Aerosol-Assisted Chemical Vapor Deposition of 2H-WS2 From Single-Source Tungsten Dithiolene Precursors.

The tungsten carbonyl dimethyldithiolene (dmdt) complexes W(CO)4(dmdt), W(CO)2(dmdt)2, and W(dmdt)3 were evaluated as potential single-source precursors for the chemical vapor deposition of WS2. The results of TGA-MS, DIP-MS, and pyrolysis with NMR analysis were consistent with a thermal decomposition pathway in which loss of 2-butyne through a retro[3+2]cycloaddition of the dithiolene ligand generated terminal sulfido ligands. Aerosol-assisted chemical vapor deposition onto silicon substrates was performed using all three complexes, yielding 2H-WS2 thin films as characterized by Raman spectroscopy and GI-XRD. Film morphology and elemental composition of the films were determined using SEM, EDS, and XPS. Four-point probe measurements afforded a film resistivity of 8.37 Ωcm for a sample deposited from W(dmdt)3 in toluene at 600 °C.

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.

Molybdenum(III) Amidinate: Synthesis, Characterization, and Vapor Phase Growth of Mo-Based Materials.

The synthesis, characterization, and thermogravimetric analysis of tris(N,N'-di-isopropylacetamidinate)molybdenum(III), Mo(iPr-AMD)3, are reported. Mo(iPr-AMD)3 is a rare example of a homoleptic mononuclear complex of molybdenum(III) and fills a longstanding gap in the literature of transition metal(III) trisamidinate complexes. Thermogravimetric analysis (TGA) reveals excellent volatilization at elevated temperatures, pointing to potential applications as a vapor phase precursor for higher temperature atomic layer deposition (ALD), or chemical vapor deposition (CVD) growth of Mo-based materials. The measured TGA temperature window was 200-314 °C for samples in the 3-20 mg range. To validate the utility of Mo(iPr-AMD)3, we demonstrate aerosol-assisted CVD growth of MoO3 from benzonitrile solutions of Mo(iPr-AMD)3 at 500 °C using compressed air as the carrier gas. The resulting films are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. We further demonstrate the potential for ALD growth at 200 °C with a Mo(iPr-AMD)3/Ar purge/300 W O2 plasma/Ar purge sequence, yielding ultrathin films which retain a nitride/oxynitride component. Our results highlight the broad scope utility and potential of Mo(iPr-AMD)3 as a stable, high-temperature precursor for both CVD and ALD processes.

Aerosol-Assisted Chemical Vapor Deposition of MoS2 with a Thiourea Sulfur Source: Single-Source Precursors vs Coreactant Mixtures.

Aerosol-assisted chemical vapor deposition of MoS2 from solutions containing the single-source precursors cis-Mo(CO)4(TMTU)2 and Mo(CO)5(TMTU) in toluene was compared to depositions from the coreactant solution containing Mo(CO)6 and uncoordinated TMTU in toluene. The results were used to assess the significance of ligand precoordination on the properties of the deposited films. Raman spectra and GI-XRD patterns of the films show that the single-source precursors produced more intense and sharper signals for 2H-MoS2 as compared to the coreactant system of Mo(CO)6 and TMTU, which is indicative of improved crystallinity. SEM and XPS were also used to assess morphology and film composition. Thermolysis of cis-Mo(CO)4(TMTU)2 and analysis of the pyrolysis products by GC-MS and 1H NMR suggested a decomposition mechanism of the TMTU ligand where a terminal sulfido is generated on the molybdenum center with loss of a heteroatom stabilized carbene.

Molybdenum(IV) dithiocarboxylates as single-source precursors for AACVD of MoS2 thin films.

Tetrakis(dithiocarboxylato)molybdenum(IV) complexes of the type Mo(S2CR)4 (R = Me, Et, iPr, Ph) were synthesized, characterized, and prescreened as precursors for aerosol assisted chemical vapor deposition (AACVD) of MoS2 thin films. The thermal behavior of the complexes as determined by TGA and GC-MS was appropriate for AACVD, although the complexes were not sufficiently volatile for conventional CVD bubbler systems. Thin films of MoS2 were grown by AACVD at 500 °C from solutions of Mo(S2CMe)4 in toluene. The films were characterized by GIXRD diffraction patterns which correspond to a 2H-MoS2 structure in the deposited film. Mo-S bonding in 2H-MoS2 was further confirmed by XPS and EDS. The film morphology, vertically oriented structure, and thickness (2.54 µm) were evaluated by FE-SEM. The Raman E12g and A1g vibrational modes of crystalline 2H-MoS2 were observed. These results demonstrate the use of dithiocarboxylato ligands for the chemical vapor deposition of metal sulfides.

Charged Particle-Induced Surface Reactions of Organometallic Complexes as a Guide to Precursor Design for Electron- and Ion-Induced Deposition of Nanostructures.

Focused electron beam-induced deposition (FEBID) and focused ion beam-induced deposition (FIBID) are direct-write fabrication techniques that use focused beams of charged particles (electrons or ions) to create 3D metal-containing nanostructures by decomposing organometallic precursors onto substrates in a low-pressure environment. For many applications, it is important to minimize contamination of these nanostructures by impurities from incomplete ligand dissociation and desorption. This spotlight on applications describes the use of ultra high vacuum surface science studies to obtain mechanistic information on electron- and ion-induced processes in organometallic precursor candidates. The results are used for the mechanism-based design of custom precursors for FEBID and FIBID.

The Role of Low-Energy Electron Interactions in cis-Pt(CO)2Br2 Fragmentation.

Platinum coordination complexes have found wide applications as chemotherapeutic anticancer drugs in synchronous combination with radiation (chemoradiation) as well as precursors in focused electron beam induced deposition (FEBID) for nano-scale fabrication. In both applications, low-energy electrons (LEE) play an important role with regard to the fragmentation pathways. In the former case, the high-energy radiation applied creates an abundance of reactive photo- and secondary electrons that determine the reaction paths of the respective radiation sensitizers. In the latter case, low-energy secondary electrons determine the deposition chemistry. In this contribution, we present a combined experimental and theoretical study on the role of LEE interactions in the fragmentation of the Pt(II) coordination compound cis-PtBr2(CO)2. We discuss our results in conjunction with the widely used cancer therapeutic Pt(II) coordination compound cis-Pt(NH3)2Cl2 (cisplatin) and the carbonyl analog Pt(CO)2Cl2, and we show that efficient CO loss through dissociative electron attachment dominates the reactivity of these carbonyl complexes with low-energy electrons, while halogen loss through DEA dominates the reactivity of cis-Pt(NH3)2Cl2.

Electron beam-induced deposition of platinum from Pt(CO)2Cl2 and Pt(CO)2Br2.

Two platinum precursors, Pt(CO)2Cl2 and Pt(CO)2Br2, were designed for focused electron beam-induced deposition (FEBID) with the aim of producing platinum deposits of higher purity than those deposited from commercially available precursors. In this work, we present the first deposition experiments in a scanning electron microscope (SEM), wherein series of pillars were successfully grown from both precursors. The growth of the pillars was studied as a function of the electron dose and compared to deposits grown from the commercially available precursor MeCpPtMe3. The composition of the deposits was determined using energy-dispersive X-ray spectroscopy (EDX) and compared to the composition of deposits from MeCpPtMe3, as well as deposits made in an ultrahigh-vacuum (UHV) environment. A slight increase in metal content and a higher growth rate are achieved in the SEM for deposits from Pt(CO)2Cl2 compared to MeCpPtMe3. However, deposits made from Pt(CO)2Br2 show slightly less metal content and a lower growth rate compared to MeCpPtMe3. With both Pt(CO)2Cl2 and Pt(CO)2Br2, a marked difference in composition was found between deposits made in the SEM and deposits made in UHV. In addition to Pt, the UHV deposits contained halogen species and little or no carbon, while the SEM deposits contained only small amounts of halogen species but high carbon content. Results from this study highlight the effect that deposition conditions can have on the composition of deposits created by FEBID.

Efficient NH3-based process to remove chlorine from electron beam deposited ruthenium produced from (η3-C3H5)Ru(CO)3Cl.

The fabrication of Ru nanostructures by focused electron beam induced deposition (FEBID) requires suitable precursor molecules and processes to obtain the pure metal. So far this is problematic because established organometallic Ru precursors contain large organic ligands, such as cyclopentadienyl anions, that tend to become embedded in the deposit during the FEBID process. Recently, (η3-C3H5)Ru(CO)3X (X = Cl, Br) has been proposed as an alternative precursor because CO can easily desorb under electron exposure. However, allyl and Cl ligands remain behind after electron irradiation and the removal of the halide requires extensive electron exposures. Auger electron spectroscopy is applied to demonstrate a postdeposition purification process in which NH3 is used as a reactant that enhances the removal of Cl from deposits formed by electron irradiation of thin condensed layers of (η3-C3H5)Ru(CO)3Cl. The loss of CO from the precursor during electron-induced decomposition enables a reaction between NH3 and the Cl ligands that produces HCl. The combined use of electron-stimulated desorption experiments and thermal desorption spectrometry further reveals that thermal reactions contribute to the loss of CO in the FEBID process but remove only minor amounts of the allyl and Cl ligands.

Identifying and Rationalizing the Differing Surface Reactions of Low-Energy Electrons and Ions with an Organometallic Precursor.

Surface reactions of electrons and ions with physisorbed organometallic precursors are fundamental processes in focused electron and ion beam-induced deposition (FEBID and FIBID, respectively) of metal-containing nanostructures. Markedly different surface reactions occur upon exposure of nanometer-scale films of (η5-Cp)Fe(CO)2Re(CO)5 to low-energy electrons (500 eV) compared to argon ions (860 eV). Electron-induced surface reactions are initiated by electronic excitation and fragmentation of (η5-Cp)Fe(CO)2Re(CO)5, causing half of the CO ligands to desorb. Residual CO ligands decompose under further electron irradiation. In contrast, Ar+-induced surface reactions proceed by an ion-molecule momentum/energy transfer process, causing the desorption of all CO ligands without significant ion-induced precursor desorption. This initial decomposition step is followed by ion-induced sputtering of the deposited atoms. The fundamental insights derived from this study can be used not only to rationalize the composition of deposits made by FEBID and FIBID but also to inform the choice of a charged particle deposition strategy and the design of new precursors for these emerging nanofabrication tools.

Dissociation of the FEBID precursor cis-Pt(CO)2Cl2 driven by low-energy electrons.

In this study, we present experimental and theoretical results on dissociative electron attachment and dissociative ionisation for the potential FEBID precursor cis-Pt(CO)2Cl2. UHV surface studies have shown that high purity platinum deposits can be obtained from cis-Pt(CO)2Cl2. The efficiency and energetics of ligand removal through these processes are discussed and experimental appearance energies are compared to calculated thermochemical thresholds. The present results demonstrate the potential effectiveness of electron-induced reactions in the deposition of this FEBID precursor, and these are discussed in conjunction with surface science studies on this precursor and the design of new FEBID precursors.

Growth of WOx from Tungsten(VI) Oxo-Fluoroalkoxide Complexes with Partially Fluorinated ß-Diketonate/ß-Ketoesterate Ligands: Comparison of Chemical Vapor Deposition to Aerosol-Assisted CVD.

Tungsten(VI) oxo complexes of the type WO(OR)3L [R = C(CH3)2CF3, C(CF3)2CH3, CH(CF3)2, L = hexafluoroacetylacetonate (hfac), ethyl trifluoroacetoacetonate (etfac), acetylacetonate (acac)] bearing partially fluorinated alkoxide and/or chelating ligands were synthesized. Thermal decomposition behavior and mass spectrometry (MS) fragmentation patterns of selected examples were studied. The thermolysis products of WO(OC(CF3)2CH3)3(hfac) were characterized by nuclear magnetic resonance and gas chromatography-MS. Studies of the sublimation behavior of the complexes demonstrated that their volatility depends on the degree of fluorination. Comparative studies of the deposition of tungsten oxide by chemical vapor deposition (CVD) and aerosol-assisted CVD were carried out using WO(OC(CF3)2CH3)3(hfac) as a single-source precursor. WOx materials were successfully deposited by both deposition methods, but the deposits differed in morphology, structure, and crystallinity.

Focused Electron Beam-Induced Deposition and Post-Growth Purification Using the Heteroleptic Ru Complex (η3-C3H5)Ru(CO)3Br.

Focused electron beam-induced deposition using the heteroleptic complex (η3-C3H5)Ru(CO)3Br as a precursor resulted in deposition of material with Ru content of 23 at. %. Transmission electron microscopy images indicated a nanogranular structure of pure Ru nanocrystals, embedded into a matrix containing carbon, oxygen, and bromine. The deposits were purified by annealing in a reactive 98% N2/2% H2 atmosphere at 300 °C, resulting in a reduction of contaminants and an increase of the Ru content to 83 at. %. Although a significant volume loss of 79% was found, the shrinkage was observed mostly for vertical thickness (around 75%). The lateral dimensions decreased much less significantly (around 9%). Deposition results, in conjunction with previous gas-phase and condensed-phase surface studies on the electron-induced reactions of (η3-C3H5)Ru(CO)3Br, provide insights into the behavior of allyl, carbonyl, and bromide ligands under identical electron beam irradiation.

Design, Synthesis, and Evaluation of CF3AuCNR Precursors for Focused Electron Beam-Induced Deposition of Gold.

The Au(I) complexes CF3AuCNMe (1a) and CF3AuCN tBu (1b) were investigated as Au(I) precursors for focused electron beam-induced deposition (FEBID) of metallic gold. Both 1a and 1b are sufficiently volatile for sublimation at 125 ± 1 mTorr in the temperature range of roughly 40-50 °C. Electron impact mass spectra of 1a-b show gold-containing ions resulting from fragmenting the CF3 group and the CNR ligand, whereas in negative chemical ionization of 1a-b, the major fragment results from dealkylation of the CNR ligand. Steady-state depositions from 1a in an Auger spectrometer produce deposits with a similar gold content to the commercial precursor Me2Au(acac) (3) deposited under the same conditions, while the gold content from 1b is less. These results enable us to suggest the likely fate of the CF3 and CNR ligands during FEBID.

Electron induced surface reactions of (η5-C5H5)Fe(CO)2Mn(CO)5, a potential heterobimetallic precursor for focused electron beam induced deposition (FEBID).

Electron-induced surface reactions of (η5-C5H5)Fe(CO)2Mn(CO)5 were explored in situ under ultra-high vacuum conditions using X-ray photoelectron spectroscopy and mass spectrometry. The initial step involves electron-stimulated decomposition of adsorbed (η5-C5H5)Fe(CO)2Mn(CO)5 molecules, accompanied by the desorption of an average of five CO ligands. A comparison with recent gas phase studies suggests that this precursor decomposition step occurs by a dissociative ionization (DI) process. Further electron irradiation decomposes the residual CO groups and (η5-C5H5, Cp) ligand, in the absence of any ligand desorption. The decomposition of CO ligands leads to Mn oxidation, while electron stimulated Cp decomposition causes all of the associated carbon atoms to be retained in the deposit. The lack of any Fe oxidation is ascribed to either the presence of a protective carbonaceous matrix around the Fe atoms created by the decomposition of the Cp ligand, or to desorption of both CO ligands bound to Fe in the initial decomposition step. The selective oxidation of Mn in the absence of any Fe oxidation suggests that the fate of metal atoms in mixed-metal precursors for focused electron beam induced deposition (FEBID) will be sensitive to the nature and number of ligands in the immediate coordination sphere. In related studies, the composition of deposits created from (η5-C5H5)Fe(CO)2Mn(CO)5 under steady state deposition conditions, representative of those used to create nanostructures in electron microscopes, were measured and found to be qualitatively consistent with predictions from the UHV surface science studies.

N,N-Disubstituted-N'-acylthioureas as modular ligands for deposition of transition metal sulfides.

First row transition metal complexes (Ni, Co, Cu, Zn) with N,N-disubstituted-N'-acylthiourea ligands have been synthesized and characterized. Bis(N,N-diisopropyl-N'-cinnamoylthiourea)nickel was found to have the lowest onset temperature for thermal decomposition. Thin film deposition of Ni, Co, and Zn sulfides by aerosol assisted chemical vapor deposition from their respective N,N-diisopropyl-N'-cinnamoylthiourea complexes at 350 °C has been demonstrated.