Heteroleptic Precursors Research Articles

Electron-induced deposition using Fe(CO)4MA and Fe(CO)5 - effect of MA ligand and process conditions.

The electron-induced decomposition of Fe(CO)4MA (MA = methyl acrylate), which is a potential new precursor for focused electron beam-induced deposition (FEBID), was investigated by surface science experiments under UHV conditions. Auger electron spectroscopy was used to monitor deposit formation. The comparison between Fe(CO)4MA and Fe(CO)5 revealed the effect of the modified ligand architecture on the deposit formation in electron irradiation experiments that mimic FEBID and cryo-FEBID processes. Electron-stimulated desorption and post-irradiation thermal desorption spectrometry were used to obtain insight into the fate of the ligands upon electron irradiation. As a key finding, the deposits obtained from Fe(CO)4MA and Fe(CO)5 were surprisingly similar, and the relative amount of carbon in deposits prepared from Fe(CO)4MA was considerably less than the amount of carbon in the MA ligand. This demonstrates that electron irradiation efficiently cleaves the neutral MA ligand from the precursor. In addition to deposit formation by electron irradiation, the thermal decomposition of Fe(CO)4MA and Fe(CO)5 on an Fe seed layer prepared by EBID was compared. While Fe(CO)5 sustains autocatalytic growth of the deposit, the MA ligand hinders the thermal decomposition in the case of Fe(CO)4MA. The heteroleptic precursor Fe(CO)4MA, thus, offers the possibility to suppress contributions of thermal reactions, which can compromise control over the deposit shape and size in FEBID processes.

Air-Stable Dinuclear Complexes of Four-Coordinate ZnII and NiII Ions with a Radical Bridge: A Detailed Look at Redox Activity and Antiferromagnetic Coupling.

Air-stable dinuclear complexes [(bmsab)NiII(tmsab)NiII(bmsab)]3- and [(bmsab)ZnII(tmsab)ZnII(bmsab)]3- (bmsab = bis(methanesulfoneamido)benzene, tmsab = tetra(methanesulfonamido)benzene) were prepared via a synthetic route based on heteroleptic precursor complexes. The new complexes combine a distorted tetrahedral coordination environment with an open-shell bridging ligand. The ZnII species was subjected to a detailed investigation of the (spectro-)electrochemical processes. The NiII species is a rare example of a complex that combines strong exchange coupling (J > 440 cm-1) with pronounced positive zero-field splitting (D = +72 cm-1). Combining SQUID magnetometry and (HF)EPR spectroscopy with ab initio calculations allowed for accurate quantification of the exchange interaction.

Chemisorption and Surface Reaction of Hafnium Precursors on the Hydroxylated Si(100) Surface

Hafnium oxide (HfO2) is widely recognized as one of the most promising high-k dielectric materials due to its remarkable properties such as high permittivity, wide band gap, and excellent thermal and chemical stability. The atomic layer deposition (ALD) of HfO2 has attracted significant attention in recent decades since it enables uniform and conformal deposition of HfO2 thin films on various substrates. In this study, we examined the initial surface reactions of a series of homoleptic hafnium precursors on hydroxylated Si(100) surfaces using density functional theory calculations. Our theoretical findings align with previous experimental studies, indicating that hafnium amides exhibit higher reactivity compared to other precursors such as hafnium alkoxides and hafnium halides in surface reactions. Interestingly, we found that the chemisorption and reactivity of hafnium precursors are considerably affected by their thermal stability and size. For alkoxide precursors, which have similar thermal stabilities, the size of alkoxide ligands is an important factor in determining their reactivity. Conversely, the reactivity of hafnium halides, which have ligands of similar sizes, is primarily governed by their thermal stability. These insights are valuable for understanding the surface reaction mechanisms of precursors on hydroxylated Si(100) surfaces and for designing new materials, particularly heteroleptic precursors, in future research.

Reaction mechanism and film properties of the atomic layer deposition of ZrO2 thin films with a heteroleptic CpZr(N(CH3)2)3 precursor

The surface reaction of a heteroleptic precursor in the atomic layer deposition (ALD) of ZrO2 has been investigated using atomically thin films for scaled-down semiconductor devices. Heteroleptic precursors have been widely used in ALD. Because different ligands in heteroleptic precursors have different reaction pathways, understanding the surface reaction is crucial. In this study, we investigate the growth mechanisms of thin films using a CpZr(N(CH3)2)3 precursor with different reactants, such as H2O and O2 plasma. Chemical composition, surface roughness, film density, and microstructures were analyzed through electron microscopy and X-ray techniques together with quantum chemical calculations to elucidate the ZrO2 growth mechanism. The results showed that the reaction pathway of Zr–Cp ligand exchange with surface hydroxyl groups is less favorable than the Zr–N(CH3)2 reaction pathway. Furthermore, H2O molecules showed less effective oxidation reactions with Cp ligands than with alkyl amide ligands. In contrast, all the ligands were completely removed, resulting in negligible impurity incorporation, as revealed via X-ray photoelectron spectroscopy, when O2 plasma was used, leading to an improved leakage current (30-fold reduction). We believe that these findings on the surface reactions of heteroleptic ALD precursors will be helpful for developing future scaled-down semiconductor devices.

Surface reaction mechanism of atomic layer deposition of niobium oxide: In situ characterization and first-principle study

We report the surface reaction mechanism of the atomic layer deposition (ALD) of niobium oxide films by combining experimental and theoretical studies. Tert-butylimido tris(dimethylamido)niobium [tBuN=Nb(NMe2)3] and ozone (O3) were studied as the precursor and co-reactant. The in situ quartz crystal microbalance (QCM) analysis predicted that two HNMe2 molecules or one H2NtBu molecule were released at 200 °C per one molecule tBuN=Nb(NMe2)3. Since the QCM alone could not distinguish two possible reaction pathways, we compared two pathways by density functional theory (DFT) calculations. We simulated the chemisorption of the precursor molecule on the –OH terminated T-Nb2O5 substrate. The release of two –NMe2 ligands was favorable due to low activation energies of 0.71 and 0.63 eV for the first and second releases. On the other hand, the release of =NtBu was not favorable due to the high activation energy of 1.50 eV. Therefore, the amido and imido ligands were actor and spectator ligands during the first half-reaction of the ALD of niobium oxide films. The combination of QCM and DFT could successfully reveal the surface reaction mechanism of the ALD process using a heteroleptic precursor.

Atomic layer deposition of Y2O3 films using a novel liquid homoleptic yttrium precursor tris(sec-butylcyclopentadienyl)yttrium [Y(sBuCp)3] and water.

Atomic layer deposition (ALD) of Y2O3 thin films was studied using a novel homoleptic yttrium ALD precursor: tris(sec-butylcyclopentadienyl)yttrium [Y(sBuCp)3]. Y(sBuCp)3 is a liquid at room temperature. The thermogravimetry curve for Y(sBuCp)3 is clean, with no indication of decomposition or residue formation. Thermogravimetry-differential thermal analysis measurements showed that Y(sBuCp)3 is stable for 18 weeks at 190 °C. Y(sBuCp)3 has a homoleptic structure. Thus, a reduction in manufacturing costs is expected compared to those associated with heteroleptic precursors because additional chemical synthesis steps are usually necessary to produce heteroleptic compounds. In addition, ALD of Y2O3 was demonstrated using Y(sBuCp)3 and water as a co-reactant. The deposition temperature was varied from 200 to 350 °C. The growth rate was 1.7 Å per cycle. In addition, neither carbon nor nitrogen contamination was detected in the Y2O3 films by X-ray photoelectron spectroscopy. Furthermore, smooth films were confirmed by X-ray secondary-electron microscopy. The root-mean-square roughness was measured to be 0.660 nm by atomic force microscopy. Metal-insulator-semiconductor (MIS) Pt/Y2O3/p-Si devices were fabricated to evaluate the electrical properties of the Y2O3 films. An electric breakdown field of -6.5 MV cm-1 and a leakage current density of ∼3.2 × 10-3 A cm-2 at 1 MV cm-1 were determined. The permittivity of Y2O3 was estimated to be 11.5 at 100 kHz. Therefore, compared with conventional solid precursors, Y(sBuCp)3 is suitable for use in ALD manufacturing processes.

Plasma-enhanced atomic layer deposition of WO3-SiO2 films using a heteronuclear precursor

Tungsten oxide–silicon dioxide (WOx–SiOy) composite thin films were deposited for the first time via the remote oxygen plasma-enhanced atomic layer deposition (ALD) process using a novel metal-organic heteronuclear and heteroleptic precursor, bis(tert-butylimido)bis(trimethylsilylmethyl)tungsten. Self-limiting ALD growth was demonstrated over a wide temperature window of 203–328 °C with growth per cycle decreasing with increasing temperature from 0.75 to 0.4 Å/cycle, respectively. Residual gas analysis revealed ligand competition and showed that ligand reaction during ALD nucleation and growth was a function of deposition temperature, thereby affecting the film composition. As the temperature increased from 203 to 328 °C, the film composition [W/(Si + W)] ranged from 0.45 to 0.53. In addition, the carbon impurity content was reduced and the refractive index increased from 1.73 to 1.96, the density increased from 4.63 to 5.6 g/cm3, and the optical bandgap decreased from 3.45 to 3.27 eV. Grazing angle x-ray diffraction indicated that as-deposited films were amorphous. Upon annealing in O2 at 500 °C or higher, depending on deposition temperature, films are crystalized into the triclinic WO3 phase. At the same time, WO3 is sublimed from the surface and films are reduced in thickness.

Extending growth inhibition during area-selective atomic layer deposition of Al2O3 on aminosilane-functionalized SiO2.

During area-selective atomic layer deposition (ALD) based on growth inhibitors, nucleation eventually occurs as the metal precursor reacts with the surface through secondary pathways. We show that ALD of Al2O3 on functionalized SiO2 can be significantly delayed by using a lower reactivity, heteroleptic precursor at well below the saturation dose.

Direct liquid injection chemical vapor deposition of ZrO2 films from a heteroleptic Zr precursor: interplay between film characteristics and corrosion protection of stainless steel

The direct liquid injection chemical vapor deposition (DLI-CVD) of uniform and dense zirconium oxide (ZrO2) thin films applicable as corrosion protection coatings (CPCs) is reported. We present the entire development chain from the rational choice and thermal evaluation of the suitable heteroleptic precursor [Zr(OiPr)2(tbaoac)2] over the detailed DLI-CVD process design and finally benchmarking the CPC behavior using electrochemical impedance spectroscopy (EIS). For a thorough development of the growth process, the deposition temperature (Tdep) is varied in the range of 400 – 700 °C on Si(100) and stainless steel (AISI 304) substrates. Resulting thin films are thoroughly analyzed in terms of structure, composition, and morphology. Grazing incidence X-ray diffractometry (GIXRD) reveals an onset of crystallization at Tdep ≥ 500 °C yielding monoclinic and even cubic phase at low temperatures. At Tdep = 400 °C, isotropic growth of XRD amorphous material is shown to feature cubic crystalline domains at the interfacial region as revealed by electron diffraction. Corrosion results obtained through EIS measurements and further immersion tests revealed improved CPC characteristic for the 400 °C processed ZrO2 coatings compared to the ones deposited at Tdep ≥ 500 °C, yielding valuable insights into the correlation between growth parameter and CPC performance which are of high relevance for future exploration of CPCs.

Comprehensive Study and Atomic Layer Deposition of HfO2 Process Development Using Novel Hf Alkoxide Precursors

Due to the scale down of the semiconductor device, the SiO2 based dielectric has been replaced with a high dielectric constant dielectric due to the decrease in the reliability of the gate oxide against electric breakdown and the increased leakage current due to the direct tunneling of electrons. Among the various high-k dielectric materials, HfO2 is considered one of the most suitable materials due to its high dielectric constant, good thermodynamic stability, and high band gap energy. Atomic layer deposition of high quality HfO2 has emerged as a key technology in ultra-thin and high-k dielectrics. The thin film deposited by ALD is formed through surface chemical reactions that occur only on the substrate surface because precursors and reactants are sequentially injected and do not contact in the gas phase during the ALD process. The surface reaction depends on the chemical properties, which can affect the growth properties and film properties. Therefore, proper selection of precursors is very important for the potential application of ALD HfO2. Numerous studies have been reported investigating the effect of Hf precursors on the growth and film properties of ALD HfO2. In the case of halide precursors, it was used to avoid electrical degradation due to carbon impurities. However, it is difficult to apply the halide precursor to the ALD HfO2 process due to the high thermal budget. On the other hands, alkylamide precursors are the most widely used metalorganic precursors owing to their low melting points and high vapor pressure, but thermal decomposition of the alkylamide precursors easily occurs at temperature of ~300 °C due to their poor thermal stability. Among the various type of Hf precursors, alkoxide precursors are considered as attractive precursor to deposit C-free films owing to its strong inherent metal-oxygen bonding. However, ALD of HfO2 using alkoxide precursors exhibits non-saturated growth characteristics caused by undesired chemical reaction. Chain reaction of alkoxide precursor hydrolysis or ligand exchange occurs continuously because vapor phase H2O, alcohol and surface hydroxyl (O-H) can be generated from ligand degradation by β-hydride elimination. In addition, the alkoxide precursor requires a relatively high deposition temperature due to the ligand exchange reaction between the alkoxide ligand and the surface OH group due to strong Hf-O bonds. It is difficult to apply alkoxide precursors for ALD process due to thermal decomposition due to high process temperature. To solve the problem, it is necessary to modify the chemical properties of the precursors, such as thermal stability, reactivity. Suitable properties of the precursor for ALD application can be obtained by changing the precursor structure. Despite numerous efforts to develop new precursors, only limited success has been achieved by previous studies. Thus, the development of new types of ligand and precursors is needed to solve non-saturation behavior and improve thermal stability. In addition, a systematic study is needed to reveal the effect of modifying the precursor molecular structure on improving growth properties and thermal stability. In this study, we report a comprehensive investigation into the synthesis and evaluation of new precursors to overcome the weaknesses of existing alkoxide precursors. Hf alkoxide precursors were newly synthesized by Korea Research Institute of Chemical Technology designed for deposition of HfO2 with low impurities. New type of β-diketonates ligand was employed to improving thermal stability and inhibition of undesired reactions in ALD process. In addition, Cyclopentadienyl ligand was employed to develop heteroleptic precursor. We also investigated the impact of ligand substitution on growth characteristics, chemical compositions, film density and crystallinity of ALD HfO2 using new precursors. The electrical properties including the dielectric constant and leakage current density were evaluated for applications. Furthermore, we theoretically investigated the effects of ligand substitution on mechanism of precursor intermolecular reactions which can cause non-saturated growth by density functional theory (DFT) calculations used for calculating the reaction energy.

Thermal stability and reactivity of titanium halide precursors for the atomic layer deposition of TiO2 on a Pt (111) surface

The atomic layer deposition (ALD) of ultrathin metal-oxide film on metal surfaces is an effective technique for protecting supported metal catalyst against sintering, leaching and coking thereby improving catalyst stability and durability. Although numerous studies have demonstrated the advantages of TiO2 overcoats in catalysis, the reactivity and thermal stabilities of TiO2 ALD precursors on metal surfaces is still relatively unexplored. In this work, density functional theory (DFT) was employed to investigate the reactivity and thermal stability of titanium halide-based precursors with emphasis on homoleptic TiF4, TiCl4 and TiI4 precursors. In conjunction with DFT, the automated enumeration tool and the bond-ligand dissociation automation tool in Schrodinger material science suite were employed to investigate and compare the thermal stability and reactivity of the precursors. The surface reactivity of these precursors on a bare and hydroxylated Pt surface was also investigated. The results obtained from the simulations were compared and correlated with available literature. The results reveal that thermal stability of the precursors is represented as TiF4>TiCl4>TiI4 by whereas the opposite is true for the reactivity TiF4<TiCl4<TiI4. Moreover, the thermal stability of all heteroleptic precursors is dependent on the number of Ti-F bonds, while the reactivity depends on the number of Ti-I bonds. However, TiCl4 provides a better balance between the two extremes when taking both thermal stability and surface reactivity into consideration.

Structure-Decomposition relationships of homoleptic and heteroleptic vanadium(IV) alkoxides for defined gas phase depositions of VO2 and V2O3 thin films

A facile high yield synthesis of a novel non-oxo heteroleptic vanadium(IV) alkoxide with chelating asymmetric fluorinated ß-heteroarylacetamides (2,2,2-trifluoro-N-(pyridine-2-yl)acetamide (H-PyTFA) (1)) with the general formula [V(OtBu)2(PyTFA)2] (4) as precursor for nanostructured vanadium oxides films deposited by chemical vapor deposition (CVD) is presented here. Single crystal X-ray analysis, electron paramagnetic resonance (EPR) spectroscopy and mass spectrometry (EI-MS) revealed the unambiguous structural and electronic features of the monomeric octahedral complex 4. Comparative thermogravimetric studies with homoleptic monomeric vanadium(IV) tert–butoxide [V(OtBu)4] (2) and trimeric [V3(μ-OEt)4(OEt)8] (3) displayed the strong influence of the alkyl function in vanadium(IV) alkoxides and chelating ligands on volatility, stability and thermal decomposition pathways to different oxide phases and morphologies. We demonstrated a controlled two-step thermal decomposition of heteroleptic vanadium(IV) alkoxide (4) which offer new opportunities for a versatile growth by the enlarged ‘CVD window’ between sublimation temperature and decomposition temperature and selective self-limited growth originated from the ‘Energy trap’ between the first and second decomposition steps. Preliminary CVD experiments proofed the successful conversion of the heteroleptic precursor 4 into vanadium dioxide with ribbon-like morphology on silicon, which revealed the impact of molecular precursor engineering on crystal growth kinetics.

(Invited) Single-Source Precursors for Gas Phase Deposition of Catalytic Coatings for Water Splitting Applications

Keywords: Single-source precursor, CVD, nanostructured films, catalytic water splitting Investigation of the interplay of metal-organic chemistry will enrich the state-of-the-art of CVD and ALD technology and open new possibilities for the applications of new Ir-based materials. Therefore new heteroleptic Janus-typed compounds exhibiting high volatility and defined thermal decomposition under CVD and ALD conditions are reported to elaborate the precursor chemistry – materials synthesis – functional property chain. The new precursors unify both reactivity and sufficient stability through its heteroleptic constitution to provide a precise control over compositional purity in CVD and ALD deposits. CVD- and ALD-grown materials were tested towards their (electro)catalytic applications, particular in the oxygen evolution reactions. In this work functional characterization of deposited materials will be reported and their catalytic behavior is examined. The deposition on various substrate materials without the need of additional reactant gases underlines the potential of heteroleptic precursor class for CVD and ALD of metallic thin films. The presented CVD data opens new possibilities in the vapor phase synthesis of materials facilitating the application of such films, for example, as electro- or photocatalyst in OER and ORR. Figure 1

Comparative study on the use of novel heteroleptic cyclopentadienyl-based zirconium precursors with H2O and O3 for atomic layer deposition of ZrO2

Three heteroleptic Zr precursors were studied for atomic layer deposition (ALD) of ZrO2. Films were deposited from Zr(Cp)(tBuDAD)(OiPr), Zr(MeCp)(TMEA), and Zr(Me5Cp)(TEA) with either water or ozone as the oxygen source {tBuDAD = N,N-bis(tertbutyl)ethene-1,2-diaminato, TMEA = tris[2-(methylamino)ethyl]aminate, TEA = triethoanolaminate}. Self-limiting film growth was confirmed for the Zr(Cp)(tBuDAD)(OiPr)/O3 process at 250 °C and for the Zr(Me5Cp)(TEA)/O3 process at 375 °C, which is among the highest temperatures for advanced heteroleptic precursors. Excellent film purity with C, H, and N levels below the detection limit of the elastic recoil detection analysis was obtained with ozone as the oxygen source. All the studied processes showed the same trend that at low deposition temperatures films were tetragonal ZrO2 and at higher temperatures mixtures of tetragonal and monoclinic ZrO2. With water, the monoclinic phase appeared at higher temperatures than with ozone. In addition to the deposition temperature, the film thickness affected the phase; thinner films favored the tetragonal phase and monoclinic peaks were more clearly seen in thicker films. The high thermal stability and excellent film purity show that from the three studied Zr precursors, Zr(Me5Cp)(TEA) is a noteworthy precursor candidate for ALD of ZrO2.

Initial stage of atomic layer deposition of 2D-MoS2 on a SiO2 surface: a DFT study.

In this study, we investigate the reactions involving Atomic Layer Deposition (ALD) of 2D-MoS2 from the heteroleptic precursor Mo(NMe2)2(NtBu)2 and H2S as the co-reagent on a SiO2(0001) surface by means of density functional theory (DFT). All dominant reaction pathways from the early stage of adsorption of each ALD reagent to the formation of bulk-like Mo and S at the surface are identified. In the metal pulse, proton transfer from terminal OH groups on the SiO2 to the physisorbed metal precursor increases the Lewis acidity of Mo and Lewis basicity of O, which gives rise to the chemical adsorption of the metal precursor. Proton transfer from the surface to the dimethylamido ligands leads to the formation and desorption of dimethylamine. In contrast, the formation and desorption of tert-butylamine is not energetically favorable. The tert-butylimido ligand can only be partially protonated in the metal pulse. In the sulphur pulse, co-adsorption and dissociation of H2S molecules give rise to the formation and desorption of tert-butylamine. Through the calculated activation energies, the cooperation between H2S molecules ('cooperative' mechanism) is shown to have a profound influence on the formation and desorption of tert-butylamine, which are crucial steps in the initial ALD deposition of 2D-MoS2 on SiO2. The cyclic ALD reactions give rise to the formation of a buffer layer which might have important consequences for the electrical and optical properties on the 2D layer formed in the subsequent homodeposition.

Heteroleptic Precursors for Atomic Layer Deposition

As the atomic layer deposition (ALD) method is based on sequential, self-limiting surface reactions the precursor chemistry is the key to a successful processing of conformal high quality thin films. ALD precursor chemistry has traditionally been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides and alkoxides. However, these precursors sometimes have drawbacks such as possible halide contamination and low thermal stabilities. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes. Here, examples of heteroleptic precursors for ALD processes of transition metals and their oxides are given. Special focus is given to oxides of the rare earths and groups 4 and 5. Trends in the properties of heteroleptic precursors are discussed. Several examples of our recent results are shown, including introduction of novel processes based on amidinate-cyclopentadienyl complexes for ALD of rare earth oxides.

Heteroleptic Precursors for Atomic Layer Deposition

As the atomic layer deposition (ALD) method is based on sequential, self-limiting surface reactions the precursor chemistry is the key to a successful processing of conformal high quality thin films. ALD precursor chemistry has traditionally been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides and alkoxides. However, these precursors may have drawbacks such as possible halide contamination and low thermal stabilities in the case of the alkylamides and alkoxides. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes.In the recent years, increasing research effort has been dedicated in development of novel ALD processes focusing especially on transition metals and their oxides using heteroleptic precursors. Interestingly, heteroleptic compounds often show different properties than their homoleptic counterparts. One type of ligand can provide reactivity and another stability, thus widening the process window with comparable film growth rate.In this presentation heteroleptic precursors for ALD processes of transition metals and their oxides will be reviewed. Special focus is given to oxides of the rare earths and groups 4 and 5. Trends in the properties of heteroleptic precursors are discussed. Several examples of our recent results are shown, including introduction of novel processes based on amidinate-cyclopentadienyl complexes for ALD of rare earth oxides.

Atomic Layer Deposition of Groups 4 and 5 Transition Metal Oxide Thin Films: Focus on Heteroleptic Precursors

The atomic layer deposition (ALD) process, an alternative to CVD, is universally appreciated for its unique advantages such as excellent repeatability, conformity, and thickness control at the atomic level. ALD precursor chemistry has mainly been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides, and alkoxides, however these precursors have drawbacks such as possible halide contamination and low thermal stabilities in the case of the alkylamides and alkoxides. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes. Nevertheless, there is no thematic review dedicated to the heteroleptic precursors and their properties, and it seems that no coherent strategy has been adopted for the development of heteroleptic precursors. This review gives a brief description of ALD and presents studies on the deposition of thin films of groups 4 and 5 metal oxides using ALD. A description of the general ALD properties of homoleptic precursors, in addition to a review on the thermal ALD of groups 4 and 5 metal oxides from heteroleptic precursors, is provided. Trends in the properties of heteroleptic ALD precursors, based on the literature review and recent experimental data, are discussed.

Initial growth mechanisms of ZrO2 and TiO2 thin films using cycloheptatrienyl–cyclopentadienyl heteroleptic precursors: A comparative study by density functional theory

The initial growth mechanisms of atomic layer deposition (ALD) of ZrO2 and TiO2 thin films using cycloheptatrienyl (CHT)–cyclopentadienyl (Cp) precursors on SiOH surface have been investigated theoretically. The reactions of CpMeZr(CHT) and CpTi(CHT) with SiOH surface proceed through similar reaction pathways. The reaction of CpTi(CHT) requires much more energies than that of CpMeZr(CHT). Chemisorption of the CHT–Cp precursors on SiOH surface could not be found. The most stable adsorption states of both CpMeZr(CHT) and CpTi(CHT) are formed via CHT rings. The previous experiments have shown that an ALD-window has been established when depositing ZrO2 thin films, but has not been established when depositing TiO2 thin films. Our calculation results show that the adsorption of CpMeZr(CHT) is energetically favorable, but the adsorption of CpTi(CHT) is thermodynamic unfavorable. One theoretical explanation is given for this observed difference from the viewpoint of precursor adsorption.

Tin(ii) amide/alkoxide coordination compounds for production of Sn-based nanowires for lithium ion battery anode materials

A series of tin(II) amide alkoxides ([(OR)Sn(NMe(2))](n)) and tin(II) alkoxides ([Sn(OR)(2)](n)) were investigated as precursors for the production of tin oxide (SnO(x)) nanowires. The precursors were synthesized from the metathesis of tin dimethylamide ([Sn(NMe(2))(2)](2)) and a series of aryl alcohols {H-OAr = H-OC(6)H(4)(R)-2: R = CH(3) (H-oMP), CH(CH(3))(2) (H-oPP), C(CH(3))(3) (H-oBP)] or [H-OC(6)H(3)(R)(2)-2,6: R = CH(3) (H-DMP), CH(CH(3))(2) (H-DIP), C(CH(3))(3) (H-DBP)]}. The 1:1 products were all identified as the dinuclear species [(OAr)Sn(μ-NMe(2))](2) where OAr = oMP (1), oPP (2), oBP (3), DMP (4), DIP (5), DBP (6). The 1:2 products were identified as either a polymer ([Sn(μ-OAr)(2)](∞) (where OAr = oMP (7), oPP (8)), dinuclear [(OAr)Sn(μ-OAr)](2) (where OAr = oBP (9), DMP (10) or DIP/HNMe(2) (11)), or mononuclear [Sn(DBP)(2)] (12) complexes. These novel families of compounds (heteroleptic 1-6, and homoleptic 7-12) were evaluated for the production of SnO(x) nanowires using solution precipitation (SPPT; oleylamine/octadecene solvent system) or electrospinning (ES; THF solvent) processing conditions. The SPPT route that employed the heteroleptic precursors yielded mixed phases of Sn(o):romarchite [1 (100:0); 2 (80:20); 3 (68:32); 4 (86:14); 5 (66:35); 6 (88:12)], with a variety of spherical sized particles [1 (350-900 nm); 2 (150-1200 nm); 3 (250-950 nm); 4 (20-180 nm); 5 (80-400 nm); 6 (40-200 nm)]. For the homoleptic precursors, similar phased [7 (80:20); 8 (23:77); 9 (15:85); 10 (34:66); 11 (77:23); 12 (77:23)] spherical nanodots were isolated [7 (50-300 nm); 8: (irregular); 10 (200-800 nm); 11 (50-150 nm); 12 (50-450 nm)], except for 9 which formed polycrystalline rods [Sn(o):romarchite (15:85)] with aspect ratios >100. From ES routes, the heteroleptic species were found to form 'tadpole-shaped' materials whereas the homoleptic species formed electrosprayed nanodots. The one exception noted was for 7, where, without use of a polymer matrix, nanowires of Sn(o), decorated with micron sized 'balls' were observed. Due to the small amount of material generated, PXRD patterns were inconclusive to the identity of the generated material; however, cyclic voltammetry on select samples was used to tentatively identify the final Sn(o) (from 7) with the other sample identified as SnO(x) (from 1).