Nanocrystalline Diamond Thin Films Research Articles

Diamond-coated fiber Bragg grating through the hot filament chemical vapor process for chemical durability improvement.

In recent years, the coating of fiber Bragg gratings (FBGs) with a specific material has opened up the possibility to broaden the limits of applicability of this technology. Diamond has a set of properties that makes it an attractive candidate to protect the optical fiber against chemically harsh environments, whose sensing is also a great challenge. One of the most used techniques to obtain these coatings is through the hot filament chemical vapor deposition (HFCVD); in this process, the temperature reaches, typically, around 850°C-900°C. In this work, the regeneration of a seed FBG during its coating with a nanocrystalline diamond thin film through the HFCVD process is presented. Simultaneously, the thermal monitoring of the process was also performed using the same grating. The resistance test in a corrosive medium reveals an improvement on the durability of the sensing properties of the diamond-coated FBG compared with an uncoated FBG, foreseeing a vast range of applications.

Electrostatic Self-Assembly of Diamond Nanoparticles onto Al- and N-Polar Sputtered Aluminum Nitride Surfaces.

Electrostatic self-assembly of diamond nanoparticles (DNPs) onto substrate surfaces (so-called nanodiamond seeding) is a notable technique, enabling chemical vapor deposition (CVD) of nanocrystalline diamond thin films on non-diamond substrates. In this study, we examine this technique onto differently polarized (either Al- or N-polar) c-axis oriented sputtered aluminum nitride (AlN) film surfaces. This investigation shows that Al-polar films, as compared to N-polar ones, obtain DNPs with higher density and more homogeneously on their surfaces. The origin of these differences in density and homogeneity is discussed based on the hydrolysis behavior of AlN surfaces in aqueous suspensions.

Structure, Electronic Properties and Electrochemical Behavior of a Boron-Doped Diamond/Quartz Optically Transparent Electrode

This presentation will review the morphology, microstructure, chemistry, electronic properties and electrochemical behavior of a boron-doped nanocrystalline diamond (BDD) thin film grown on quartz. Diamond optically transparent electrodes (OTEs) are useful for transmission spectroelectrochemical measurements offering excellent stability during anodic and cathodic polarization and exposure to a variety of chemical environments.1,2 An example will be shown of how the OTE in a spectroelectrochemical measurement can be used to study the redox mechanism of the model analyte, chlorpromazine.1 We will also report on the characterization of the BDD OTE by AFM, optical spectroscopy, Raman spectroscopic mapping, AC Hall Effect measurements, XPS and electrochemical methods. Cyclic voltammetry was used to investigate the background current response as a function of potential, scan rate and electrolyte composition. Cyclic voltammetry and digital simulation were used to determine the heterogeneous electron-transfer rate constants (ko) for IrCl6 2-/3-, Fe(CN)6 3-/4-, ferrocene carboxylic acid, Ru(NH3)6 3+/2+ and methyl viologen. The diamond OTE supports relatively rapid electron transfer for a wide range of redox systems with formal potentials from ca. 0.9 to -1.0 V vs. Ag/AgCl. Capacitance-potential measurements were made in aqueous electrolyte solutions and ionic liquids to better understand the organization of electrified interfaces formed at this electrode. The work constitutes the first comprehensive study of the relationship between the physical and chemical structure and electronic properties of a diamond OTE, and the electrode’s electrochemical activity. 1. Stotter, J; Zak J; Behler Z; Show Y; Swain GM. Optical and Electrochemical Properties of Optically Transparent, Boron-Doped Diamond Thin Films on Quartz. Anal. Chem. 2002, 74, 5924-5930. 2. Stotter J; Show Y; Wang S; Swain GM. Comparison of the Electrical, Optical and Electrochemical Properties of Diamond and Indium Tin Oxide Thin-Film Electrodes. Chem. Mater. 2005, 17, 4880-4888.

Structure, Electronic Properties, and Electrochemical Behavior of a Boron-Doped Diamond/Quartz Optically Transparent Electrode.

The morphology, microstructure, chemistry, electronic properties, and electrochemical behavior of a boron-doped nanocrystalline diamond (BDD) thin film grown on quartz were evaluated. Diamond optically transparent electrodes (OTEs) are useful for transmission spectroelectrochemical measurements, offering excellent stability during anodic and cathodic polarization and exposure to a variety of chemical environments. We report on the characterization of a BDD OTE by atomic force microscopy, optical spectroscopy, Raman spectroscopic mapping, alternating-current Hall effect measurements, X-ray photoelectron spectroscopy, and electrochemical methods. The results reported herein provide the first comprehensive study of the relationship between the physical and chemical structure and electronic properties of a diamond OTE and the electrode's electrochemical activity.

Influence of non-adherent yeast cells on electrical characteristics of diamond-based field-effect transistors

Diamond thin films provide unique features as substrates for cell cultures and as bio-electronic sensors. Here we employ solution-gated field effect transistors (SGFET) based on nanocrystalline diamond thin films with H-terminated surface which exhibits the sub-surface p-type conductive channel. We study an influence of yeast cells (Saccharomyces cerevisiae) on electrical characteristics of the diamond SGFETs. Two different cell culture solutions (sucrose and yeast peptone dextrose–YPD) are used, with and without the cells. We have found that transfer characteristics of the SGFETs exhibit a negative shift of the gate voltage by −26mV and −42mV for sucrose and YPD with cells in comparison to blank solutions without the cells. This effect is attributed to a local pH change in close vicinity of the H-terminated diamond surface due to metabolic processes of the yeast cells. The pH sensitivity of the diamond-based SGFETs, the role of cell and protein adhesion on the gate surface and the role of negative surface charge of yeast cells on the SGFETs electrical characteristics are discussed as well.

Influence of nanocrystalline diamond on resonant properties of gold plasmonic antennas

We report on the influence of a nanocrystalline diamond (NCD) thin film or its segments on optical response of gold plasmonic single and dimer antennas in the visible and near‐infrared. We have simulated optical response of these antennas to the electromagnetic radiation in the wavelength range 500–1500 nm. As the response, we take the localized plasmon resonant wavelength and electric field enhancement around the antennas, localized on the top of NCD films or close to NCD segments. Compared to the antennas on a bare glass substrate, the presence of NCD near the antennas causes a significant red‐shift of the antenna resonant wavelength and slightly decreases the antenna resonant scattering cross section. As for the antenna near‐field, it is more enhanced at the antenna–NCD interface than at the antenna–glass one; however, this field is more confined to the interface.

Low temperature growth of diamond films on optical fibers using Linear Antenna CVD system

It is not trivial to achieve a good quality diamond-coated fibre interface due to a large difference in the properties and composition of the diamond films (or use coating even) and the optical fibre material, i.e. fused silica. One of the biggest problems is the high temperature during the deposition which influences the optical fibre or optical fibre sensor structure (e.g. long-period gratings (LPG)). The greatest advantage of a linear antenna microwave plasma enhanced chemical vapor deposition system (LA MW CVD) is the fact that it allows to grow the diamond layers at low temperature (below 300°C) [1].High quality nanocrystalline diamond (NCD) thin films with thicknesses ranging from 70 nm to 150 nm, were deposited on silicon, glass and optical fibre substrates [2]. Substrates pretreatment by dip-coating and spin coating process with a dispersion consisting of detonation nanodiamond (DND) in dimethyl sulfoxide (DMSO) with polyvinyl alcohol (PVA) has been applied. During the deposition process the continuous mode of operation of the LA MW CVD system was used, which produces a continuous wave at a maximum power of 1.9 kW (in each antenna). Diamond films on optical fibres were obtained at temperatures below 350°C, providing a clear improvement of results compared to our earlier work [3].The samples were characterized by scanning electron microscopy (SEM) imaging to investigate the morphology of the nanocrystalline diamond films. The film growth rate, film thickness, and optical properties in the VIS-NIR range, i.e. refractive index and extinction coefficient will be discussed based on measurements on reference quartz plates by using spectroscopic ellipsometry (SE).

Improved nanodiamond seeding on chromium by surface plasma pretreatment

Surface pretreatment by gas discharge plasmas of N2, O2 and CF4 is studied for enhanced diamond nucleation on Cr surfaces. The seeding density following the interaction of water-dispersed nanodiamonds (NDs) and the Cr surface is enhanced due to chemical modification of a surface. The surface that is untreated or pretreated with N2 plasma possesses a suppressed electrostatic attraction of NDs, while the pretreatment with O2 or CF4 plasmas render a strong electrostatic attraction and high seeding density. Finally, by this method thin nanocrystalline diamond films are achieved on Cr surfaces after O2 and CF4 plasma pretreatments.

Thermal etching of nanocrystalline diamond films

A dry process for selective etching of nanocrystalline diamond thin films has been developed as an alternative to plasma etching. This process relies on subjecting masked diamond films to a controlled oxygen atmosphere at temperatures of 700–800°C to controllably etch both vertically through the film and laterally underneath the mask. SiO2, SiNx, and Al2O3 films constitute viable mask materials for this process, provided that the underlying diamond film is fully outgassed before mask deposition and diamond etching. As expected, etching occurred more rapidly at higher temperatures. The etch rate was higher in the lateral direction than the vertical direction, which has been attributed to accelerated etching along disordered grain boundaries and the underlying nucleation layer. Similar activation energies (136–140kJ/mol) were obtained for both lateral and vertical etching from 700 to 800°C. Using the dry etch process developed in this research, diamond films can be removed from exposed features and undercut masked regions at a controlled rate, as indicated by microscopy and Raman spectroscopy.

Further improvements of nano-diamond structures on unheated substrates by optimization of parameters with secondary plasma in MW-PECVD

In order to provide a particular energy threshold for the required reactions to take place, formation of good quality nanocrystalline diamond (NCD) films generally needs high substrate temperature (typically ~700°C), high H2-dilution of the CHn precursors and elevated electrical power to the plasma. For low temperature deposition, high electrical power appears essential to sufficiently aggravate the dynamics of the precursors; however, the growing surface of the NCD film simultaneously gets damaged by the direct high energy ion collision. A buffering technique in the high power zone has been proposed to mitigate the surface damage by introducing secondary plasma in MW-PECVD, through a simple but tricky approach. The present investigation demonstrates how by proper and systematic optimization of the parametric conditions, including load matching through significant reduction in volume of the tubular reactor, core-shell like NCD structures of size in the range 5–14nm with (111) crystallographic orientation in the core and ~85.4% sp3 content in the overall network, have been grown on unheated substrates, at P=500W and p=50Torr, and at significantly low H2 dilution of ~50% with CH4. In the course of further improvements at elevated MW power, P=1000W, and optimum H2-dilution, NCD thin films with grains of diameter in the range 7–13nm and prominent <111> and <200> crystalline diamond planes, along with the ~84.8% sp3 content in the overall carbon network were obtained on unheated substrates, by pursuing its growth under secondary plasma of (CH4+H2) generated within stainless steel grid-like multiple mask assembly, while simultaneously protecting the surface from vigorous high energy ionic bombardments in MW-PECVD. To the best of our knowledge this exclusive method of synthesis of NCDs using secondary plasma has never been reported earlier, which may greatly enhance the understanding of the growth process and open up a new field of NCD synthesis with numerous application possibilities.

Estimation of Requirements for the Formation of Nanocrystalline Diamond Driven by Electron Beam Ablation

Conditions for the formation of thin films of nanocrystalline diamond by electron-beam ablation of a graphite target are studied. The analysis is based on the predictions of the analytical solutions of two models describing the expansion of the plasma plume into a background gas and toward a substrate. The models allow the calculation of the pressure and temperature of ablated nanoparticles upon impact with the substrate where the film is deposited. The calculation data are reported on the phase diagram of carbon and are used to assess conditions under which the diamond phase is likely to form on the substrate. The results show that decreasing gas pressure and target to substrate distance, over the practical range of accelerating voltage, is conducive to diamond formation.

Photoemission properties of nanocrystalline diamond thin films on silicon

The authors have built up a dedicated ultrahigh vacuum setup to measure ultraviolet (266 nm photons) photoemission properties of nanocrystalline diamond thin films obtained by chemical vapor deposition on silicon substrates. The authors validated their setup by measuring polycrystalline copper quantum efficiency of ∼10−6, which is in good agreement with literature. The authors also measured quantum efficiency of bare silicon (highly p and n doped) and demonstrate strong influence of doping type. The authors then measured quantum efficiency of silicon samples coated with submicron (50 and 100 nm thick) nanocrystalline diamond layers. This coating reveals to have major influence on the photoemission properties when deposited on highly n-doped silicon samples. The authors obtain quantum yield as high as 1.60 × 10−5. The relatively high quantum efficiency of such structure associated with its high stability in air and easy processing make it a good candidate as fast electron source for electron gun based systems such as scanning/transmission electron microscopes or x-ray sources.

Si‐related color centers in nanocrystalline diamond thin films

The successful growth of nanocrystalline diamond (NCD) thin films with optically active Si‐related color centers was realized on glass and molybdenum substrates by the microwave plasma chemical vapor deposition (CVD) with focused or linear antenna plasma reactors. Diamond coatings were characterized by Raman spectroscopy, scanning electron microscopy, and photoluminescence (PL) spectroscopy. Increased of a‐Si interlayer thickness resulted in reduction of stress in NCD film and increased renucleation of NCD films. The PL spectra showed that the Si‐color center is only observed in the focused plasma system. The influence of the substrate material as well as the a‐Si interlayer on the density of Si‐related color center was not confirmed in our setup.

Epithelial cell morphology and adhesion on diamond films deposited and chemically modified by plasma processes.

The authors show that nanocrystalline diamond (NCD) thin films prepared by microwave plasma enhanced chemical vapor deposition apparatus with a linear antenna delivery system are well compatible with epithelial cells (5637 human bladder carcinoma) and significantly improve the cell adhesion compared to reference glass substrates. This is attributed to better adhesion of adsorbed layers to diamond as observed by atomic force microscopy (AFM) beneath the cells. Moreover, the cell morphology can be adjusted by appropriate surface treatment of diamond by using hydrogen and oxygen plasma. Cell bodies, cytoplasmic rims, and filopodia were characterized by Peakforce AFM. Oxidized NCD films perform better than other substrates under all conditions (96% of cells adhered well). A thin adsorbed layer formed from culture medium and supplemented with fetal bovine serum (FBS) covered the diamond surface and played an important role in the cell adhesion. Nevertheless, 50-100 nm large aggregates formed from the RPMI medium without FBS facilitated cell adhesion also on hydrophobic hydrogenated NCD (increase from 23% to 61%). The authors discuss applicability for biomedical uses.

Functionalization of Boron-Doped Nanocrystalline Diamond with N3 Dye Molecules

N3 dye molecules [cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II)] are covalently attached to boron-doped nanocrystalline diamond (B:NCD) thin films through a combination of coupling chemistries, i.e., diazonium, Suzuki, and EDC-NHS. X-ray and ultraviolet photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy are used to verify the covalent bonding of the dye on the B:NCD surface (compared to a hydrogen-terminated reference). The spectroscopic results confirm the presence of a dense N3 chromophore layer, and the positions of the frontier orbitals of the dye relative to the band edge of the B:NCD thin film are inferred as well. Proof-of-concept photoelectrochemical measurements show a strong increase in the photocurrent compared to non-dye-functionalized B:NCD films. This study opens up the possibility of applying N3-sensitized B:NCD thin films as hole conductors in dye-sensitized solar cells.

Substitutional phosphorus incorporation in nanocrystalline CVD diamond thin films

Nanocrystalline diamond (NCD) thin films were produced by chemical vapor deposition (CVD) and doped by the addition of phosphine to the gas mixture. The characterization of the films focused on probing the incorporation and distribution of the phosphorus (P) dopants. Electron microscopy evaluated the overall film morphology and revealed the interior structure of the nanosized grains. The homogeneous films with distinct diamond grains featured a notably low sp2:sp3-ratio as confirmed by Raman spectroscopy. High resolution spectroscopy methods demonstrated a homogeneous P-incorporation, both in-depth and in-plane. The P concentration in the films was determined to be in the order of 1019 cm–3 with a significant fraction integrated at substitutional donor sites. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Structural and electrical characterization of diamond films deposited in nitrogen/oxygen containing gas mixture by linear antenna microwave CVD process

Abstract In this paper we demonstrate the large area deposition of n-type conductive nanocrystalline diamond thin films on p-type Si substrates. We show that adding N 2 into CH 4 /H 2 /CO 2 gas mixture allows a wide controlling of diamond film morphology from micro- to nanocrystalline, blockstone-like or porous-like morphology potentially suitable for various applications. Moreover, after adding we found that the diamond films revealed a deep level with activation energy of 0.44 ± 0.03 eV.

Development of a templated approach to fabricate diamond patterns on various substrates.

We demonstrate a robust templated approach to pattern thin films of chemical vapor deposited nanocrystalline diamond grown from monodispersed nanodiamond (mdND) seeds. The method works on a range of substrates, and we herein demonstrate the method using silicon, aluminum nitride (AlN), and sapphire substrates. Patterns are defined using photo- and e-beam lithography, which are seeded with mdND colloids and subsequently introduced into microwave assisted chemical vapor deposition reactor to grow patterned nanocrystalline diamond films. In this study, we investigate various factors that affect the selective seeding of different substrates to create high quality diamond thin films, including mdND surface termination, zeta potential, surface treatment, and plasma cleaning. Although the electrostatic interaction between mdND colloids and substrates is the main process driving adherence, we found that chemical reaction (esterification) or hydrogen bonding can potentially dominate the seeding process. Leveraging the knowledge on these different interactions, we optimize fabrication protocols to eliminate unwanted diamond nucleation outside the patterned areas. Furthermore, we have achieved the deposition of patterned diamond films and arrays over a range of feature sizes. This study contributes to a comprehensive understanding of the mdND-substrate interaction that will enable the fabrication of integrated nanocrystalline diamond thin films for microelectronics, sensors, and tissue culturing applications.

Influence of hydrogen and hydrogen/methane plasmas on AlN thin films

Polycrystalline aluminum nitride (AlN) thin films are exposed to hydrogen and hydrogen/methane plasmas at different conditions. The latter plays an indispensable role in the subsequent deposition of nanocrystalline diamond thin films on AlN. The changes of AlN properties are investigated by means of Fourier transform infrared (FTIR) and Raman spectroscopies as well as atomic force microscopy. The E1(TO) and E22 phonon mode frequencies blue-shift after the exposure to plasmas. The damping constant of E1(TO) phonon, calculated from FTIR transmission spectra using the factorized model of a damped oscillator, and the width of E22 peak in Raman spectra decrease with increasing substrate temperature till the decomposition of AlN thin film becomes notable. It is proven that these changes are driven by the plasmas as annealing in vacuum does not induce them.

High-Q optomechanical circuits made from polished nanocrystalline diamond thin films

We demonstrate integrated optomechanical circuits with high mechanical quality factors prepared from nanocrystalline diamond thin films. Using chemomechanical polishing, the RMS surface roughness of as grown polycrystalline diamond films is reduced below 3nm to allow for the fabrication of high-quality nanophotonic circuits. By integrating free-standing nanomechanical resonators into integrated optical devices, efficient read-out of the thermomechanical motion of diamond resonators is achieved with on-chip Mach-Zehnder interferometers. Mechanical quality factors up to 28,800 are measured for four-fold clamped optomechanical resonators coupled to the evanescent near-field of nanophotonic waveguides. Our platform holds promise for large-scale integration of optomechanical circuits for on-chip metrology and sensing applications.