Spatially controlled fabrication of a bright fluorescent nanodiamond-array with enhanced far-red Si-V luminescence.

We demonstrate a novel approach to precisely pattern fluorescent nanodiamond-arrays with enhanced far-red intense photostable luminescence from silicon-vacancy (Si-V) defect centers. The precision-patterned pre-growth seeding of nanodiamonds is achieved by a scanning probe 'dip-pen' nanolithography technique using electrostatically driven transfer of nanodiamonds from 'inked' cantilevers to a UV-treated hydrophilic SiO2 substrate. The enhanced emission from nanodiamond dots in the far-red is achieved by incorporating Si-V defect centers in a subsequent chemical vapor deposition treatment. The development of a suitable nanodiamond ink and mechanism of ink transport, and the effect of humidity and dwell time on nanodiamond patterning are investigated. The precision patterning of as-printed (pre-CVD) arrays with dot diameter and dot height as small as 735 nm ± 27 nm and 61 nm ± 3 nm, respectively, and CVD-treated fluorescent ND-arrays with consistently patterned dots having diameter and height as small as 820 nm ± 20 nm and, 245 nm ± 23 nm, respectively, using 1 s dwell time and 30% RH is successfully achieved. We anticipate that the far-red intense photostable luminescence (~738 nm) observed from Si-V defect centers integrated in spatially arranged nanodiamonds could be beneficial for the development of next generation fluorescence-based devices and applications.

High Entropy Borides Synthesized by the Thermal Reduction of Metal Oxides in a Microwave Plasma.

Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf0.2, Zr0.2, Ti0.2, Ta0.2, Mo0.2)B2 made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon.

Sputtered tungsten-based ternary and quaternary layers for nanocrystalline diamond deposition.

Many of today's demanding applications require thin-film coatings with high hardness, toughness, and thermal stability. In many cases, coating thickness in the range 2-20 microm and low surface roughness are required. Diamond films meet many of the stated requirements, but their crystalline nature leads to a high surface roughness. Nanocrystalline diamond offers a smoother surface, but significant surface modification of the substrate is necessary for successful nanocrystalline diamond deposition and adhesion. A hybrid hard and tough material may be required for either the desired applications, or as a basis for nanocrystalline diamond film growth. One possibility is a composite system based on carbides or nitrides. Many binary carbides and nitrides offer one or more mentioned properties. By combining these binary compounds in a ternary or quaternary nanocrystalline system, we can tailor the material for a desired combination of properties. Here, we describe the results on the structural and mechanical properties of the coating systems composed of tungsten-chromium-carbide and/or nitride. These WC-Cr-(N) coatings are deposited using magnetron sputtering. The growth of adherent nanocrystalline diamond films by microwave plasma chemical vapor deposition has been demonstrated on these coatings. The WC-Cr-(N) and WC-Cr-(N)-NCD coatings are characterized with atomic force microscopy and SEM, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and nanoindentation.

Improved adhesion of ultra-hard carbon films on cobalt-chromium orthopaedic implant alloy.

While interfacial graphite formation and subsequent poor film adhesion is commonly reported for chemical vapor deposited hard carbon films on cobalt-based materials, we find the presence of O(2) in the feedgas mixture to be useful in achieving adhesion on a CoCrMo alloy. Nucleation studies of surface structure before formation of fully coalesced hard carbon films reveal that O(2) feedgas helps mask the catalytic effect of cobalt with carbon through early formation of chromium oxides and carbides. The chromium oxides, in particular, act as a diffusion barrier to cobalt, minimizing its migration to the surface where it would otherwise interact deleteriously with carbon to form graphite. When O(2) is not used, graphitic soot forms and films delaminate readily upon cooling to room temperature. Continuous 1 µm-thick nanostructured carbon films grown with O(2) remain adhered with measured hardness of 60 GPa and show stable, non-catastrophic circumferential micro-cracks near the edges of indent craters made using Rockwell indentation.

Bias-Enhanced Formation of Metastable and Multiphase Boron Nitride Coating in Microwave Plasma Chemical Vapor Deposition.

Boron nitride (BN) is primarily a synthetically produced advanced ceramic material. It is isoelectronic to carbon and, like carbon, can exist as several polymorphic modifications. Microwave plasma chemical vapor deposition (MPCVD) of metastable wurtzite boron nitride is reported for the first time and found to be facilitated by the application of direct current (DC) bias to the substrate. The applied negative DC bias was found to yield a higher content of sp3 bonded BN in both cubic and metastable wurtzite structural forms. This is confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Nano-indentation measurements reveal an average coating hardness of 25 GPa with some measurements as high as 31 GPa, consistent with a substantial fraction of sp3 bonding mixed with the hexagonal sp2 bonded BN phase.

Selective Deposition of Hard Boron-Carbon Microstructures on Silicon.

Boron-rich B-C compounds with high hardness have been recently synthesized by the chemical vapor deposition (CVD) method. In this paper, we present our successful efforts in the selective growth of microstructures of boron-carbon compounds on silicon substrates. This was achieved by combining microfabrication techniques such as maskless lithography and sputter deposition with the CVD technique. Our characterization studies on these B-C microstructures showed that they maintain structural and mechanical properties similar to that of their thin-film counterparts. The methodology presented here paves the way for the development of microstructures for microelectromechanical system (MEMS) applications which require custom hardness and strength properties. These hard B-C microstructures are an excellent choice as support structures in MEMS-based devices.

Superhard Boron-Rich Boron Carbide with Controlled Degree of Crystallinity.

Superhard boron-rich boron carbide coatings were deposited on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) under controlled conditions, which led to either a disordered or crystalline structure, as measured by X-ray diffraction. The control of either disordered or crystalline structures was achieved solely by the choice of the sample being placed either directly on top of the sample holder or within an inset of the sample holder, respectively. The carbon content in the B-C bonded disordered and crystalline coatings was 6.1 at.% and 4.5 at.%, respectively, as measured by X-ray photoelectron spectroscopy. X-ray diffraction analysis of the crystalline coating provided a good match with a B50C2-type structure in which two carbon atoms replaced boron in the α-tetragonal B52 structure, or in which the carbon atoms occupied different interstitial sites. Density functional theory predictions were used to evaluate the dynamical stability of the potential B50C2 structural forms and were consistent with the measurements. The measured nanoindentation hardness of the coatings was as high as 64 GPa, well above the 40 GPa threshold for superhardness.

First-Principles Predictions and Synthesis of B50C2 by Chemical Vapor Deposition.

Density functional theory predictions have been combined with the microwave-plasma chemical vapor deposition technique to explore metastable synthesis of boron-rich boron-carbide materials. A thin film synthesis of high-hardness (up to 37 GPa) B50C2 via chemical vapor deposition was achieved. Characterization of the experimental crystal structure matches well with a new theoretical model structure, with carbon atoms inserted into the boron icosahedra and 2b sites in a α-tetragonal B52 base structure. Previously reported metallic B50C2 structures with carbons inserted only into the 2b or 4c sites are found to be dynamically unstable. The newly predicted structure is insulating and dynamically stable, with a computed hardness value and electrical properties in excellent agreement with the experiment. The present study thus validates the density functional theory calculations of stable crystal structures in boron-rich boron-carbide system and provides a pathway for large-area synthesis of novel materials by the chemical vapor deposition method.

Non-equilibrium organosilane plasma polymerization for modulating the surface of PTFE towards potential blood contact applications.

We report a novel and facile organosilane plasma polymerization method designed to improve the surface characteristics of poly(tetrafluoroethylene) (PTFE). We hypothesized that the polymerized silane coating would provide an adhesive surface for endothelial cell proliferation due to a large number of surface hydroxyl groups, while the large polymer networks on the surface of PTFE would hinder platelet attachment. The plasma polymerized PTFE surfaces were then systematically characterized via different analytical techniques such as FTIR, XPS, XRD, Contact angle, and SEM. The key finding of the characterization is the time-dependent deposition of an organosilane layer on the surface of PTFE. This layer was found to provide favorable surface properties to PTFE such as a very high surface oxygen content, high hydrophilicity and improved surface mechanics. Additionally, in vitro cellular studies were conducted to determine the bio-interface properties of the plasma-treated and untreated PTFE. The important results of these experiments were rapid endothelial cell growth and decreased platelet attachment on the plasma-treated PTFE compared to untreated PTFE. Thus, this new surface modification technique could potentially address the current challenges associated with PTFE for blood contact applications, specifically poor endothelial cell growth and risk of thrombosis.

Nanomechanical properties of electrospun composite scaffolds based on polycaprolactone and hydroxyapatite.

Fibrous nanocomposite scaffolds were electrospun from dispersions of hydroxyapatite nanoparticles (nanoHA) in polycaprolactone (PCL) with varying nanoHA contents (from 0% to 50% by weight). Such scaffolds were produced to mimic the nano-features of the extracellular matrix (ECM) for natural bone tissue regeneration. NanoHA was found to be well dispersed in the PCL fibers up to the addition of 30 wt%, whereas beads and agglomeration of HA particles was observed above this nanoHA concentration. The structural and morphological characterizations were evaluated by scanning electron microscopy (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD). The average fiber diameter decreased with increased nanoHA concentration. The nanomechanical properties of the as-spun fibrous scaffolds as well as pressure-consolidated (pelletized) composites were evaluated by nanoindentation. Elastic modulus increased with increasing HA content, but was especially pronounced for 40-50% HA content where the indenter tip is more likely to probe agglomerated HA particles.

Molecularly Imprinted Polyacrylamide with Fluorescent Nanodiamond for Creatinine Detection.

Creatinine measurement in blood and urine is an important diagnostic test for assessing kidney health. In this study, a molecularly imprinted polymer was obtained by incorporating fluorescent nanodiamond into a creatinine-imprinted polyacrylamide hydrogel. The quenching of peak nanodiamond fluorescence was significantly higher in the creatinine-imprinted polymer compared to the non-imprinted polymer, indicative of higher creatinine affinity in the imprinted polymer. Fourier transform infrared spectroscopy and microscopic imaging was used to investigate the nature of chemical bonding and distribution of nanodiamonds inside the hydrogel network. Nanodiamonds bind strongly to the hydrogel network, but as aggregates with average particle diameter of 3.4 ± 1.8 µm and 3.1 ± 1.9 µm for the non-imprinted and molecularly imprinted polymer, respectively. Nanodiamond fluorescence from nitrogen-vacancy color centers (NV- and NV0) was also used to detect creatinine based on nanodiamond-creatinine surface charge interaction. Results show a 15% decrease of NV-/NV0 emission ratio for the creatinine-imprinted polymer compared to the non-imprinted polymer, and are explained in terms of changes in the near-surface band structure of diamond with addition of creatinine. With further improvement of sensor design to better disperse nanodiamond within the hydrogel, fluorescent sensing from nitrogen-vacancy centers is expected to yield higher sensitivity with a longer range (Coulombic) interaction to imprinted sites than that for a sensor based on acceptor/donor resonance energy transfer.

Mesenchymal stem cell interaction with ultra-smooth nanostructured diamond for wear-resistant orthopaedic implants.

Ultra-smooth nanostructured diamond (USND) can be applied to greatly increase the wear resistance of orthopaedic implants over conventional designs. Herein we describe surface modification techniques and cytocompatibility studies performed on this new material. We report that hydrogen (H)-terminated USND surfaces supported robust mesenchymal stem cell (MSC) adhesion and survival, while oxygen- (O) and fluorine (F)-terminated surfaces resisted cell adhesion, indicating that USND can be modified to either promote or prevent cell/biomaterial interactions. Given the favorable cell response to H-terminated USND, this material was further compared with two commonly used biocompatible metals, titanium alloy (Ti-6Al-4V) and cobalt chrome (CoCrMo). MSC adhesion and proliferation were significantly improved on USND compared with CoCrMo, although cell adhesion was greatest on Ti-6Al-4V. Comparable amounts of the pro-adhesive protein, fibronectin, were deposited from serum on the three substrates. Finally, MSCs were induced to undergo osteoblastic differentiation on the three materials, and deposition of a mineralized matrix was quantified. Similar amounts of mineral were deposited onto USND and CoCrMo, whereas mineral deposition was slightly higher on Ti-6Al-4V. When coupled with recently published wear studies, these in vitro results suggest that USND has the potential to reduce debris particle release from orthopaedic implants without compromising osseointegration.

Rapid Growth of Nanocrystalline Diamond on Single Crystal Diamond for Studies on Materials under Extreme Conditions.

Early stage nucleation morphologies of spatially localized nanocrystalline diamond (NCD) micro-anvils grown on (100)-oriented single crystal diamond (SCD) anvil surfaces were analyzed and investigated for applications in high pressure studies on materials. NCD was grown on SCD using Microwave Plasma Chemical Vapor Deposition (MPCVD) for brief time intervals ranging from 1-15 minutes. Early stage film morphologies were characterized using scanning electron microscopy (SEM) and Raman spectroscopy and were compared to films grown for several hours. Rapid nucleation and growth of NCD on SCD is demonstrated without any pre-growth seeding of the substrate surface. As grown NCD diamond micro-anvils on SCD were used to generate static pressure of 0.5 Terapascal (TPa) on a tungsten sample as measured by synchrotron x-ray diffraction in a diamond anvil cell. Atomic force microscopy (AFM) analysis after decompression from ultrahigh pressures showed that the detachment of the NCD stage occurred in the bulk of the SCD and not at the interface, suggesting significant adhesive bond strength between nanocrystalline and single crystal diamond.

Computational Predictions and Microwave Plasma Synthesis of Superhard Boron-Carbon Materials.

Superhard boron-carbon materials are of prime interest due to their non-oxidizing properties at high temperatures compared to diamond-based materials and their non-reactivity with ferrous metals under extreme conditions. In this work, evolutionary algorithms combined with density functional theory have been utilized to predict stable structures and properties for the boron-carbon system, including the elusive superhard BC5 compound. We report on the microwave plasma chemical vapor deposition on a silicon substrate of a series of composite materials containing amorphous boron-doped graphitic carbon, boron-doped diamond, and a cubic hard-phase with a boron-content as high as 7.7 at%. The nanoindentation hardness of these composite materials can be tailored from 8 GPa to as high as 62 GPa depending on the growth conditions. These materials have been characterized by electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and nanoindentation hardness, and the experimental results are compared with theoretical predictions. Our studies show that a significant amount of boron up to 7.7 at% can be accommodated in the cubic phase of diamond and its phonon modes and mechanical properties can be accurately modeled by theory. This cubic hard-phase can be incorporated into amorphous boron-carbon matrices to yield superhard materials with tunable hardness values.

Mesenchymal stem cell adhesion and spreading on microwave plasma-nitrided titanium alloy.

Improved methods to increase surface hardness of metallic biomedical implants are being developed in an effort to minimize the formation of wear debris particles that cause local pain and inflammation. However, for many implant surface treatments, there is a risk of film delamination due to the mismatch of mechanical properties between the hard surface and the softer underlying metal. In this article, we describe the surface modification of titanium alloy (Ti-6Al-4V), using microwave plasma chemical vapor deposition to induce titanium nitride formation by nitrogen diffusion. The result is a gradual transition from a titanium nitride surface to the bulk titanium alloy, without a sharp interface that could otherwise lead to delamination. We demonstrate that vitronectin adsorption, as well as the adhesion and spreading of human mesenchymal stem cells to plasma-nitrided titanium is equivalent to that of Ti-6Al-4V, while hardness is improved 3- to 4-fold. These in vitro results suggest that the plasma nitriding technique has the potential to reduce wear, and the resulting debris particle release, of biomedical implants without compromising osseointegration; thus, minimizing the possibility of implant loosening over time.

Ultra-smooth nanostructured diamond films deposited from He/H2/CH4/N2 microwave plasmas.

Addition of He to a high CH4 content (10.7 vol%) H2/CH4/N2 feedgas mixture for microwave plasma chemical vapor deposition produced hard (58-72 GPa), ultra-smooth nanostructured diamond films on Ti-6AI-4V alloy substrates. Upon increase in He content up to 71 vol%, root mean squared (RMS) surface roughness of the film decreased to 9-10 nm and average diamond grain size to 5-6 nm. Our studies show that increased nanocrystallinity with He addition in plasma is related to plasma dilution, enhanced fragmentation of carbon containing species, and enhanced formation of CN radical.

Synthesis of ultrasmooth nanostructured diamond films by microwave plasma chemical vapor deposition using a He/H(2)/CH(4)/N(2) gas mixture.

Ultrasmooth nanostructured diamond (USND) films were synthesized on Ti-6Al-4V medical grade substrates by adding helium in H(2)/CH(4)/N(2) plasma and changing the N(2)/CH(4) gas flow from 0 to 0.6. We were able to deposit diamond films as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 µm(2). Grain size was 4-5 nm at 71% He in (H(2) + He) and N(2)/CH(4) gas flow ratio of 0.4 without deteriorating the hardness (~50-60 GPa). The characterization of the films was performed with AFM, scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond films. The plasma species during deposition were monitored by optical emission spectroscopy. With increasing N(2)/CH(4) feedgas ratio (CH(4) was fixed) in He/H(2)/CH(4)/N(2) plasma, a substantial increase of CN radical (normalized by Balmer H(α) line) was observed along with a drop in surface roughness up to a critical N(2)/CH(4) ratio of 0.4. The CN radical concentration in the plasma was thus correlated to the formation of ultrasmooth nanostructured diamond films.

A wear simulation study of nanostructured CVD diamond-on-diamond articulation involving concave/convex mating surfaces.

Using microwave-plasma Chemical Vapor Deposition (CVD), a 3-micron thick nanostructured-diamond (NSD) layer was deposited onto polished, convex and concave components that were machined from Ti-6Al-4V alloy. These components had the same radius of curvature, 25.4mm. Wear testing of the surfaces was performed by rotating articulation of the diamond-deposited surfaces (diamond-on-diamond) with a load of 225N for a total of 5 million cycles in bovine serum resulting in polishing of the diamond surface and formation of very shallow, linear wear grooves of less than 50nm depth. The two diamond surfaces remained adhered to the components and polished each other to an average surface roughness that was reduced by as much as a factor of 80 for the most polished region located at the center of the condyle. Imaging of the surfaces showed that the initial wearing-in phase of diamond was only beginning at the end of the 5 million cycles. Atomic force microscopy, scanning electron microscopy, Raman spectroscopy, and surface profilometry were used to characterize the surfaces and verify that the diamond remained intact and uniform over the surface, thereby protecting the underlying metal. These wear simulation results show that diamond deposition on Ti alloy has potential application for joint replacement devices with improved longevity over existing devices made of cobalt chrome and ultra-high molecular weight polyethylene (UHMWPE).

Nanoindentation on porous bioceramic scaffolds for bone tissue engineering.

We report nanoindentation mechanical properties measurements on porous ceramic scaffolds made for tissue engineering applications. The scaffolds have been made from tricalcium phosphate (TCP), hydroxyapatite (HA) nanopowder and mixed powders of HA (50 wt%) and TCP (50 wt%) using the polyurethane sponge method, which produces open porous ceramic scaffolds through replication of a porous polymer template. The scaffolds prepared by this method have a controllable pore size and interconnected pore structure. The crystal structures and morphology of porous scaffolds were determined by X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. Nanoindentation measurements to a depth of 600 nm showed a Young's modulus value of 10.3 GPa for HA+TCP composite scaffolds and 1.5 GPa for TCP scaffolds. The hardness values were 240 MPa for HA+TCP composites and 21 MPa for TCP sample respectively. The results showed that the mechanical properties of the biodegradable scaffolds can be considerably enhanced with the addition of HA while maintaining the interconnected open pores and pore geometry desirable for bone tissue engineering.

Mesenchymal stem cell adhesion and spreading on nanostructured biomaterials.

Bone marrow-derived human mesenchymal stem cells were seeded in serum-free media onto ion beam-deposited nanostructured metalloceramic (Ti-Cr-N) films and plasma-nitrided titanium disks, which were left uncoated as well as precoated with fetal bovine serum. Precoating the disks with serum appears to stimulate cell spreading on both the titanium nitride and metalloceramic materials for as little as 1 hour incubation time. The implication is that both of these materials can adsorb serum proteins in amounts sufficient to influence cell adhesion and spreading for potentially improved in vivo response of orthopedic and dental implants. The materials in this study may prove to exhibit enhanced biological and mechanical properties when compared to conventional micron-scale implant materials such as titanium or cobalt-chrome alloys.