Low Nucleation Density Research Articles

Controllable growth of large 1T-NbTe2 nanosheets on mica by chemical vapor deposition and its magnetic properties

Two-dimensional (2D) magnetic materials have recently attracted broad attention for their potential technological applications. Here, we report controllable growth of NbTe2 nanosheets on mica substrate by atmospheric chemical vapor deposition. Different from the SiO2/Si substrates, dangling bonds on the surface of the mica are absent, which induces low nucleation density and higher lateral growth rate. As a result, the lateral size of NbTe2 nanosheets on mica substrates can be significantly larger than those on SiO2/Si substrates. Large single crystal NbTe2 nanosheets with a lateral size of up to 151 µm were fabricated on a mica. High-resolution transmission electron microscope image, XRD patterns and Raman spectra reveal the NbTe2 nanosheets adopt the single crystal with 1T phase. Furthermore, the NbTe2 nanosheets on mica demonstrate ferromagnetic properties with a Curie temperature of up to 162 K. Our work paves the way for the atmospheric chemical vapor deposition growth and applications of many more 2D magnets.

High-temperature tribological properties and wear mechanisms of multilayered cubic-BN/nanocrystalline diamond tool coatings

Wear failure of cutting inserts is the main form of failure that occurs when cutting difficult-to-machine materials at high speeds. To obtain cutting inserts with outstanding tribological properties, multilayered cubic boron nitride (cBN) and nanocrystalline diamond (NCD) coatings (cBN/NCD) were prepared on WC-6 wt% Co tool substrates. The tribological properties of the multilayered cBN/NCD coatings were studied at different temperatures, and the results showed that the coefficient of friction (COF) and wear rate of the coatings were closely interrelated to the temperatures, and the COF gradually decrease and the wear rate gradually increase as the temperature increased from room temperature (25 °C) to 600 °C. The COF and wear rate of the multilayer coatings at 600 °C were ∼0.05 and 8.43 × 10−6 mm3/(N⋅m), respectively. The wear resistance of the cBN/NCD coatings was higher than that of TiAlN coatings and PcBN tool materials by more than 3 and 4.5 times, respectively. The high-temperature wear failure modes of the multilayered cBN/NCD coatings were abrasive wear, diamond graphitization, and BN hydrolysis. Coating wear accelerated when pores were formed on the cBN surface of a multilayered cBN/NCD coating due to the low nucleation density of diamond. These results would guide the application of cBN/NCD-coated cutting tools in high-speed cutting processes.

High-quality MAPbI3 film prepared by drip-pressing method in the air for perovskite solar cells

Perovskite solar cells are generally fabricated by one-step or two-step solution deposition in an inert atmosphere. It is obvious that large-scale perovskite film preparation process in the air is the future trend. However, perovskite materials are easily degraded by water, resulting in low nucleation density and incomplete coverage of perovskite film. Here, we choose the drip-pressing method to regulate the quantity of moisture, which can also balance the grains growth and solvent volatility. The perovskite films were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), UV–visible absorption, steady-state photoluminescence (PL) and in situ PL measurement. The orientation and quality of perovskite crystal are improved significantly in drip pressing, compared with traditional spin-coating. In addition, it facilitates to adjust the crystallization kinetics of perovskite by altering the imposed pressure in drip pressing. The grain size increases first and then decreases as pressure increases, reaching a maximum at 10 kPa. Meanwhile, the films become more and more compact as pressure increases. Perovskite films show great photoelectric performance at 30 kPa. The photoelectric conversion efficiency of the device in air can be improved from 10.99% to 13.21%. The results indicate that the drip-pressing method is in favor of high-quality of perovskite crystal in air.

Relationship between graphene nucleation density and epitaxial growth orientation on Cu(111) surfaces

Crystal domain alignment is required to produce large, single-crystal, two-dimensional materials which are synthesized by epitaxial growth, but it has been unclear what is needed to achieve the alignment. It is presumed to be possible when the symmetries of the two-dimensional material and substrate are consistent. However, experiments show that although graphene and the Cu(111) surface have the same symmetry, graphene domains grown on Cu(111) are not always well aligned. We developed a physical model and used first principles calculations to explain these observations. It is found that small hexagonal graphene domains are always misorientated on the Cu(111) substrate because the vertexes of the graphene domains have preferred binding sites. These misorientation angles vary with the domain size, with near-perfect alignment of the domains on the substrate only being achieved when they become hundreds of nanometers in size. Our results indicate that low nucleation density of graphene on Cu(111) surfaces is necessary to achieve epitaxial alignment of graphene domains.

Growth and characterization of uniformly distributed triangular single-crystalline hexagonal boron nitride grains on liquid copper surface

Single-layer hexagonal boron nitride (h-BN) is a two-dimensional (2D) material with a wide band gap and extraordinary mechanical, thermal and optical properties. It has promising applications in optoelectronics, electronics and photovoltaics. Low-pressure (150 Pa) chemical vapor deposition (LPCVD) was used to prepare uniformly distributed single-crystalline triangular h-BN grains and continuous film on liquid copper surface. A series of microscopic and spectroscopic methods were performed to characterize the morphology, crystalline structure and quality, component and thickness. It was found that h-BN grains turned into circles at higher background pressure (1000 Pa), which was due to the change in the growth mechanism from equilibrium controlled to deposition controlled. Small islands formed on primary h-BN grains because of higher concentration of precursor product. Compared to the higher background pressure, the synthetic h-BN films at low background pressure exhibited larger domain size, lower nucleation density, and no adlayer growth. Due to randomly distributed polar h-BN grains, the grain boundaries formed along the h-BN domains after merging. Typical patterns formed via the coalescence of triangular h-BN grains indicating the merging mode of growth, including edge-to-edge and edge-to-point modes. This work provides a pathway for the preparation of uniformly distributed single-crystalline h-BN grains and an in-depth understanding of the growth and merging process on liquid Cu surface.

Impact of nitrogen doping on homoepitaxial diamond (111) growth

The impacts of nitrogen (N) doping on the lateral growth mode during two-dimensional (2D) nucleation, on the growth rate, and the incorporation of nitrogen (concentration [N]) of (111)-oriented diamond films were investigated by modulating the [N2]/[CH4] gas admixture ratio. The 2D nucleation density first increased with increasing [N2]/[CH4] ratio between 0.02 and 20%. Further increase of the [N2]/[CH4] ratio to up 200% caused a decrease of the nucleation density. The growth rates showed the similar N-doping dependence as the nucleation density variation. This is attributed to an initial increase of CN radicals in the regime 0.02 to 20%, followed by a reduction of CHx radicals in the regime 20 to 200%. The nitrogen incorporation concentration increases with increasing the [N2]/[CH4] ratio. The highest nitrogen concentration with 2 × 1020 atoms/cm3 is detected with [N2]/[CH4] = 200% and a relatively low nucleation density is achieved. These results are beneficial for the optimized formation of nitrogen-vacancy centers used in a quantum metrology and for device application such as inversion-channel diamond MOSFETs.

Direct van deer Waals epitaxy of multiband-emitting InGaN-based LEDs on graphene for phosphor-free white light illumination

The white light-emitting diodes (LEDs), a kind of representative multiband light emitters, which are generally manufactured by coating phosphor with InGaN-based blue LED chips, are widely used in illumination and display. However, the degradation and wavelength-conversion loss of phosphor are still issues which shorten the life of these LEDs. Herein, we demonstrate a convenient way to fabricate phosphor-free multiband-emitting LEDs without any external process or additional dielectric interlayers, utilizing only graphene as substrate for direct van deer Waals epitaxy (vdWE) of multiple-facet InGaN-based LEDs. Taking advantage of the low nucleation density of GaN on graphene, the surface morphology of the films was controlled as complete voids without flat. The morphology of InGaN/GaN multi quantum wells followed with the multiple-facet GaN, which caused the variation of indium concentration in different lattice plane of multi quantum well and led to multiband emission. Those findings reveal the roles of graphene in growth of III-nitride and provide valuable insights into the application of graphene for phosphor-free white light illumination.

Effects of the Phase Content on Spallation Damage Behavior in Dual-Phase Steel

The dual-phase steel (martensite (M)/ferrite (F)) samples with different phase content were dynamic loaded simultaneously by one-stage light gas gun. The velocity of free surface particles was measured by Doppler pin system (DPS) during the loading experiment. The soft recovered samples were investigated with optical microscopy, nanoindentation, and EBSD techniques to study the effect of the phase content on dynamic damage evolution in dual-phase steel. The results show that the sample two (1000°C/60 min + 780°C/30 min+quenching) has a higher M area percentage (77.2%), larger M size, and smaller number of M and less M/F interfaces compared with the sample one (1000°C/60 min + 740°C/30 min+quenching, with M area percentage of 45.51%). Due to the reflection and transmission of shock wave at M/F interface, tensile stress will be generated inside M with higher shock impedance. Under the same dynamic loading conditions, the more M/F interface means the greater the probability of void nucleation inside M. Thus the sample two with less M/F interfaces has lower nucleation density and lower spallation strength. The microcrack propagation resistance increases with the increase in the area percentage of M and the size of M, which results in the lower damage evolution rate of the sample two. Meanwhile, the size of M will affect the direction of microcracks propagation. Each M with larger size in the sample two is composed of several prior austenite (A) grains, and the orientation of the martensite packets in prior A grains is very different. Therefore, the microcrack propagation in the sample two is limited to different regions, the direction of microcracks propagation is easy to be deflected.

Hexagonal Boron Nitride Crystal Growth from Iron, a Single Component Flux.

The highest quality hexagonal boron nitride (hBN) crystals are grown from molten solutions. For hBN crystal growth at atmospheric pressure, typically the solvent is a combination of two metals, one with a high boron solubility and the other to promote nitrogen solubility. In this study, we demonstrate that high-quality hBN crystals can be grown at atmospheric pressure using pure iron as a flux. The ability to produce excellent-quality hBN crystals using pure iron as a solvent is unexpected, given its low solubility for nitrogen. The properties of crystals produced with this flux matched the best values ever reported for hBN: a narrow Raman E2g vibration peak (7.6 cm-1) and strong phonon-assisted peaks in the photoluminescence spectra. To further test their quality, the hBN crytals were used as a substrate for WSe2 epitaxy. WSe2 was deposited with a low nucleation density, indicating the low defect density of the hBN. Lastly, the carrier tunneling through our hBN thin layers (3.5 nm) follows the Fowler-Nordheim model, with a barrier height of 3.7 eV, demonstrating hBN's superior electrical insulating properties. This ability to produce high-quality hBN crystals in such a simple, environmentally friendly and economical process will advance two-dimensional material research by enabling integrated devices.

Mechanism of Additive-Assisted Room-Temperature Processing of Metal Halide Perovskite Thin Films.

Perovskite solar cells have received substantial attention due to their potential for low-cost photovoltaic devices on flexible or rigid substrates. Thiocyanate (SCN)-containing additives, such as MASCN (MA = methylammonium), have been shown to control perovskite film crystallization and the film microstructure to achieve effective room-temperature perovskite absorber processing. Nevertheless, the crystallization pathways and mechanisms of perovskite formation involved in MASCN additive processing are far from clear. Using in situ X-ray diffraction and photoluminescence, we investigate the crystallization pathways of MAPbI3 and reveal the mechanisms of additive-assisted perovskite formation during spin coating and subsequent N2 drying. We confirm that MASCN induces large precursor aggregates in solution and, during spin coating, promotes the formation of the perovskite phase with lower nucleation density and overall larger initial nuclei size, which forms upon reaching supersaturation in solution, in addition to intermediate solvent-complex phases. Finally, during the subsequent N2 drying, MASCN facilitates the dissociation of these precursor aggregates and the solvate phases, leading to further growth of the perovskite crystals. Our results show that the nature of the intermediate phases and their formation/dissociation kinetics determine the nucleation and growth of the perovskite phase, which subsequently impact the film microstructure. These findings provide mechanistic insights underlying room-temperature, additive-assisted perovskite processing and help guide further development of such facile room-temperature synthesis routes.

Nitrogen doped ultrananocrystalline diamond conductive layer grown on InGaN-based light-emitting diodes using nanopattern enhanced nucleation

This study grew nitrogen doped ultrananocrystalline diamond (N-UNCD) conductive layer on InGaN-based light emitting diodes (LEDs) using the concave nanopattern (CNP) enhanced nucleation. The low nucleation density of the N-UNCD on bare LEDs (Br-LED) resulted in an island morphology. The nucleation density of the N-UNCD increased significantly from 1.8 × 108 cm−2 for Br-LED to 3.6 × 109 cm−2 for LEDs with a CNP density of 6.1 × 109 cm−2 (CNP-LED). The N-UNCD preferred to nucleate inside the CNP during the initial growth stage. The N-UNCD islands then merged and grew laterally to form a continuous thin film within a thickness of 300 nm. The N-UNCD/CNP-LED exhibited a stable electroluminescence peak wavelength of ~447.1 nm in the injection current range of 10–100 mA. The decrease in the compressive stress due to removing the p-GaN layer (i.e. the CNP structure) resulted in a quantum confined Stark effect (QCSE) mitigation in the multi-quantum wells (MQWs). In addition, the lower turn on voltage caused a lower electric field in the MQWs and diminished the screen of the QCSE. The N-UNCD prepared by the proposed nucleation technique demonstrated a promising conductive layer for InGaN-based LEDs.

GaN Single Crystalline Domain Growth on Graphene Substrate

Graphene is very well known 2D nano-material which have novel physical and chemical properties even in a few layer thick form. Recently, there have been extensive studies on hetero-structures based on these Van-der-Waals 2D crystals like graphene, transition metal dichalcogenide (TMD), boron nitride (BN) and so on. In the case of graphene, this 2D layer can be stable and can be transferred to other target substrate with ease. Therefore, graphene can be a good candidate materials for growing other active materials on it. In this study, we tried to grow GaN micro-domain on graphene substrate using hydride vapor phase epitaxy (HVPE) method. GaN epitaxial behavior can be affected by the condition of graphene substrate and we compared the growth behavior of GaN micro-domain on graphene changing the preparation condition. And, several experimental parameters could be controlled like flow rate of Ga and N source with carrier gas, temperature and time to get the proper morphology of GaN micro-crystals. Despite of lattice mis-match of hexagonal GaN on graphene, we could grow single crystalline GaN micro-domain having the lateral size of a few or lager microns. We characterized these micro-crystalline GaN domains using optical microscopy, secondary electron microscope (SEM), micro-Raman spectroscopy and x-ray diffraction (XRD) which confirmed the crystalline phase of each domain. As a typical Van-der-Waals crystal, pristine non-defective graphene may have lacking dangling bonds leading to very low nucleation density for GaN growth and can be used even as a mask layer. When grown on the transferred graphene, however, we expect to have enough nucleation sites originating from the physical defects like grain boundary, vacancy and ripples of poly-crystalline graphene layer. This experimental finding is very interesting to further studies of graphene applications for hetero-structures of graphene and other complementary active materials because graphene has disadvantage in the applications of switching devices, optical elements and can be a potential candidate to overcome these weak points by utilizing very efficiently active GaN together.[1] Y. Kim et al., Nature, 544, p.340 (2017)[2] V. Kumaresan et al,, Nano Letters, 16, p.4895 (2016)

The Role of Demixing and Crystallization Kinetics on the Stability of Non-Fullerene Organic Solar Cells.

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10-23 and 2.0 × 10-22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

Growth and Stress Analysis of Spontaneous Nucleation c‐Plane Bulk AlN Crystals by a PVT Method

AbstractAlN is a kind of promising semiconductor material for high‐efficiency optoelectronic devices and high‐power high‐frequency electronic devices. In this work, high‐quality c‐plane AlN single crystal with low‐stress is grown on tungsten (W) substrate using a spontaneous nucleation physical vapor transport (PVT) method. Temperature field simulation and theoretical analysis provide a theoretical guide for the AlN crystals growth experiment. The hexagonal appearance c‐plane AlN crystal is obtained with a size up to 3 mm × 3 mm. A large radial temperature gradient and suitable supersaturation lead to a low nucleation density and the growth of AlN crystal with large size, which can be used to regulate nucleation density and size of AlN crystals on W substrate. X‐ray diffraction (XRD), Raman and electron backscatter diffraction (EBSD) results confirm that the crystal is grown along the c‐axis direction with hexagonal wurtzite structure and high crystalline quality. The Raman micro‐mapping results indicate that the grown AlN crystal has small tensile stress, close to free‐stress, and the biaxial stress decreases from interface to surface along with the increasing of crystal thickness. This work will greatly contribute to the research of AlN crystal growth on W substrate.

Millimeter sized graphene domains through in situ oxidation/reduction treatment of the copper substrate

In chemical vapor deposition of graphene, low nucleation density and large domain sizes are desirable. In the particular case of growth on copper, throughout recent years, several works have reported the oxidation of the substrate through various means, which leads to a reduced nucleation density. In this work, an in situ oxidation and reduction treatment is explored, establishing its connection with the removal of carbon impurities from the copper bulk, as well as clarifying the role of these impurities as graphene nucleation sites. The extent of the contribution of these impurities to the growth of graphene is inferred by experiments using carbon-13 methane feed. The effect of the oxidation and reduction treatment on the substrate’s surface roughness is also addressed. Lastly, the synthesis recipe based on this treatment is supplemented with a growth stage splitting, an increased argon flowrate and a short etching stage, resulting in millimeter sized hexagonal graphene domains (diameters 2–3 mm) with improved electrical transport properties. Thus, the deposition process presented here stands out on account of its streamlined nature, which makes it an attractive candidate for industrial scale-up.

Fabrication of high porosity Nanocellular polymer foams based on PMMA/PVDF blends

Due to special micro-nano scale heat transfer effect and the ultra-large specific area, nanocellular polymer foams (NPFs), especially NPFs with high porosity, have great application potential in the fields of heat insulation, filtration, and catalysis. However, since the thermodynamic instability of nanoscale bubbles causes lower nucleation density and cells rupture, developing a green and cost-effective method for fabricating high porosity NPFs remains a challenge. Hence, we proposed a novel method for fabricating high porosity NPFs based the design of materials system and foaming process, and the NPFs with a porosity of over 70%, an average cell size of 287 nm, and a nucleation density of over 1014/cm3 were obtained. Firstly, based on the excellent compatibility between poly (methyl methacrylate) (PMMA) and poly (vinylidene fluoride) (PVDF), homogeneous PMMA/PVDF blends were prepared by melt compounding. Then, the PMMA/(PVDF-microcrystals) system was formed by self-assembly through CO2 induced crystallization. Thanks to numerous homo-dispersed PVDF crystallites' facilitating heterogeneous nucleation and enhancing polymer strength, high porosity NPFs with tailored cellular structures were fabricated by heating foaming. This method provides a simple, flexible, and environmentally friendly way for the preparation of high porosity NPFs.

Unprecedented differences in the diamond nucleation density between carbon- and silicon-faces of 4H-silicon carbides

4H-silicon carbides deposited by diamond films have wide applications in many fields such as semiconductor heterojunction, heat sink and mechanical sealing. Nucleation plays a critical role in the deposition of the diamond film on 4H-silicon carbides. Nevertheless, as a typical polar material, the fundamental mechanism of diamond nucleation on different faces of 4H-silicon carbides has not been fully understood yet. In this contribution, nucleation of diamond was performed on the carbon- and silicon-faces of 4H-silicon carbides in a direct current chemical vapor deposition device. The nucleation density on the carbon-face is higher by 2–3 orders of magnitude compared to the silicon-face. Transmission electron microscopy verifies that there are high density diamond nuclei on the interface between the carbon-face and the diamond film, which is different from columnar diamond growth structure on the silicon-face. Transition state theory calculation reveals that the unprecedented distinction of the nucleation density between the carbon-face and the silicon-face is attributed to different desorption rates of the absorbed hydrocarbon radicals. In addition, kinetic model simulations demonstrate that it is more difficult to form CH2(s)-CH2(s) dimers on silicon-faces than carbon-faces, resulting in much lower nucleation densities on silicon-faces.

Growth and carrier transport performance of single-crystalline monolayer graphene over electrodeposited copper film on quartz glass

The synthesis of single crystal graphene domains on cold rolling Cu foil has recently been reported. However, the cold rolling Cu foils have many rolling lines that increase the roughness of the foil. In this work, we developed an electrodeposited method to achieve high-quality Cu film on which the nucleation density of graphene decreased greatly. The Cu film was electrodeposited over chemical plated silver film on quartz glass. The roughness and thickness of the Cu film were positively correlated with the deposition temperature and current density, respectively. High-quality single crystal graphene with low nucleation density was achieved on electrodeposited Cu film. With increasing deposition temperature of Cu film from 20 to 65 °C, the nucleation density of graphene increased from 124 to 448 mm−2. The nucleation density of graphene increased from 124 to 192 mm−2 with increasing current density from 0.03 to 0.07 A/cm−2. Finally, a back-gated graphene field effect transistor (FET) was fabricated with a carrier transport performance of μh = ~4041 cm2V−1s−1 and μe = ~2580 cm2V−1s−1 at room temperature.

Controlled Nucleation of Graphene Domains on Copper With an Oxide Layer by Atmospheric Pressure Chemical Vapor Deposition

The lack of effective control on the defects and layers of graphene restricts its advanced technological applications. The synthesis of high-quality graphene requires a low nucleation density. By the pre-oxidation of copper foil and subsequent annealing in reducing atmosphere with different time, the effect of surface morphology variation on the nucleation density of graphene domains were discussed, as well as on the domain size. The obtained domain is the combination of sub-millimeter-size single-crystal graphene layer and thick multilayer graphene branches. The formation mechanism of the special structure was explored, as the accumulation of carbon atoms at the surface impurities with the help of oxygen. These results provide directions for the synthesis of controllable high-quality graphene films.

Temperature‐Dependent Nucleation and Growth of Dendrite‐Free Lithium Metal Anodes

AbstractIt is essential to develop a facile and effective method to enhance the electrochemical performance of lithium metal anodes for building high‐energy‐density Li‐metal based batteries. Herein, we explored the temperature‐dependent Li nucleation and growth behavior and constructed a dendrite‐free Li metal anode by elevating temperature from room temperature (20 °C) to 60 °C. A series of ex situ and in situ microscopy investigations demonstrate that increasing Li deposition temperature results in large nuclei size, low nucleation density, and compact growth of Li metal. We reveal that the enhanced lithiophilicity and the increased Li‐ion diffusion coefficient in aprotic electrolytes at high temperature are essential factors contributing to the dendrite‐free Li growth behavior. As anodes in both half cells and full cells, the compact deposited Li with minimized specific surface area delivered high Coulombic efficiencies and long cycling stability at 60 °C.