states in ferroelectric HfO2. Through the combination of conventional voltage measurements, hysteresis temperature dependence analysis, piezoelectric force microscopy, first-principles calculations, and Monte Carlo simulations, we suggest that the unprecedented stability of intermediate states in ferroelectric HfO2 is due to the small critical volume size for nucleation and the large activation energy for ferroelectric dipole flipping. This work demonstrates the potential of ferroelectric HfO2 for analogue device applications enabling neuromorphic computing.

Ferroelectric Polarization-Switching Dynamics and Wake-Up Effect in Si-Doped HfO2.

The ferroelectricity in ultrathin HfO2 offers a viable alternative to ferroelectric memory. A reliable switching behavior is required for commercial applications; however, many intriguing features of this material have not been resolved. Herein, we report an increase in the remnant polarization after electric field cycling, known as the "wake-up" effect, in terms of the change in the polarization-switching dynamics of a Si-doped HfO2 thin film. Compared with a pristine specimen, the Si-doped HfO2 thin film exhibited a partial increase in polarization after a finite number of ferroelectric switching behaviors. The polarization-switching behavior was analyzed using the nucleation-limited switching model characterized by a Lorentzian distribution of logarithmic domain-switching times. The polarization switching was simulated using the Monte Carlo method with respect to the effect of defects. Comparing the experimental results with the simulations revealed that the wake-up effect in the HfO2 thin film is accompanied by the suppression of disorder.

Expanding the Chemistry of Molecular U(2+) Complexes: Synthesis, Characterization, and Reactivity of the {[C5 H3 (SiMe3 )2 ]3 U}(-) Anion.

The synthesis of new molecular complexes of U(2+) has been pursued to make comparisons in structure, physical properties, and reactivity with the first U(2+) complex, [K(2.2.2-cryptand)][Cp'3 U], 1 (Cp'=C5 H4 SiMe3 ). Reduction of Cp''3 U [Cp''=C5 H3 (SiMe3 )2 ] with KC8 in the presence of 2.2.2-cryptand or 18-crown-6 generates [K(2.2.2-cryptand)][Cp''3 U], 2-K(crypt), or [K(18-crown-6)(THF)2 ][Cp''3 U], 2-K(18c6), respectively. The UV/Vis spectra of 2-K and 1 are similar, and they are much more intense than those of U(3+) analogues. Variable temperature magnetic susceptibility data for 1 and 2-K(crypt) reveal lower room temperature χM T values relative to the experimental values for the 5f(3) U(3+) precursors. Stability studies monitored by UV/Vis spectroscopy show that 2-K(crypt) and 2-K(18c6) have t1/2 values of 20 and 15 h at room temperature, respectively, vs. 1.5 h for 1. Complex 2-K(18c6) reacts with H2 or PhSiH3 to form the uranium hydride, [K(18-crown-6)(THF)2 ][Cp''3 UH], 3. Complexes 1 and 2-K(18c6) both reduce cyclooctatetraene to form uranocene, (C8 H8 )2 U, as well as the U(3+) byproducts [K(2.2.2-cryptand)][Cp'4 U], 4, and Cp''3 U, respectively.

Synthesis, Structure, and Reactivity of the Ethyl Yttrium Metallocene, (C5Me5)2Y(CH2CH3), Including Activation of Methane.

(C5Me5)2Y(µ-Ph)2BPh2, 1, reacted with ethyllithium at -15 °C to make (C5Me5)2Y(CH2CH3), 2, which is thermally unstable at room temperature and formed the C-H bond activation product, (C5Me5)2Y(µ-H)(µ-η(1):η(5)-CH2C5Me4)Y(C5Me5), 3, containing a metalated (C5Me5)(1-) ligand. Spectroscopic evidence for 2 was obtained at low temperature, and trapping experiments with (i)PrNCN(i)Pr and CO2 gave the Y-CH2CH3 insertion products, (C5Me5)2Y[(i)PrNC(Et)N(i)Pr-κ(2)N,N'], 4, and [(C5Me5)2Y(µ-O2CEt)]2, 5. Although 2 is highly reactive, low temperature isolation methods allowed the isolation of single crystals which revealed an 82.6(2)° Y-CH2-CH3 bond angle consistent with an agostic structure in the solid state. Complex 2 reacted with benzene and toluene to make (C5Me5)2YPh, 7, and (C5Me5)2YCH2Ph, 8, respectively. The reaction of 2 with [(C5Me5)2YCl]2 formed (C5Me5)2Y(µ-Cl)(µ-η(1):η(5)-CH2C5Me4)Y(C5Me5) in which a (C5Me5)(1-) ligand was metalated. C-H bond activation also occurred with methane which reacted with 2 to make [(C5Me5)2YMe]2, 9.

Structural, spectroscopic, and theoretical comparison of traditional vs recently discovered Ln(2+) ions in the [K(2.2.2-cryptand)][(C5H4SiMe3)3Ln] complexes: the variable nature of Dy(2+) and Nd(2+).

The Ln(3+) and Ln(2+) complexes, Cp'3Ln, 1, (Cp' = C5H4SiMe3) and [K(2.2.2-cryptand)][Cp'3Ln], 2, respectively, have been synthesized for the six lanthanides traditionally known in +2 oxidation states, i.e., Ln = Eu, Yb, Sm, Tm, Dy, and Nd, to allow direct structural and spectroscopic comparison with the recently discovered Ln(2+) ions of Ln = Pr, Gd, Tb, Ho, Y, Er, and Lu in 2. 2-La and 2-Ce were also prepared to allow the first comparison of all the lanthanides in the same coordination environment in both +2 and +3 oxidation states. 2-La and 2-Ce show the same unusual structural feature of the recently discovered +2 complexes, that the Ln-(Cp' ring centroid) distances are only about 0.03 Å longer than in the +3 analogs, 1. The Eu, Yb, Sm, Tm, Dy, and Nd complexes were expected to show much larger differences, but this was observed for only four of these traditional six lanthanides. 2-Dy and 2-Nd are like the new nine ions in this tris(cyclopentadienyl) coordination geometry. A DFT-based model explains the results and shows that a 4f (n)5d(1) electron configuration is appropriate not only for the nine recently discovered Ln(2+) ions in 2 but also for Dy(2+) and Nd(2+), which traditionally have 4f (n+1) electron configurations like Eu(2+), Yb(2+), Sm(2+), and Tm(2+). These results indicate that the ground state of a lanthanide ion in a molecule can be changed by the ligand set, a previously unknown option with these metals due to the limited radial extension of the 4f orbitals.

Identification of the +2 oxidation state for uranium in a crystalline molecular complex, [K(2.2.2-cryptand)][(C5H4SiMe3)3U].

Flash reduction of Cp'3U (Cp' = C5H4SiMe3) in a column of potassium graphite in the presence of 2.2.2-cryptand generates crystalline [K(2.2.2-cryptand)][Cp'3U], the first isolable molecular U(2+) complex. To ensure that this was not the U(3+) hydride, [K(2.2.2-cryptand)][Cp'3UH], which could be crystallographically similar, the hydride complex was synthesized by addition of KH to Cp'3U and by reduction of H2 by the U(2+) complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f(3)6d(1) quintet ground state for the [Cp'3U](-) anion and match the observed strong transitions in its optical spectrum.

Completing the series of +2 ions for the lanthanide elements: synthesis of molecular complexes of Pr2+, Gd2+, Tb2+, and Lu2+.

The first examples of crystallographically characterizable complexes of Tb(2+), Pr(2+), Gd(2+), and Lu(2+) have been isolated, which demonstrate that Ln(2+) ions are accessible in soluble molecules for all of the lanthanides except radioactive promethium. The first molecular Tb(2+) complexes have been obtained from the reaction of Cp'3Ln (Cp' = C5H4SiMe3, Ln = rare earth) with potassium in the presence of 18-crown-6 in Et2O at -35 °C under argon: [(18-crown-6)K][Cp'3Tb], {[(18-crown-6)K][Cp'3Tb]}n, and {[K(18-crown-6)]2(µ-Cp')}{Cp'3Tb}. The first complex is analogous to previously isolated Y(2+), Ho(2+), and Er(2+) complexes, the second complex shows an isomeric structural form of these Ln(2+) complexes, and the third complex shows that [(18-crown-6)K](1+) alone is not the only cation that will stabilize these reactive Ln(2+) species, a result that led to further exploration of cation variants. With 2.2.2-cryptand in place of 18-crown-6 in the Cp'3Ln/K reaction, a more stable complex of Tb(2+) was produced as well as more stable Y(2+), Ho(2+), and Er(2+) analogs: [K(2.2.2-cryptand)][Cp'3Ln]. Exploration of this 2.2.2-cryptand-based reaction with the remaining lanthanides for which Ln(2+) had not been observed in molecular species provided crystalline Pr(2+), Gd(2+), and Lu(2+) complexes. These Ln(2+) complexes, [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, Pr, Gd, Tb, Ho, Er, Lu), all have similar UV-vis spectra and exhibit Ln-C(Cp') bond distances that are ~0.03 Å longer than those in the Ln(3+) precursors, Cp'3Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4f(n)5d(1) description of the electron configuration in these Ln(2+) ions is more appropriate than 4f(n+1).

Expanding rare-earth oxidation state chemistry to molecular complexes of holmium(II) and erbium(II).

The first molecular complexes of holmium and erbium in the +2 oxidation state have been generated by reducing Cp'(3)Ln [Cp' = C(5)H(4)SiMe(3); Ln = Ho (1), Er (2)] with KC(8) in the presence of 18-crown-6 in Et(2)O at -35 °C under argon. Purification and crystallization below -35 °C gave isomorphous [(18-crown-6)K][Cp'(3)Ln] [Ln = Ho (3), Er (4)]. The three Cp' ring centroids define a trigonal-planar geometry around each metal ion that is not perturbed by the location of the potassium crown cation near one ring with K-C(Cp') distances of 3.053(8)-3.078(2) Å. The metrical parameters of the three rings are indistinguishable within the error limits. In contrast to Ln(2+) complexes of Eu, Yb, Sm, Tm, Dy, and Nd, 3 and 4 have average Ln-(Cp' ring centroid) distances only 0.029 and 0.021 Å longer than those of the Ln(3+) analogues 1 and 2, a result similar to that previously reported for the 4d(1) Y(2+) complex [(18-crown-6)K][Cp'(3)Y] (5) and the 5d(1) La(2+) complex [K(18-crown-6)(Et(2)O)][Cp″(3)La] [Cp″ = 1,3-(Me(3)Si)(2)C(5)H(3)]. Surprisingly, the UV-vis spectra of 3 and 4 are also very similar to that of 5 with two broad absorptions in the visible region, suggesting that 3-5 have similar electron configurations. Density functional theory calculations on the Ho(2+) and Er(2+) species yielded HOMOs that are largely 5d(z(2)) in character and supportive of 4f(10)5d(1) and 4f(11)5d(1) ground-state configurations, respectively.

Synthesis of a crystalline molecular complex of Y2+, [(18-crown-6)K][(C5H4SiMe3)3Y].

The La(2+) complex [K(18-crown-6)(OEt(2))][Cp″(3)La] (1) [Cp″ = C(5)H(3)(SiMe(3))(2)-1,3], can be synthesized under N(2), but in the presence of KC(5)Me(5), 1 reduces N(2) to the (N═N)(2-) product [(C(5)Me(5))(2)(THF)La](2)(µ-η(2):η(2)-N(2)). This suggests a dichotomy in terms of ligands that optimize isolation of reduced dinitrogen complexes versus isolation of divalent complexes of the rare earths. To determine whether the first crystalline molecular Y(2+) complex could be isolated using this logic, Cp'(3)Y (2) (Cp' = C(5)H(4)SiMe(3)) was synthesized from YCl(3) and KCp' and reduced with KC(8) in the presence of 18-crown-6 in Et(2)O at -45 °C under argon. EPR evidence was consistent with Y(2+) and crystallization provided the first structurally characterizable molecular Y(2+) complex, dark-maroon-purple [(18-crown-6)K][Cp'(3)Y] (3).

Coordination and reductive chemistry of tetraphenylborate complexes of trivalent rare earth metallocene cations, [(C5Me5)2Ln][(µ-Ph)2BPh2].

The reactivity of the tetraphenylborate salts of the rare earth metallocene cations [(C(5)Me(5))(2)Ln][(µ-Ph)(2)BPh(2)] (Ln = Y, 1; Sm, 2) has been investigated with substrates that undergo reduction with f element complexes to probe metal-substrate interactions prior to reduction. Results with NaN(3), 1-adamantyl azide, acetone, benzophenone, phenanthroline, pyridine, azobenzene, and phenazine are described. Not only were coordination complexes isolated, but substrate reduction by (BPh(4))(-) was also observed. Complex 1 reacts with NaN(3) to form the azide [(C(5)Me(5))(2)YN(3)](x), 3, which crystallizes as [(C(5)Me(5))(2)Y(µ-N(3))](3), 4, when obtained from 1 and 1-adamantyl azide. The samarium analogue [(C(5)Me(5))(2)SmN(3)](x), 5, can be produced similarly from 2 and NaN(3) and crystallized from MeCN as [(C(5)Me(5))(2)Sm(NCMe)(µ-N(3))](3), 6, and {[(C(5)Me(5))(2)Sm(µ-N(3))][(C(5)Me(5))(2)Sm(NCMe)(µ-N(3))]}(n), 7. Complexes 1 and 2 react with stoichiometric amounts of acetone and benzophenone to form the ketone adducts [(C(5)Me(5))(2)Ln(OCMe(2))(2)][BPh(4)] (Ln = Y, 8; Sm, 9) and [(C(5)Me(5))(2)Ln(OCPh(2))(2)][BPh(4)] (Ln = Y, 10; Sm, 11), respectively. Phenanthroline (phen) coordinates to 1 to form [(C(5)Me(5))(2)Y(phen)][BPh(4)], 12. Complexes 1 and 2 react with pyridine (py) to form [(C(5)Me(5))(2)Ln(py)(2)][BPh(4)], (Ln = Y, 13; Sm, 14). Complexes 3, 8, 10, and 12 can also be made from the solvated cation [(C(5)Me(5))(2)Y(THF)(2)][BPh(4)]. The reaction of 1 with PhNNPh yields the diamagnetic adduct [(C(5)Me(5))(2)Y(PhNNPh)][BPh(4)], 15, which transforms in benzene to the radical anion complex (C(5)Me(5))(2)Y(PhNNPh), 16, via a one electron reduction by (BPh(4))(-). Complex 1 similarly reacts with phenazine (phz) to produce the first rare earth phenazine radical anion complex {[(C(5)Me(5))(2)Y](2)(phz)}{BPh(4)}, 17. Further reduction of phenazine by (BPh(4))(-) in 17 yields [(C(5)Me(5))(2)Y](2)(phz), 18, which contains the common (phz)(2-) dianion. The reduction of fluorenone by (BPh(4))(-) is also reported.