Magnetic Field-assisted Chemical Vapor Deposition of Transition Metal Oxides and in situ Investigations on Electronic Structure by X-ray

与传统的化学气相沉积(CVD)实验参数(如温度、压力和前驱体化学)相比，对外在参数(如磁场)影响的研究较少。结果表明，外加磁场对薄膜的相组成、晶粒生长和致密化过程有很强的影响，显著改变了薄膜的形貌、化学形貌和功能性质。磁场辅助化学气相沉积(MfCVD)显示了从反铁磁Cr2O_3(零场)到铁磁Cr3C2(1.0特斯拉)的化学成分的显著变化。同样，氧化铁在mfCVD中的生长表现出强烈的各向异性效应，从而提高了薄膜的光电化学性能。这项应用的科学目标是(I)通过使用更高的场强(高达4特斯拉)和更高的前体通量(科隆大学)将场效应与均匀和非均匀成核相关联，以及(Ii)通过基于同步加速器的Operando X射线光谱分析(PGI-6，Jülich研究中心)在原子水平上了解生长过程和磁场效应。为此，对同时含有d-和p-嵌段元素的尖晶石前体分子([CoAl2(OR)8]vs.系统地研究了[CoFe2(OR)8]转变为固体薄膜时的相分离和相变现象。这将把分子离子和粒子的内在属性(例如，磁性)与它们的宏观表现(颗粒大小和晶体结构)联系起来。MfCVD实验与薄膜生长过程中的操纵面X射线吸收光谱相结合，将破译外加磁场如何影响成核、表面扩散、氧化还原过程和晶格级相互作用，从而提供对生长氧化物中表面化学和沉淀现象的整体理解。本研究将论证外加磁场能否扩大化学气相沉积的实验空间，并为影响原生薄膜特性提供一个额外的控制参数。此外，还将研究带有顺磁和抗磁中心原子的过渡金属尖晶石(CoFe2O4和CoAl2O4)中的磁场辅助相分离和界面反应。为了验证化学形貌和表面性质的变化，将在场增强和零场条件下生长的薄膜上进行小分子(例如H2O、CO2)活化的操纵面X射线光谱分析。项目活动得到SPP成员在以下领域的互补专业知识的支持：mfCVD样品的高级磁化测量(RG Gutfleisch)、通过固态核磁共振研究薄膜样品的磁性有序(RG Wurmehl)以及通过高分辨率扫描电子显微镜/透射电子显微镜(RG Volkert)进行微结构变化。

Compared to conventional experimental parameters of chemical vapor deposition (CVD) such as temperature, pressure and precursor chemistry, investigations on effects of extrinsic parameters (e.g., magnetic field) are less studied. The results obtained in the initial phase of the SPP on the CVD of transition metal demonstrated strong effects of external magnetic fields on the phase composition, grain growth and densification processes significantly altering the morphology, chemical topography and functional properties of the obtained films. Magnetic field-assisted CVD (mfCVD) of Cr(OBut)4 showed a remarkable modulation of the chemical composition from antiferromagnetic Cr2O3 (zero field) to ferromagnetic Cr3C2 (1.0 Tesla). Similarly, the growth of iron oxide in mfCVD showed strong anisotropy effects in films responsible for enhanced photoelectrochemical properties. The scientific objectives of this application is to (i) correlate the field effects to homogeneous and heterogeneous nucleation by using higher field strengths (up to 4 Tesla) and higher precursor fluxes (University of Cologne) and (ii) understand the growth process and magnetic field effect on an atomistic level by synchrotron based operando X-Ray spectroscopy (PGI-6, Research Center Jülich). For this purpose, conversion of spinel precursor molecules containing both d- and p-block elements ([CoAl2(OR)8] vs. [CoFe2(OR)8] into solid films will be systematically studied with respect to phase segregation and transformation phenomena. This will link the intrinsic properties (e.g., magnetism) of molecular ions and particles to their macroscopic manifestation (grain size and crystallographic structure). The mfCVD experiments together with operando X-ray absorption spectroscopy during film growth will decipher how nucleation, surface diffusion, redox processes and lattice-level interactions are influenced by applied magnetic fields and thus provide a holistic understanding of surface chemistry and precipitation phenomena in growing oxides. This research will demonstrate whether external magnetic fields can expand the experimental space of CVD and provide an additional control parameter for influencing the characteristics of as-grown thin films. Moreover, magnetic field-assisted phase segregation and interfacial reactions in transition metal spinels carrying paramagnetic and diamagnetic central atoms (CoFe2O4 vs. CoAl2O4) will be investigated. For the validation of changes in the chemical topography and surface properties, operando X-Ray spectroscopy of the activation of small molecules (e.g., H2O, CO2) will be performed on films grown in field-enhanced and zero-field conditions. The project activities are supported by the complementary expertise of the SPP members in the field of advanced magnetization measurements of mfCVD samples (RG Gutfleisch), magnetic ordering in thin film samples by solid-state NMR studies (RG Wurmehl) and microstructural changes via high-resolution SEM/TEM (RG Volkert).