Designing Stability into Thermally Reactive Plumbylenes

We complete the picture of thermally stable and volatile <i>N-</i>heterocyclic metallylenes with the synthesis, structural analysis, and thermal characterization of <i>rac</i>-<i>N</i>²,<i>N</i>³-di-<i>tert</i>-butylbutane-2,3-diamido lead(II) (<b>1Pb</b>). Transamination of bis[bis(trimethylsilyl)amido] lead(II) with the free diamino ligand yields <b>1Pb</b> in high yield, whereas salt-metathesis leads to oxidative cleavage of the butane backbone and production of acetaldehyde-<i>tert-</i>butylimine. <b>1Pb</b> itself undergoes [2+2+1] cycloreversion at 150 °C to the same imine, but with a vapour pressure of 1 Torr at 94 °C a wide thermal window is available for use as a vapour deposition precursor.<div><br></div><div>We contrast this with the the extreme instability of its sisters <i>N</i>²,<i>N</i>³-di-<i>tert</i>-butylethane-2,3-diamido lead(II) (<b>2Pb</b>) and <i>N</i>²,<i>N</i>³-di-<i>tert</i>-butylethylene-2,3-diamido lead(II) (<b>3Pb</b>), which both reductively eliminate Pb(0) at or below room temperature. This is also in start contrast to the stability of the lighter Si, Ge and Sn congeners.</div

Designing Stability into Thermally Reactive Plumbylenes

Lead analogues of <i>N</i>-heterocyclic carbenes (NHPbs) are the least understood members of this increasingly important class of compounds. Here we report the design, preparation, isolation, structure, volatility, and decomposition pathways of a novel aliphatic NHPb: <i>rac</i>-<i>N</i> ²,<i>N</i> ³-di-<i>tert</i>-butylbutane-2,3-diamido lead­(II) (<b>1Pb</b>). The large steric bulk of the <i>tert</i>-butylamido moieties and <i>rac</i>-butane backbone successfully hinder redox decomposition pathways observed for diamidoethylene and -ethane backbone analogues, pushing the onset of thermal decomposition from below 0 °C to above 150 °C. With an exceptionally high vapor pressure of 1 Torr at 94 ± 2 °C and excellent thermal stability among Pb­(II) complexes, <b>1Pb</b> is a promising precursor for the chemical vapor deposition (CVD) and atomic layer deposition (ALD) of functional lead-containing materials

Growth and Crystallization of TiO₂ Thin Films by Atomic Layer Deposition Using a Novel Amido Guanidinate Titanium Source and Tetrakis-dimethylamido-titanium

We studied the growth of TiO₂ by liquid injection atomic layer deposition (ALD) utilizing two different amide-based titanium sources, tetrakis-dimethylamido-titanium [(NMe₂)₄-Ti, TDMAT] and its recently developed derivative, tris-(dimethylamido)-mono-(<i>N</i>,<i>N</i>′-diisopropyl-dimethyl-amido-guanidinato)-titanium {[(N-iPr)₂NMe₂]­Ti­(NMe₂)₃, TiA₃G₁}, with water vapor as counterreactant. A clear saturation of growth with an increasing precursor supply was found for TDMAT between 150 and 300 °C and for TiA₃G₁ between 150 and 330 °C. Representative growth per cycle (GPC) values at 250 °C were 0.041 and 0.044 nm/cycle, respectively. Compared to that of TDMAT, ALD of TiA₃G₁ exhibited a significantly higher stability in the GPC values up to 300 °C coinciding with an improved temperature stability of the precursor. Both processes showed a minimum of the growth rate as a function of temperature. In all cases, the residual carbon and nitrogen contents of the TiO₂ films were <3 atom %. Conformal growth was demonstrated on three-dimensional pinhole structures with an aspect ratio of around 1:30. Deposition temperatures of ≤200 °C led to quasi-amorphous films. At higher growth temperatures, the anatase phase developed, accompanied by the brookite and/or the rutile phase depending on process conditions, deposition temperature, and film thickness

Low-Temperature Atomic Layer Deposition of Low-Resistivity Copper Thin Films Using Cu(dmap)₂ and Tertiary Butyl Hydrazine

Herein, we describe a process for the low-temperature atomic layer deposition of copper using Cu­(dmap)₂ (dmap = dimethylamino-2-propoxide). The use of tertiary butyl hydrazine (TBH) as the reducing agent was found to have a significant improvement on the purity and the resistivity of the Cu films compared to previous processes. Our process was studied at low temperatures of 80–140 °C on native oxide terminated Si. At 120 °C, self-limiting Cu deposition was demonstrated with respect to both Cu­(dmap)₂ and TBH pulse lengths. During the initial stages of the deposition (125–1000 cycles), a growth rate of 0.17 Å/cycle was measured. Once the substrate surface was completely covered, deposition proceeded with a more moderate growth rate of 0.05 Å/cycle. According to X-ray diffraction, the films were crystalline cubic Cu with a slight preference toward (111) orientation. Based on scanning electron micrographs, the Cu films were relatively smooth with the roughness increasing as a function of both increasing temperature and thickness. A 54 nm film deposited at the low temperature of 120 °C exhibited a low resistivity of 1.9 μΩ·cm. Composition analysis on this film showed a remarkably high purity of approximately 99.4 at.%, with the rest being hydrogen and oxygen. The films could be deposited also on hydrogen terminated Si, glass, Al₂O₃, TiN, and Ru, extending the suitability of the process to a wide range of applications

Rational Development of Cobalt β‑Ketoiminate Complexes: Alternative Precursors for Vapor-Phase Deposition of Spinel Cobalt Oxide Photoelectrodes

A series of six cobalt ketoiminates, of which one was previously reported but not explored as a chemical vapor deposition (CVD) precursor, namely, bis­(4-(isopropylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(^<i>i</i>pki)₂], <b>1</b>), bis­(4-(2-methoxyethylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(meki)₂], <b>2</b>), bis­(4-(2-ethoxyethylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(eeki)₂], <b>3</b>), bis­(4-(3-methoxy-propylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(mpki)₂], <b>4</b>), bis­(4-(3-ethoxypropylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(epki)₂], <b>5</b>), and bis­(4-(3-isopropoxypropylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(^<i>i</i>ppki)₂], <b>6</b>) were synthesized and thoroughly characterized. Single-crystal X-ray diffraction (XRD) studies on compounds <b>1</b>–<b>3</b> revealed a monomeric structure with distorted tetrahedral coordination geometry. Owing to the promising thermal properties, metalorganic CVD of CoO_<i>x</i> was performed using compound <b>1</b> as a representative example. The thin films deposited on Si(100) consisted of the spinel-phase Co₃O₄ evidenced by XRD, Rutherford backscattering spectrometry/nuclear reaction analysis, and X-ray photoelectron spectroscopy. Photoelectrochemical water-splitting capabilities of spinel CoO_<i>x</i> films grown on fluorine-doped tin oxide (FTO) and TiO₂-coated FTO revealed that the films show p-type behavior with conduction band edge being estimated to −0.9 V versus reversible hydrogen electrode. With a thin TiO₂ underlayer, the CoO_<i>x</i> films exhibit photocurrents related to proton reduction under visible light

Integrating AlN with GdN Thin Films in an in Situ CVD Process: Influence on the Oxidation and Crystallinity of GdN

The application potential of rare earth nitride (REN) materials has been limited due to their high sensitivity to air and moisture leading to facile oxidation upon exposure to ambient conditions. For the growth of device quality films, physical vapor deposition methods, such as molecular beam epitaxy, have been established in the past. In this regard, aluminum nitride (AlN) has been employed as a capping layer to protect the functional gadolinium nitride (GdN) from interaction with the atmosphere. In addition, an AlN buffer was employed between a silicon substrate and GdN serving as a seeding layer for epitaxial growth. In pursuit to grow high-quality GdN thin films by chemical vapor deposition (CVD), this successful concept is transferred to an in situ CVD process. Thereby, AlN thin films are included step-wise in the stack starting with Si/GdN/AlN structures to realize long-term stability of the oxophilic GdN layer. As a second strategy, a Si/AlN/GdN/AlN stacked structure was grown, where the additional buffer layer serves as the seeding layer to promote crystalline GdN growth. In addition, chemical interaction between GdN and the Si substrate can be prevented by spatial segregation. The stacked structures grown for the first time with a continuous CVD process were subjected to a detailed investigation in terms of structure, morphology, and composition, revealing an improved GdN purity with respect to earlier grown CVD thin films. Employing thin AlN buffer layers, the crystallinity of the GdN films on Si(100) could additionally be significantly enhanced. Finally, the magnetic properties of the fabricated stacks were evaluated by performing superconducting quantum interference device measurements, both of the as-deposited films and after exposure to ambient conditions, suggesting superparamagnetism of ferromagnetic GdN grains. The consistency of the magnetic properties precludes oxidation of the REN material due to the amorphous AlN capping layer

Homoleptic Gadolinium Amidinates as Precursors for MOCVD of Oriented Gadolinium Nitride (GdN) Thin Films

Five new homoleptic gadolinium tris-amidinate complexes are reported, which were synthesized via the salt-elimination reaction of GdCl₃ with 3 equiv of lithiated symmetric and asymmetric amidinates at ambient temperature. The Gd-tris-amidinates [Gd­{(N^<i>i</i>Pr)₂­CR}₃] [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>t</i>Bu (<b>3</b>), ^<i>n</i>Bu (<b>4</b>)] and [Gd­{(NEt)­(N^<i>t</i>Bu)­CMe}₃] (<b>5</b>) are solids at room temperature and sublime at temperatures of about 125 °C (6 × 10^–2 mbar) with the exception of compound <b>4</b>, which is a viscous liquid at room temperature. According to X-ray diffraction analysis of <b>3</b> and <b>5</b> as representative examples of the series, the complexes adopt a distorted octahedral structure in the solid state. Mass spectrometric (MS) data confirmed the monomeric structure in the gas phase, and high-resolution MS allowed the identification of characteristic fragments, such as [{(N^<i>i</i>Pr)₂CR}GdCH₃]⁺ and [{(N^<i>i</i>Pr)₂CR}GdNH]⁺. The alkyl substitution patterns of the amidinate ligands clearly show an influence on the thermal properties, and specifically, the introduction of the asymmetric carbodiimide leads to a lowering of the onset of volatilization and decomposition. Compound <b>5</b>, which is the first Gd complex with an asymmetric amidinate ligand system to be reported, was, therefore, tested for the MOCVD of GdN thin films. The as-deposited GdN films were capped with Cu in a subsequent MOCVD process to prevent postdeposition oxidation of the films. Cubic GdN on Si(100) substrates with a preferred orientation in the (200) direction were grown at 750 °C under an ammonia atmosphere and exhibited a columnar morphology and low levels of C or O impurities according to scanning electron microscopy, Rutherford backscattering, and nuclear reaction analysis

Homoleptic Gadolinium Amidinates as Precursors for MOCVD of Oriented Gadolinium Nitride (GdN) Thin Films

Five new homoleptic gadolinium tris-amidinate complexes are reported, which were synthesized via the salt-elimination reaction of GdCl₃ with 3 equiv of lithiated symmetric and asymmetric amidinates at ambient temperature. The Gd-tris-amidinates [Gd­{(N^<i>i</i>Pr)₂­CR}₃] [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>t</i>Bu (<b>3</b>), ^<i>n</i>Bu (<b>4</b>)] and [Gd­{(NEt)­(N^<i>t</i>Bu)­CMe}₃] (<b>5</b>) are solids at room temperature and sublime at temperatures of about 125 °C (6 × 10^–2 mbar) with the exception of compound <b>4</b>, which is a viscous liquid at room temperature. According to X-ray diffraction analysis of <b>3</b> and <b>5</b> as representative examples of the series, the complexes adopt a distorted octahedral structure in the solid state. Mass spectrometric (MS) data confirmed the monomeric structure in the gas phase, and high-resolution MS allowed the identification of characteristic fragments, such as [{(N^<i>i</i>Pr)₂CR}GdCH₃]⁺ and [{(N^<i>i</i>Pr)₂CR}GdNH]⁺. The alkyl substitution patterns of the amidinate ligands clearly show an influence on the thermal properties, and specifically, the introduction of the asymmetric carbodiimide leads to a lowering of the onset of volatilization and decomposition. Compound <b>5</b>, which is the first Gd complex with an asymmetric amidinate ligand system to be reported, was, therefore, tested for the MOCVD of GdN thin films. The as-deposited GdN films were capped with Cu in a subsequent MOCVD process to prevent postdeposition oxidation of the films. Cubic GdN on Si(100) substrates with a preferred orientation in the (200) direction were grown at 750 °C under an ammonia atmosphere and exhibited a columnar morphology and low levels of C or O impurities according to scanning electron microscopy, Rutherford backscattering, and nuclear reaction analysis

Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors

A novel hybrid core–shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni–NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g^–1), good rate capability (80.5%) under high current charge–discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g^–1). These attractive capacitive behaviors are mainly attributed to the unique core–shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications

F-Doped Co₃O₄ Photocatalysts for Sustainable H₂ Generation from Water/Ethanol

p-Type Co₃O₄ nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H₂ from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co₃O₄ results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co₃O₄ films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes