PacSurf2014 Session TF-MoM: Self-Organized and Nanostructured Thin Films
Monday, December 8, 2014 8:40 AM in Makai
TF-MoM-1 Growth Kinetics, Structure, and Properties of 2D Layered Materials
Suneel Kodambaka (University of California, Los Angeles)
Two-dimensional (2D) layered materials owing to a wide range of properties (e.g., graphene is metallic, h-BN is insulating, and MoS2 is semiconducting) have attracted immense attention over the past decade for a variety of optoelectronic and nanoelectronic applications. Recent efforts have focused on vertical integration of 2D layers of dissimilar materials (e.g., graphene/h-BN and graphene/MoS2). In these heterostructures, due to relatively weak van der Waals interactions, orientational registry between the layers is not expected and is often difficult to control. This talk will focus on the effect of interlayer orientation on the electronic structure of the resulting heterostructures. Using a combination of in situ low-energy electron microscopy (LEEM) and density functional theory (DFT) calculations, we investigated the electronic properties of graphene/Pd(111). From the LEEM images we determined the graphene growth kinetics and measured graphene work function as a function of orientation and layer thickness. Recently, we extended our DFT calculations to study the electronic structures of h-BN/Ni(111) and graphene/MoS2 heterostructured layers. We found that hBN can chemisorb or physisorb on Ni(111), with metallic or insulating properties, respectively and these properties are not altered when graphene is placed atop hBN. For graphene on MoS2, we found that rotating graphene layer by 30o with respect to MoS2 changes the MoS2 band gap from 1.68 eV direct to 1.56 eV indirect. We attribute the observed orientation-dependent bandgap to the variation in the S-S interplanar distance with the MoS2-graphene interlayer orientation.
TF-MoM-3 Deposition and Tribomechanical Properties of Hf-B-C Thin Films
Elham Mohimi, Tanil Ozkan, Shaista Babar, Peter Sempsrott (University of Illinois at Urbana-Champaign, USA); Andreas Polycarpou (Texas A&M University); Gregory Girolami, John Abelson (University of Illinois at Urbana-Champaign, USA)
The tribomechanical properties of thin film coatings can be enhanced by alloying to afford a multicomponent nanostructure. Our group previously reported the conformal growth and favorable mechanical properties of HfB2 and Hf-B-N hard coatings by chemical vapor deposition (CVD) at low substrate temperature. Here, we alloy C into HfB2 in order to reduce the friction coefficient and enhance the hardness. A useful analogue is C-alloyed TiB2, which exhibits super hardness and good thermal stability. However, there have been no previous studies of Hf-B-C alloys.
Hf-B-C nanocomposite coatings are deposited by CVD onto Si or stainless steel discs using the precursor hafnium borohydride, Hf(BH4)4, with a co-flow of dimethylbutene (DMB), (CH3)3CCH=CH2, as the carbon source. Depositions are performed in a high vacuum chamber with base pressure of 10-8 Torr, hafnium borohydride pressure 0.1-0.5 mTorr and DMB pressure 0.05-0.4 mTorr at substrate temperatures of 250-600°C. DMB also acts as growth inhibitor – it reduces the film growth rate by a factor of 2-6 compared to growth using the precursor alone, an effect which enhances conformality. For higher temperature growth, DMB increases the film density and decreases the surface roughness. XPS analysis indicates a mixture of HfB2, HfCx and B4C phases, however, this is uncertain due to the small shifts between different bonding states.
As-deposited films are XRD amorphous with hardness values of 9-26 Gpa and reduced modulus of 99-208 Gpa. Upon annealing at 700°C for 3 hours under inert gas atmosphere, the films transform to a partially nanocrystalline structure, which increases the hardness to 17-34 Gpa and the elastic modulus to 158-248 Gpa. The tribological properties of Hf-B-C films are superior to those of HfB2 films. This is attributed to graphitic attachment of carbon atoms on contacting surfaces as evidenced by EDS analysis of the wear scar surface. Summarizing, this system affords conformal growth at low growth temperatures, suitable for the fabrication of complex structures such as MEMS.
TF-MoM-4 Effect of Chemical Reaction on Low Friction of Diamond-Like Carbon in Water Lubrication : A Theoretical Study
Shandan Bai, Yasunori Niiyama, Yoshihiko Kobayashi, Yuji Higuchi, Nobuki Ozawa, Koshi Adachi, Shigeyuki Mori, Kazue Kurihara, Momoji Kubo (Tohoku University, Japan)
[Introduction] Diamond-Like Carbon (DLC) coatings have low friction and anti-wear tribological performances. Furthermore, water lubrication improves the friction properties of DLC films and reduces the emission of CO2. The friction coefficient of the DLC films drastically changes under water lubrication, since some tribo-chemical reactions occur during the sliding. However, tribo-chemical reactions are difficult to be revealed only by experimental analyses in details. The computational technique is efficient method to investigate the low friction mechanism . In this study, we reveal the tribo-chemical reaction between DLC film and water using the computational method on an atomic scale.
[Method] To clarify the tribo-chemical reactions of DLC films under water lubrication, we use our tight-binding quantum chemical molecular dynamics (TB-QCMD) method . We construct the sliding simulation model consisting of 80 water molecules and two DLC substrates. The thickness of water is approximately 1.0 nm. The friction simulation is performed for 100,000 steps with the time step of 0.1 fs. We apply contact pressures of 0.5 and 5 GPa on the top layer of upper substrate of DLC films, while it is forcibly slid with a horizontal velocity of 10 m/s. The simulation temperature is set at 300 K, achieved by velocity scaling method.
[Results and Discussion] We perform our TB-QCMD calculations to investigate the low friction properties of DLC in water lubrication. Under a contact pressure of 0.5 GPa, one C-OH bond is generated on the DLC surface at 0.045 ps during the sliding, because of the dissociation of a water molecule. Furthermore, at 3.655 ps, we observe another C-OH bond generation on the surface. The result indicates that OH terminates the DLC surface under a contact pressure of 0.5 GPa. Under a contact pressure of 5 GPa, generation of a C-OH bond is observed on the surface at 1.380 ps. Furthermore, at 3.880 ps, it is very interesting to see the generation of C-O-C on the DLC surface, which is a different chemical reaction with that under pressure of 0.5 GPa. The friction coefficients are 0.81 and 0.05 under contact pressures of 0.5 and 5 GPa, respectively. Those results indicate that the friction coefficient decreases with increasing a contact pressure. We think that the chemical reaction leads to the structure change on the DLC surface and the low friction properties of DLC in water lubrication under high contact pressures.
 S. Bai, M. Kubo et al., J. Phys. Chem. C, 116, 12559, (2012).
 S. Bai, M. Kubo et al., RSC Adv., DIO: 10.1039/c4ra04065a.
TF-MoM-6 Growth of Large-Area 2D Transition Metal Dichalcogenides
Lain-Jong Li (King Abdullah University of Science and Technology, Saudi Arabia)
The direct-gap property of the semiconducting transition metal dichalcogenide (TMD) monolayers is attractive for optoelectronics and energy harvesting. Here I would like to discuss the synthetic approaches to obtain crystalline and sub-mm sized MoS2, WSe2 and WS2 monolayers directly on arbitrary substrates using vapor phase reaction between metal oxides and S or Se powders.1-2 These layer materials can be transferred to desired substrates, making them suitable building blocks for constructing multilayer stacking structures. By using micro-beam X-ray photoelectron spectroscopy, we report the determination of band offsets in TMD heterostructures.3 These physical quantities are fundamentally important for novel devices based on heterostructures formed between dissimilar TMDs. Some possible applications based on TMD heterostructures will be discussed.
1 Huang, J. K. et al. Large-Area Synthesis of Highly Crystalline WSe2 Monolayers and Device Applications. Acs Nano 8, 923 (2014).
2 Lee, Y. H. et al. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Advanced Materials 24, 2320 (2012).
3 M.-H. Chiu et al. Determination of band alignment in transition metal dichalcogenides heterojunctions. arXiv:1406.5137
TF-MoM-8 Influence of Testing Conditions on the Tribological Behaviour of C(N)-WS2 Self Lubricating Thin Films
Albano Cavaleiro, Manuel Evaristo (University of Coimbra, Portugal); Tomas Polcar (University of Southampton, UK)
Transition metal dichalcogenides (TMD) have a layered structure and weak inter-layer bonding allowing to display very low friction coefficient when a tangential force is applied. Being sliding contact a surface phenomenon, these materials have been largely studied in the form of thin coatings. Whenever conditions exist for establishing stronger bonds between the layers, the friction coefficient can significantly increase. This is the reason why the industrial applicability of these coatings is still very limited due to the deficient tribological behavior in humid atmospheres, for which strong bonds can be formed through oxygen. In order to overcome this problem different approaches were followed based on alloying TMD with different elements. Among these elements, our group has developed a deep study on the addition of carbon and nitrogen. We have proved, as it was already known, that the friction coefficient could increase from the range [0.005 – 0.05] up to [0.05 – 0.3] when the coatings were tested in dry or in humid conditions, respectively.The aim of this talk is to present a systematic study concerning the influence of humidity in the tribological behavior of TMD+C coatings. We deposited W-S-C coatings with increasing C content up to 60 at.% with two S/W ratios, close to 1.4 and 1.0. These coatings were tested by pin-on-disk in different humidity range from RH=20% up to RH=95%. Contrarily to what we have observed in previous studies, we could not find any case where the friction coefficient went down lower than 0.1. Furthermore, there was no clear trend on the effect of either the humidity or the S/W ratio on the friction coefficient. The detailed analysis of the sliding surfaces allowed to conclude that, in all tests, orientation of WS2 crystals in the top sliding contact could not be achieved. The comparison with previous deposited W-S-C coatings allowed to conclude that the different tribological behavior could be attributed to a different nanostructure arrangement in the as-deposited conditions.
TF-MoM-9 High-Rate Deposition and Interface Structure Control of Microcrystalline Silicon/Germanium Films
Yi Shi (Nanjing University, China)
Hydrogenated microcrystalline silicon and germanium (μc-Si/Ge:H) films have attracted intense interests in electronic and photovoltaic applications. μc-Si/Ge:H films are generally grown with plasma enhanced chemical vapor deposition using very high hydrogen diluted SiH4/GeH4 as source gas, which makes the deposition rate very lower than that of a-Si:H. The films are usually prepared on foreign substrate at low substrate temperatures. Under this condition, μc-Si experiences an incubation phase to nucleate, leaving an incubation layer, at film/substrate interface. The interface between the film and underlying layer involved the transportation and the recombination of carriers has important influence on device performance. This talk is mainly devoted to the deposition of µc-Si/Ge film at high rate and the control on film/substrate interface structure. We developed the technique combining very high frequency inductively coupled plasma with gas-jet to deposit µc-Si/Ge films, and a high deposition rate of over 20 nm/s was obtained at a low substrate temperature. The high generation rate and the rapid transportation of film growth precursors contribute to this high deposition rate. In jet-ICPCVD, abundant hydrogen atoms are generated, which induces a high crystallinity in film through a hydrogen-induced chemical annealing. Furthermore, we researched the control over the interface structure of µc-Si/substrate to eliminate the amorphous incubation layer. Firstly, we realized the deposition of μc-Si film free of amorphous interface layer and homogeneous in the growth direction at a high rate and a low substrate temperature in the jet-ICPCVD system. The results reveal that high-density plasma, spatial plasma potential, and a high pressure are of substantive benefit to the generation of abundant energetic hydrogen atoms, which can reach and anneal the incubation layer through hydrogen chemical annealing, resulting in the crystallization of the amorphous incubation layer during the growth process. Secondly, it was found that the nucleation of μc-Si is closely correlated with the thickness of the H-treated a-Si underlying layer. On a H-treated ultrathin a-Si:H layer, very thin P-doped μc-Si:H films (~20 nm) with high conductivity (> 1 S/cm) were obtained owing to rapid nucleation. H2 plasma treatment results in an increased compressive stress. Consequently, abundant strained Si-Si bonds and SiHn complexes are generated, promoting the rapid nucleation of μc-Si. Additionally, we investigated the evolution characteristics of μc-Si/Ge:H films during annealing process.