Low Nucleation Density Research Articles

Role of rough substrate on the growth of large single-crystal MoS2 by chemical vapor deposition

Two-dimensional transition metal dichalcogenides show a potential application in photoelectronic devices due to their excellent electronic and optical properties. These excellent properties benefit from large single crystals. Here, we demonstrate that large single crystal monolayer MoS2 can be synthesized on rough SiO2/Si substrate via chemical vapor deposition (CVD) method. Rough SiO2/Si substrate has two advantages. The first one is low nucleation density. The rough substrate with “hill and valley” structures can effectively suppress nucleation density. The lower nucleation density seems to be caused by the reduced surface concentrations of reactants, which has been verified by solving the diffusion equation. Another important advantage is the higher growth rate. We found that the free-standing MoS2 films were successfully grown over the “valley” regions of the rough surface. The calculated growth rate of free-standing MoS2 is three times as that on polished surface, which comes from the introduction of temperature gradient during suspended growth. Our findings have provided new insights into the mechanisms underlying CVD MoS2 growth on rough surfaces and are expected to accelerate the development of directly suspended growth of two-dimensional material for further device applications.

Growth of graphene with large single-crystal domains by Ni foam-assisted structure and its high-gain field-effect transistors.

High-quality graphene materials and high-performance graphene transistors have attracted much attention in recent years. To obtain high-performance graphene transistors, large single-crystal graphene is needed. The synthesis of large-domain-sized single-crystal graphene requires low nucleation density; this can lead to a lower growth rate. In this study, a Ni-foam assisted structure was developed to control the nucleation density and growth rate of graphene by tuning the flow dynamics. Lower nucleation density and high growth rate (∼50 μm min−1) were achieved with a 4 mm-gap Ni foam. With the graphene transistor fabrication process, a pre-deposited Au film as the protective layer was used during the graphene transfer. Graphene transistors showed good current saturation with drain differential conductance as low as 0.04 S mm−1 in the strong saturation region. For the devices with gate length of 2 μm, the intrinsic cut-off frequency fT and maximum oscillation frequency fmax were 8.4 and 16.3 GHz, respectively, with fmax/fT = 1.9 and power gain of up to 6.4 dB at 1 GHz. The electron velocity saturation induced by the surface optical phonons of SiO2 substrates was analyzed. Electron velocity saturation and ultra-thin Al2O3 gate dielectrics were thought to be the reasons for the good current saturation and high power gain of the graphene transistors.

Effects of Initial Structure and Reversion Temperature on Austenite Nucleation Site in Pearlite and Ferrite–Pearlite

Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface > pearlite block > colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels.

Large-area diamond thin film on Q-carbon coated crystalline sapphire by HFCVD

The growth of diamond on transparent substrates like sapphire presents a great challenge because of the large thermal misfit between the film and the substrate, absence of any carbide layer during diamond growth, and low nucleation density during chemical vapor deposition (CVD) growth process. In this study, we report on the use and the role of Q-carbon as an intermediate layer to successfully deposit large-area diamond film on c-sapphire by hot filament chemical vapor deposition (HFCVD). The Q-carbon consists of very high-density diamond tetrahedra which act as the embryo for diamond nucleation. Different techniques such as X-ray diffraction, scanning electron microscopy, and Raman spectroscopy show that continuous diamond films with good crystallinity and without any impurity phase can be deposited on the Q-carbon coated single crystal sapphire substrate. The Q-carbon layer is very adherent and it negates the thermal mismatch between the diamond film and the sapphire substrate. A small blue shift in the Raman peak of the diamond from its equilibrium position suggests the deposition of the CVD diamond film with minimal stress (1.14 GPa). This technique of growing large-area continuous diamond thin film with excellent crystalline quality on a single crystal sapphire substrate can serve as a platform for the development of next-generation corrosion and erosion resistant infrared windows, state-of-the-art optoelectronic devices, and advanced scanning probe microscopy systems.

Monitoring the crystallization of water-saturated granitic melts in real time using the hydrothermal diamond anvil cell

Critical aspects of the crystallization dynamics of H2O-saturated melts of a typical granitic composition as well as granitic melts enriched in lithium (8800 ppm) were investigated in real-time experiments using the hydrothermal diamond anvil cell at 480–700 °C and 220–960 MPa. Complete crystallization of the charges was achieved within 5–118 h with average crystal growth rates ranging from 3 to 41 cm/year for quartz and from 18 to 58 cm/year for alkali feldspars, demonstrating that crystals formed from a silicate melt in the presence of a coexisting aqueous phase crystallize rapidly. The combination of substantial nucleation delays, low nucleation densities, and rapid growth rates for quartz and alkali feldspars led to the formation of euhedral megacrysts of these minerals in the vicinity of clusters comprised of much smaller muscovite or α-spodumene crystals. Subsolidus replacement processes initiated during crystal–fluid interactions after the silicate melt was consumed were directly observed in the experiments. The experimental results underscore the important role of water as a medium for the transport of essential elements such as Si, Al, Na, and K from the silicate melt to the newly formed crystals, and provide important insights into the crystallization of miarolitic pegmatites.

How Low Nucleation Density of Graphene on CuNi Alloy is Achieved.

CuNi alloy foils are demonstrated to be one of the best substrates for synthesizing large area single‐crystalline graphene because a very fast growth rate and low nucleation density can be simultaneously achieved. The fast growth rate is understood to be due the abundance of carbon precursor supply, as a result of the high catalytic activity of Ni atoms. However, a theoretical understanding of the low nucleation density remains controversial because it is known that a high carbon precursor concentration on the surface normally leads to a high nucleation density. Here, the graphene nucleation on the CuNi alloy surfaces is systematically explored and it is revealed that: i) carbon atom dissolution into the CuNi alloy passivates the alloy surface, thereby drastically increasing the graphene nucleation barrier; ii) carbon atom diffusion on the CuNi alloy surface is greatly suppressed by the inhomogeneous atomic structure of the surface; and iii) a prominent increase in the rate of carbon diffusion into the bulk occurs when the Ni composition is higher than the percolation threshold. This study reveals the key mechanism for graphene nucleation on CuNi alloy surfaces and provides a guideline for the catalyst design for the synthesis of graphene and other 2D materials.

Diamond nucleation and growth on WC-Co inserts with Cr2O3-Cr interlayer

Cr2O3-Cr interlayer was prepared by magnetron sputtering to restrict graphite formation on WC-Co before diamond nucleation. Continuous diamond coatings containing little graphite phase were successfully deposited on single Cr2O3 interlayered WC-Co substrates. For Cr2O3/Cr duplex interlayer, a low nucleation density was observed at a low CH4 concentration, but a continuous diamond coating was readily obtained with an increased CH4 concentration during nucleation stage. X-ray diffraction and X-ray absorption analysis reveal that the Cr top layer was carburized to Cr3C2 and Cr7C3 after diamond deposition while Cr2O3 layer kept unchanged. In addition, the indentation test shows that the diamond coating has poor adhesion to the substrate with a single Cr2O3 interlayer, while the adhesion was significantly enhanced with the Cr2O3/Cr duplex interlayer combined with a high CH4 concentration for nucleation.

Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve.

To grow small molecule semiconductor thin films with domain size larger than modern-day device sizes, we evaporate the material through a dense array of small apertures, called a stencil nanosieve. The aperture size of 0.5 μm results in low nucleation density, whereas the aperture-to-aperture distance of 0.5 μm provides sufficient crosstalk between neighboring apertures through the diffusion of adsorbed molecules. By integrating the nanosieve in the channel area of a thin-film transistor mask, we show a route for patterning both the organic semiconductor and the metal contacts of thin-film transistors using one mask only and without mask realignment.

Gas phase infiltration of carbon nanotubes in Ni Nanofoam via liquid injection chemical vapor deposition

We demonstrate a new approach for improving the specific surface area of Ni nanofoam (NiNF) by exploiting its high void fraction for gas-phase infiltration of carbon nanotubes (CNTs) via liquid injection chemical vapor deposition (LICVD) process. To address the poor thermal stability of NiNF at CNT growth temperatures and low CNT nucleation density, a two-step pretreatment approach is implemented. First, a thin graphitic coating is deposited to stabilize NiNF via CH4 CVD at 550°C. Second, Fe catalyst is activated by depositing a thin layer of AlxOy (~2nm) on the graphite-coated NiNF. The infiltration of CNTs at 750°C increases the BET specific surface area of the resulting hybrid NiNF by a factor of over 20. A major appeal of this approach is that it can be utilized for tuning the structural properties of other nanostructures with relatively low temperature tolerance.

Porosity-induced weakening of piezoresponse in GaN layers by means of piezoelectric force microscopy

Nanoporous (NP) GaN thin films with low nucleation density were fabricated via UV-assisted electrochemical etching approach. Under different applied biases, two NP samples with different porosities were fabricated. Piezoelectric characteristics of the fabricated samples were investigated using piezoresponse force microscopy. It was found that the nanopores have great influence on its piezoelectric characteristics such as weakening of piezoresponse and change of polarization orientation. During the performance of devices, the weakening of piezoresponse should be propitious to the optoelectronic and electronic stabilities of GaN-based devices.

Behavior and role of superficial oxygen in Cu for the growth of large single-crystalline graphene

Decreasing the nucleation density of graphene grown on copper (Cu) foil by chemical vapor deposition (CVD) is essential for the synthesis of large-area single-crystalline graphene. Here, the behavior of the copper oxide layer and its impact on the graphene growth have been investigated. We found that a small amount of oxygen dissolves into the Cu when the oxide layer decomposes during the heating up in a non-reducing Ar environment. The remaining oxygen in the Cu foil can play an important role in decreasing the graphene nucleation density. The dissolved oxygen can withstand at high temperatures even in reducing H2 environments without completely losing its effectiveness for maintaining a low graphene nucleation density. However, heating up in a H2 environment significantly reduces the copper oxide layer during the very first moments of the process at low temperatures, preventing the oxygen to dissolve into the Cu and significantly increasing the nucleation density. These findings will help to improve the graphene growth on Cu catalyst by increasing the grain size while decreasing the grain density.

Improved quality of graphene in the absence of hydrogen in a low-temperature growth process using an alcohol precursor

We present the results of low-temperature growth of graphene on polycrystalline copper foil surfaces at 800 °C by using low-pressure chemical-vapor deposition of alcohol precursors. The structural quality of the graphene sample was found to depend significantly on the ambient conditions during the annealing and the growth processes. The improved quality of graphene grown in an oxidizing environment was found to be associated with a lower nucleation density, suggesting that chemisorbed oxygen atoms play a critical role in determining the quality of graphene.

Concurrent fast growth of sub-centimeter single-crystal graphene with controlled nucleation density in a confined channel.

The synthesis of large-domain-sized graphene requires a low nucleation density, which inevitably leads to a reduced growth rate. To achieve both a large domain size and high growth rate, we designed a simple channel structure that allowed us to control the nucleation density by tuning the flow dynamics and by introducing an additional catalyst inside to control the growth kinetics at the same time. The designed channel structure plays three roles in the growth of graphene: (1) it retains oxygen to passivate the active nucleation sites; (2) it restricts the mass transfer of CH4 to control the supersaturation for nucleation; and (3) it provides additional catalytic sites for the decomposition of CH4 to boost the graphene growth rate. Our strategy allowed the successful preparation of sub-centimeter-domain-sized graphene in 1 h with an average growth rate of 70 μm min-1, and with a hole mobility of 5500 cm2 V-1 S-1, which is sufficient for practical applications. Our method paves the way for the large-scale production of single-crystal graphene or other 2D materials at a highly efficient level.

Supercooling-dependent morphology evolution of an organic nucleating agent in poly(l-lactide)/poly(d-lactide) blends

The morphology evolution of an organic nucleating agent (NA) in poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blends was investigated under a series of multi-step isothermal and nonisothermal processes at identical NA solubility achieved via the same NA concentration and final molten temperature of heating (Tf). It was found that the morphology of NA frameworks depended on the supercooling determined by the recrystallization temperature of the NA in the polymer melt, except for NA solubility. When the NA partly dissolved in the matrix, the residuum of the original flat-plate NA acting as a nucleus induced recrystallization at a low degree of supercooling, and small sheet-like NA particles formed in the melt. When the NA dissolved in the matrix absolutely, NA aggregation increased in the melt due to the low nucleation density of the NA itself. The shape of the NA aggregation depended on the recrystallization temperature. With the increase in the supercooling, the large aggregated sheet-like NA transformed into a dendritic-like framework accompanied by a decrease in the stem size of the NA frameworks. Moreover, the stereocomplex efficiency of the blend was influenced by the effective nucleation surface induced by the shape of the NA aggregation. For PLLA/PDLA blends with 1.0 wt% NA, the crystallization half time (t1/2) of stereocomplex (sc) crystallites at the crystallization temperature of 180 °C is as long as 79.8 min for the sample with a large aggregated sheet-like NA, with 1 order of magnitude reduction for that with a dendritic-like NA framework, leading to a t1/2 of 7.9 min. The morphological control of NA frameworks is of great significance to guide the formation of various crystal morphologies of sc crystallites. The established relationship between the self-assembled morphology of NA and NA concentration, NA solubility and supercooling affords guidance in practical processing to control the crystal morphology and polymorphic crystalline morphology of PLLA/PDLA blends via tuning the crystal morphology of NA in the polymer matrix.

QM/MD studies on graphene growth from small islands on the Ni(111) surface.

Quantum chemical molecular dynamics simulations of graphene growth from small island precursors in different carbon nucleation densities on the Ni(111) surface at high temperatures have been conducted. The results indicate that small islands are not static, i.e. lateral diffusion and vertical fluctuation are frequently observed. In the case of low carbon nucleation density, carbon atoms or small carbon patches diffuse and attach to the edge of the nuclei to expand the size of the growing carbon network. The growth of graphene precursors is accompanied by the corresponding changes in the bonding of nickel atoms with the precipitation of subsurface carbon atoms. This is because the carbon-carbon interaction is stronger than the nickel-carbon interaction. In the case of high carbon nucleation densities, the dominant ripening mechanism depends on different growth stages. In the initial stage, the coalescence of carbon islands takes place via the Smoluchowski ripening mechanism. In the later stage the Smoluchowski ripening process is damped owing to the higher diffusion barrier of larger clusters and the restriction of movement by self-assembled nickel step edges. The cross-linking mechanism eventually takes over by the coalescence of extended polyyne chains between graphene islands. In either case, the Ostwald ripening process is not found in our molecular dynamics simulations due to the stability of carbon-carbon bonds within the islands. These investigations should be instructive to the control of graphene growth in experiments.

Influence of biodegradation in thermophilic anaerobic aqueous conditions on crystallization of poly(butylene succinate)

This work investigates the biodegradation of poly(butylene succinate) (PBS) in an environment of thermophilic (55 °C) and mesophilic (37 °C) anaerobic sludge. The thermophilic anaerobic sludge was prepared in a laboratory from mesophilic sludge sourced from a municipal waste water treatment plant and placed under thermophilic conditions. It was confirmed that PBS failed to decompose under mesophilic anaerobic conditions (2 wt. %), while the degree of biodegradation achieved under thermophilic conditions reached 24.8 wt. %. Abiotic hydrolysis of PBS in an environment of phosphate buffer (pH = 7) at 55 °C reached 17.8%, leading to the conclusion that hydrolytic enzymes present in thermophilic anaerobic sludge are involved in the biodegradation process as well as abiotic hydrolysis. Nonisothermal crystallization kinetics of PBS before and after biodegradation was investigated by differential scanning calorimetry (DSC). After biodegradation, a large shift in melting temperature Tm was observed, while shift of crystallization temperature Tc was even more pronounced. Lower Tm means smaller crystals. Crystallization kinetics was approximately 50% slower, which could be explained by lower nucleation density. Scanning electron microscopy (SEM) revealed a beautiful spherulitic structure after biodegradation at elevated temperature. During biodegradation, microorganisms assimilated mostly in the amorphous part of the polymer, and elevated temperature helped healing of crystal imperfections. By X-ray diffraction (XRD), it was found that biodegradation does not have a large effect on crystal form of PBS.

Evolution of magnetic domain structure formed by ion-irradiation of B2-Fe0.6Al0.4.

Magnetic domains and magnetization reversal in 40 nm thick films of Fe0.6Al0.4, have been studied by longitudinal magneto-optical Kerr effect. By varying the Ne(+) ion-energy E between 2 and 30 keV (keeping a constant fluence), we varied the depth-penetration of the ions, and thereby influenced the homogeneity of the induced saturation magnetization M(s). The dependence of coercivity on ion energy shows maximum for 5 keV Ne(+). Considerable differences in the magnetic domain formation and magnetization reversal processes were observed: at low E (≤ 5keV), the reversal process is dominated by domain nucleation mechanism (high density of domain nucleation sites), consistent with the occurrence of an inhomogeneous M(s). Films irradiated with E > 5keV ions exhibit significantly low domain nucleation density, and the reversal is dominated by domain propagation mechanism, suggesting homogeneity in induced M(s). These results demonstrate the tunability of magnetization reversal behavior in materials possessing disorder induced magnetic phase transitions.

Tailoring the Growth Rate and Surface Facet for Synthesis of High-Quality Continuous Graphene Films from CH4 at 750 °C via Chemical Vapor Deposition

Previous work has demonstrated that the growth temperature for the chemical vapor deposition of graphene can be decreased by tailoring the precursor composition. Here, we fix the precursor as CH4 and instead explore the effects of the catalyst facet, synthesis conditions, and growth rate on the quality of graphene grown at reduced temperature on Cu. We find that in order to obtain graphene films with low defect density, it is critical to maintain a slow growth rate, which is achieved using a low CH4 partial pressure. Furthermore, growth on Cu(111) is more efficient than on other low index Cu facets. Using optimized growth conditions, we achieve high-quality continuous monolayer graphene films, with a low nucleation density of 4 × 10–2 μm–2 and a negligible Raman D:G ratio, at 750 °C, which is 125 °C cooler than previous reports achieving full graphene coverage on Cu. The field-effect mobility is 2600 cm2 V–1 s–1 at a carrier concentration of 1012 cm–2, and low-energy electron microscopy indicates that there is one predominant graphene crystallographic orientation with respect to the underlying Cu(111). These results demonstrate that by tailoring the catalyst facet and growth rate, the temperature required to achieve high-quality continuous films, using a given precursor, can be reduced without sacrificing the excellent properties of graphene.

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

Study on fracture behavior of PLLA transcrystallization: Effect of crystalline morphology

ABSTRACTIn the present work, the fracture behavior of transcrystallization (TC) of poly(l‐lactic acid) (PLLA) in thin film and the dependence of mechanical properties on the morphology of TC have been studied. The nucleation density of TC was merely determined by the annealing temperatures of the fibers which used for inducing nucleation, and the crystallization temperature and time of the samples were completely identical. By using in situ polarized optical microscopy, the fracture process of TCs was characterized. For the TC with high nucleating density (TC‐H), lots of cracks were generated from the TC bulk during fracture. But only few cracks were observed on the TC with low nucleation density (TC‐L), and the final fracture of TC‐L always occurred in the junctions of crystal segments. Compared to the samples which do not contain TC, the fracture strength was enhanced by 8.1% because of the presence of TC‐H. On the contrary, the presence of TC‐L can reduce the fracture strength of the samples. The fracture surfaces of TC were characterized by scanning electron microscope. It was observed that the fracture surface of TC‐H exhibited obvious fibrillation and cavitation, but the fracture surface of TC‐L was smooth and featureless. The possible fracture mechanism for two TCs was discussed in view of their intrinsic crystal organizations. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41273.