金属氧化物与现代微电子学:过渡金属前体化合物及转化为材料的化学过程

金属氧化物薄膜如HfO₂（被称为高k电介质）是现代微电子器件的关键组件,广泛用于计算机（平板电脑,笔记本电脑和台式机）、智能电话、智能电视、汽车和医疗设备中。具有大介电常数（k）的金属氧化物已经取代了介电常数小的SiO₂（k=3.9）,从而使得微电子元件进一步小型化。过渡金属化合物在化学气相沉积（CVD）和原子层沉积（ALD）中被广泛用作前体,通过与O₂、H₂O或O₃的反应生成金属氧化物薄膜。微电子金属氧化物膜是纳米材料最广泛应用的一个领域。本文概观该领域的最新进展,包括我们对d⁰过渡金属配合物与O₂反应的研究。     

金属氧化物与现代微电子学——过渡金属前体化合物及转化为材料的化学过程（英文）

金属氧化物薄膜如HfO_2(被称为高k电介质)是现代微电子器件的关键组件,广泛用于计算机(平板电脑,笔记本电脑和台式机)、智能电话、智能电视、汽车和医疗设备中。具有大介电常数(k)的金属氧化物已经取代了介电常数小的SiO_2(k=3.9),从而使得微电子元件进一步小型化。过渡金属化合物在化学气相沉积(CVD)和原子层沉积(ALD)中被广泛用作前体,通过与O2、H_2O或O_3的反应生成金属氧化物薄膜。微电子金属氧化物膜是纳米材料最广泛应用的一个领域。本文概观该领域的最新进展,包括我们对d0过渡金属配合物与O_2反应的研究。     

Criegee自由基CH2O2与H2O反应机理及动力学的理论研究

在6-311＋G(2d,2p)水平下, 采用密度泛函理论(DFT)的B3LYP方法, 研究了Criegee 自由基CH₂O₂与H₂O的反应. 结果表明反应存在三个通道: CH₂O₂＋H₂O®HOCH₂OOH (R1); CH₂O₂＋H₂O®HCO＋OH＋H₂O (R2); CH₂O₂＋H₂O®HCHO＋H₂O₂ (R3), 各通道的势垒高度分别为43.35, 85.30和125.85 kJ/mol. 298 K下主反应通道(R1)的经典过渡态理论(TST)与变分过渡态理论(CVT)的速率常数k^TST与k^CVT均为2.47×10^－17 cm³•molecule^－1•s^－1, 而经小曲率隧道效应模型(SCT)校正后的速率常数k^CVT/SCT为 5.22×10^－17 cm³•molecule^－1•s^－1. 另外, 还给出了200～2000 K 温度范围内拟合得到的速率常数随温度变化的三参数Arrhenius方程.     

室温下制备以{211}晶面为主的Ti1-xVxO2薄膜及其可见光催化性能

以金属Ti和V作为靶材,采用直流反应共溅射技术在室温下制备了以{211}晶面为主的锐钛矿相Ti_1-xV_xO₂薄膜,研究了不同V靶功率对Ti_1-xV_xO₂薄膜的薄膜成分、晶相结构和可见光催化性能的影响。研究表明,Ti_1-xV_xO₂薄膜的晶相结构为锐钛矿相,择优取向为(211),而结晶度受V靶功率的影响。随着V靶功率的增加,薄膜中V元素含量逐渐增加,同时,晶粒和沉积速率也逐渐增加。另外,当V靶功率为150W时,薄膜的表面粗糙度值有一个最大值。V的掺杂导致薄膜的能带间隙变窄,对光的吸收向可见光区偏移,从而有效地改善了薄膜的可见光催化能力。当V靶功率为150W时,Ti_1-xV_xO₂薄膜的能带间隙值为2.82eV,其在2h的可见光照射下分解了80%的RhB染料。这被归结于能带间隙窄,高能晶面{211}和结晶度高的共同作用。     

室温下制备以{211}晶面为主的Ti1-xVxO2薄膜及其可见光催化性能

以金属Ti和V作为靶材,采用直流反应共溅射技术在室温下制备了以{211}晶面为主的锐钛矿相Ti_1-xV_xO₂薄膜,研究了不同V靶功率对Ti_1-xV_xO₂薄膜的薄膜成分、晶相结构和可见光催化性能的影响。研究表明,Ti_1-xV_xO₂薄膜的晶相结构为锐钛矿相,择优取向为(211),而结晶度受V靶功率的影响。随着V靶功率的增加,薄膜中V元素含量逐渐增加,同时,晶粒和沉积速率也逐渐增加。另外,当V靶功率为150 W时,薄膜的表面粗糙度值有一个最大值。V的掺杂导致薄膜的能带间隙变窄,对光的吸收向可见光区偏移,从而有效地改善了薄膜的可见光催化能力。当V靶功率为150 W时,Ti_1-xV_xO₂薄膜的能带间隙值为 2.82 eV,其在2 h的可见光照射下分解了80%的RhB染料。这被归结于能带间隙窄,高能晶面{211}和结晶度高的共同作用。     

CH3S自由基H迁移异构化及脱H2反应的直接动力学研究

采用密度泛函方法(MPW1PW91)在6-311G(d,p)基组水平上研究了CH₃S自由基H迁移反应CH₃S→CH₂SH (R1), 脱H₂反应CH₃S→HCS＋H₂ (R2)以及脱H₂产物HCS异构化反应HCS→CSH (R3)的微观动力学机理. 在QCISD(t)/6- 311＋＋G(d,p)//MPW1PW91/6-311G(d,p)＋ZPE水平上进行了单点能校正. 利用经典过渡态理论(TST)与变分过渡态理论(CVT)分别计算了各反应在200～2000 K温度区间内的速率常数k^TST和k^CVT, 同时获得了经小曲率隧道效应模型(SCT)校正后的速率常数k^CVT/SCT. 结果表明, 反应 R1, R2 和R3的势垒△E^≠分别为160.69, 266.61和241.63 kJ/mol, R1为反应的主通道. 低温下CH₃S比CH₂SH稳定, 高温时CH₂SH比CH₃S更稳定. 另外, 速率常数计算结果显示, 量子力学隧道效应在低温段对速率常数的计算有显著影响, 而变分效应在计算温度段内对速率常数的影响可以忽略.     

d^0过渡金属化合物对氧气反应性：合成和反应机理研究

众所周知,过渡金属如卟啉中的铁与氧气的结合和反应对许多生物功能和催化氧化至关重要．在这些反应中,过渡金属一般含d价电子,并且金属被氧化往往是其中一个重要的反应步骤．近年来,氧气与d^0过渡金属化合物如Hf（NR2）4（R=烷基）的反应被广泛用来制备金属氧化物薄膜以作为新型微电子器件中的栅（门）绝缘材料．这篇专题文章讨论我们近期对这些反应以及TiO2薄膜形成的研究．在许多氧气与d^0过渡金属化合物的反应中,总是金属被氧化．然而,在d^0过渡金属化合物如Hf（NMe2）4和Ta（NMe2）4（SiR3）与氧气的反应中通常是配体被氧化．如-NMe2和--SIR3配体分别形成了-0NMe2和--OSiR3配体．反应机理和理论方面的研究显示了微电子金属氧化物薄膜形成的途径．     

多金属氧酸盐修饰的过渡金属配合物的催化效果和电化学行为

基于水热技术,合成了多金属氧酸盐修饰的2种新的过渡金属配合物[Cu（dmbipy）₂（SiW₁₂O₄₀）][Cu（dmbipy）（H₂O）₃]·3H₂O（1）和{（Hdmphen）₂[Ag₂（dmphen）₂（SiW₁₂O₄₀）]}_n（2）（dmbipy=4,4＇-二甲基-2,2＇-联吡啶, dmphen=2,9-二甲基-1,10-菲咯啉）。配合物1的最小不对称单元包含2种不同的Cu（Ⅱ）单元,其中Cu（Ⅱ）离子都采用了五配位的模式,并且水分子也参与了配位。氢键将不同的Cu（Ⅱ）单元连接形成了一维链,并延伸至二维层。在配合物2中,多金属氧酸盐连接Ag（Ⅱ）离子形成一维链。氢键和π…π堆积作用将一维链拓展成了二维层。     

Cr(III)-取代磷钨杂多配合物对4-甲基吡啶的电催化氧化作用

利用Cr(III)-取代磷钨杂多配合物PW₁₁O₃₉Cr^III(H₂O)^4－的内球电子转移特性, 通过与反应活性中心Cr^III(H₂O)第六配位水分子的交换反应, 将4-甲基吡啶分子络合修饰到该活性中心上进行阳极催化氧化. 可见吸收光谱证实4-甲基吡啶和Cr^III(H₂O)中心进行配体交换反应生成PW₁₁O₃₉Cr^III(NC₆H₇)^4－; 而循环伏安和恒电位电解实验结果表明, 修饰在Cr(III)活性中心上的4-甲基吡啶分子每一步都经历2电子氧化, 依次生成吡啶-4-甲醇, 吡啶-4-甲醛和吡啶-4-甲酸. 由此提出了一个关于这类反应的分子内电催化模板机制, 为过渡金属取代杂多配合物作为间接氧化电催化剂的应用开辟了一条新途径.     

新型反相微乳液制备纳米结构甲烷催化燃烧催化剂La0.95Ba0.05MnAl11O19-α的研究

提出一种由水, 异丙醇, 正丁醇组成的新型微乳液, 并以其为反应介质制备了甲烷高温燃烧La_0.95Ba_0.05MnAl₁₁O_19-a催化剂. 采用¹H NMR, FT-IR, 电导法及激光粒度散射法研究了新型微乳液中水的结构及相特性. 新型微乳液中水的体积分数小于一定值时, 电导率与水含量成非线性关系, k～φ曲线上存在一临界值(φ^P＝0.15). 水质子的化学位移随水含量的降低移向高场. 加入4% D₂O测定的O—D键的伸缩振动随水含量增加而向高波数方向移动. 异丙醇铝在新型微乳液中水解形成的Al(OH)₃ 胶体粒子的粒径范围为226～329 nm. 采用新型微乳液作为反应介质制备的Ba_0.05La_0.95MnAl₁₁O_19-α催化剂的粒径在30 nm, 明显小于纯水制备的样品(100 nm). BET 比表面积为65 m²/g, 比纯水制备样品高出约一倍. XRD结果显示, 1200 ℃焙烧10 h即可获得含单一β-Al₂O₃相的催化剂. Ba_0.05La_0.95MnAl₁₁O_19-α催化剂甲烷催化燃烧的T₁₀为420 ℃, 比纯水制备样品下降了90 ℃. 甲烷催化燃烧活性提高是由于含有较多Mnⁿ+纳米结构六铝酸盐的形成.     

Reactivities of d<Superscript>0</Superscript> transition metal complexes toward oxygen: Synthetic and mechanistic studies

Transition metals such as Fe in porphyrin complexes are known to bind or react with O₂, and such reactions are critical to many biological functions and catalytic oxidation using O₂. The transition metals in these reactions often contain valence d electrons, and oxidation of metals is an important step. In recent years, reactions of O₂ with d⁰ transition metal complexes such as Hf(NR₂)₄ (R = alkyl) have been used to make metal oxide thin films as insulating gate materials in new microelectronic devices. This feature article discusses our recent studies of such reactions and the formation of TiO₂ thin films. In contrast to the reactions of many d ⁿ complexes where metals are often oxidized, reactions of d⁰ complexes such as Hf(NMe₂)₄ and Ta(NMe₂)₄(SiR₃) with O₂ usually lead to the oxidation of ligands, forming, e.g., -ONMe₂ and -OSiR₃ from -NMe₂ and -SiR₃ ligands, respectively. Mechanistic and theoretical studies of these reactions have revealed pathways in the formation of the metal oxide thin films as microelectronic materials.     

Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

Phenomenological kinetics of irreversible electrochemical dissolution of metal-oxide microparticles

The electrochemical dissolution of metal oxides and other stable solid phases composed of nano- to micro-crystalline particles is generally a complex process. It can be simplified by distinguishing two main contributions to the reactivity of the solid: the potential-dependent rate coefficient k(E), and the conversion-dependent function f(y). These contributions can be evaluated by a combination of potentiostatic and potentiodynamic experiments. Both k(E) and f(y) were obtained experimentally for the dissolution of iron and chromium oxides, and theoretical consequences of their particular forms are discussed. A peak-shaped function k(E) was observed in the case of γ-Fe₂O₃, whereas, for α-Cr₂O₃ and CrO₂, a different model based on intermediate surface complexes is proposed. This model also explains the complete electrochemical dissolution of metal oxides regardless of their low intrinsic electric conductivity. Received: 2 July 1997 / Accepted: 3 November 1997     

Composition-property relationships in high-κ LaxZr1-xOy thin films from aqueous solution

Miniaturization of microelectronic devices has reached a fundamental scaling limit; parasitic electron tunneling through the ultrathin gate dielectric has become a major obstacle to continued device performance. One method for overcoming this limitation is to replace SiO₂ gate dielectrics with thicker high-κ metal oxides. La₂O₃ and ZrO₂ are two such materials that have received significant interest, but low stability to post-anneal water absorption and low-crystallization temperatures, respectively, have limited their widespread use. We recently reported an aqueous, all-inorganic route to high-κ lanthanum zirconium oxide dielectric films (1/1 La/Zr), which mitigates the disadvantages of the binary oxides but maintains their high-κ properties. In this contribution, we vary the La/Zr ratio of the aqueous precursor to optimize the properties of the resulting films. We find that the La_0.20Zr_0.80O_y composition is optimal for providing a high dielectric constant (∼18.2 at 600 °C) while maintaining excellent film morphology and stability. 20% La was necessary to prevent crystallization up to 600 °C, but films with higher La content displayed diminished dielectric constants and decreased stability towards post-anneal water absorption.     

Correlating the Surface Basicity of Metal Oxides with Photocatalytic Hydroxylation of Boronic Acids to Alcohols

Photoredox catalysis provides opportunities in harnessing clean and green resources such as sunlight and O₂, while the acid and base surface sites of metal oxides are critical for industrial catalysis such as oil cracking. The contribution of metal oxide surfaces towards photocatalytic aerobic reactions was elucidated, as demonstrated through the hydroxylation of boronic acids to alcohols. The strength and proximity of the surface base sites appeared to be two key factors in driving the reaction; basic and amphoteric oxides such as MgO, TiO₂, ZnO, and Al₂O₃ enabled high alcohol yields, while acidic oxides such as SiO₂ and B₂O₃ gave only low yields. The reaction is tunable to different irradiation sources by merely selecting photosensitizers of compatible excitation wavelengths. Such surface complexation mechanisms between reactants and earth abundant materials can be effectively utilized to achieve a wider range of photoredox reactions.     

A secondary ion mass spectrometric study of thallium oxide thin films grown via metal organic chemical vapour deposition

Within a comprehensive programme including synthesis via metal organic chemical vapour deposition (MOCVD) and characterization of inorganic compounds and materials of possible interest in technologies based on thin films, results concerning the deposition of metal oxides by means of volatile organometal precursors are reported. In particular, thallium oxide films obtained by the MOCVD technique and commercial powders of Tl₂O₃ and Tl₂O adsorbed on several metal substrates (stainless steel, Si, Cu, Mo, Pt) were studied by secondary ion mass Spectrometry (SIMS) under ion beam bombardment at different ion energies. The positive- and negative-ion mass spectra exhibit typical isotopic patterns of several ionic species produced by interesting interfacial reactions, and the analysis of their relative abundances provides a measure of oxide reactivity towards different substrates. SIMS measurements of metal substrates were also performed. The ability and limits of SIMS in the reactivity study of thallium oxide powders and films and, in addition, in the identification of reaction products evidencing impurity species that, in turn, can be ascribed to the substrates or to the precursors used for the oxide synthesis is pointed out.     

Fine Control of the Redox Reactivity of a Nonheme Iron(III)–Peroxo Complex by Binding Redox‐Inactive Metal Ions

Redox‐inactive metal ions are one of the most important co‐factors involved in dioxygen activation and formation reactions by metalloenzymes. In this study, we have shown that the logarithm of the rate constants of electron‐transfer and C−H bond activation reactions by nonheme iron(III)–peroxo complexes binding redox‐inactive metal ions, [(TMC)Fe^III(O₂)]⁺‐M^{n +} (M^{n +}=Sc³⁺, Y³⁺, Lu³⁺, and La³⁺), increases linearly with the increase of the Lewis acidity of the redox‐inactive metal ions (ΔE ), which is determined from the g_zz values of EPR spectra of O₂^.−‐M^{n +} complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in nucleophilic reactions with aldehydes decreases linearly as the ΔE value increases. Thus, the Lewis acidity of the redox‐inactive metal ions bound to the mononuclear nonheme iron(III)–peroxo complex modulates the reactivity of the [(TMC)Fe^III(O₂)]⁺‐M^{n +} complexes in electron‐transfer, electrophilic, and nucleophilic reactions.     

Molecular Mixed‐Metal Manganese Oxido Cubanes as Precursors to Heterogeneous Oxygen Evolution Catalysts

Well‐defined mixed‐metal [CoMn₃O₄] and [NiMn₃O₄] cubane complexes were synthesized and used as precursors for heterogeneous oxygen evolution reaction (OER) electrocatalysts. The discrete clusters were dropcasted onto glassy carbon (GC) and indium tin oxide (ITO) electrodes, and the OER activities of the resulting films were evaluated. The catalytic surfaces were analyzed by various techniques to gain insight into the structure‐function relationships of the electrocatalysts’ heterometallic composition. Depending on preparation conditions, the Co‐Mn oxide was found to change metal composition during catalysis, while the Ni–Mn oxides maintained the NiMn₃ ratio. XAS studies provided structural insights indicating that the electrocatalysts are different from the molecular precursors, but that the original NiMn₃O₄ cubane‐like geometry was maintained in the absence of thermal treatment ( 2‐Ni ). In contrast, the thermally generated 3‐Ni develops an oxide‐like extended structure. Both 2‐Ni and 3‐Ni undergo structural changes upon electrolysis, but they do not convert into the same material. The observed structural motifs in these heterogeneous electrocatalysts are reminiscent of the biological oxygen‐evolving complex in Photosystem II, including the MMn₃O₄ cubane moiety. The reported studies demonstrate the use of discrete heterometallic oxide clusters as precursors for heterogeneous water oxidation catalysts of novel composition and the distinct behavior of two sets of mixed metal oxides.     

Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe₂O₃ and the corresponding ferrites in 0.1 mol dm⁻³ oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (k_Me; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k_−w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me-O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe₂O₄>CoFe₂O₄?NiFe₂O₄. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe₂O₃ and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni²⁺ labilizes less the corresponding ferrite. The correlation between log k_M and log k_−w is somewhat blurred and displaced to lower k_M values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.