WO2019042484A1

WO2019042484A1 - Method of manufacturing a porous diamond layer and a nanofiber supported thick porous diamond layer

Info

Publication number: WO2019042484A1
Application number: PCT/CZ2017/050053
Authority: WO
Inventors: Vincent Mortet; Andrew Taylor; Ladislav Kavan; Otakar Frank; Zuzana VLCKOVA; Hana Krysova; Vaclav Petrak
Original assignee: Fyzikalni Ustav Av Cr, V.V.I.; Ustav Fyzikalni Chemie J. Heyrovskeho Av Cr, V. V. I.
Priority date: 2017-08-29
Filing date: 2017-11-01
Publication date: 2019-03-07
Also published as: CZ2017500A3; CZ307885B6

Abstract

The present invention discloses a method of manufacturing a porous diamond layer (6) and a nanofiber supported porous diamond body (7). The method comprising step of seeding of diamond nanoparticles into a nanofibers of any material susceptible withstand of plasma-enhanced deposition conditions. Seeded nanofibers are then mixed in a sacrificial material. This mixture is then applied on a substrate (2) and dried to form a solid nanofibers/sacrificial material composite film. Resulting composite film is then subjected to plasma enhanced chemical vapour deposition of the diamond under conditions where the sacrificial material is decomposed. These steps can be repeated for forming a nanofibers supported porous diamond layer (6) of a desired thickness. The diamond can be doped by boron. Such a conductive porous boron doped diamond layers (1, 6) serve for microelectronical (MEMS) applications where chemical stability is required. In particularly, porous conductive boron-doped diamond films can be employed as a sensors, supercapacitor and/or a filter to separate organic electrochemical substances.

Description

Method of manufacturing a porous diamond layer and a nanofiber supported thick porous diamond layer

Field of the art

Present invention pertains to a method of manufacturing a porous diamond layer by means of plasma-enhanced chemical vapour deposition (PECVD) of the diamond simultaneously with decomposition of a sacrificial material. The invention pertains also to a nanofiber supported thick porous diamond layer which can be obtained by the invented method.

Diamond is a material which has excellent electrochemical properties. Several methods have been developed to fabricate porous (or large surface to volume ratio) diamond electrodes using either top-down (etching) or bottom up (growth on 3D substrates) approach.

Etching methods are limited due to the etching depth of diamond. The etching depth is limited by isotropy of oxygen plasma etching and simultaneous etching of etching mask, i.e. the depth of the fabricated porous material stops when the mask is completely consumed during the etching process. Metals (Al, Ni etc.) are generally used as a mask for diamond etching. To increase the surface to volume ratio using etching method, it is necessary to achieve a high density of diamond nano-rods/nano-pillars with a small diameter (few tens of nanometers) which requires the fabrication of a corresponding etching mask. Such metallic masks might be produced by means of electronic lithography, self-assemble metallic islands obtained from a few nanometers thick annealed metal layers (e.g. Ni, Co, Au) [W. Smirnov et al., Diam Relat Mater 19 (2010), 186], or mono-layer coating of metallic nanoparticles. Diamond nanoparticles have also be used as etching mask [N. Yang et al., Nano Lett 2008 (2008) 3572]. Additionally, the resulting “porous” material suffers from the high electrical resistance of the long and narrow diameter of diamond rods or pillars and limited conductivity of boron doped diamond [C. Hebert et al., carbon 90 (2015) 102].

Deposition of diamond thin film methods on 3D substrates with high aspect ratio is limited by the quality and homogeneity of diamond seeding as well as the non-conformal deposition capability of CVD deposition techniques of diamond. As a matter of fact, upper layers of 3D templates (e.g. glass fibers filters) act as a mask for diamond deposition in the “bulk” of the porous template (typically few micrometers in depth). Unless the 3D template can be repeated after diamond deposition (see for instance F. Gao, M. T. Wolfer, C. E. Nebel, Carbon 80 (2014) 833), the surface to volume ratio increased of the fabricated porous material is limited.

Document US 2013156974 describes a method of manufacturing a thick porous diamond layer by plasma-enhanced chemical vapour deposition of the diamond layer on a sacrificial material and decomposition of the sacrificial material. This method includes provision of a layer made of a sacrificial material having a porous three-dimensional structure capable of gradual decomposing upon contact with a plasma and growth by plasma-enhanced chemical vapour deposition of a diamond layer.

A new method combining growth of diamond on substrates using a supporting structure on one hand with decomposition of a sacrificial material on the other hand has been found.

This method is characterized by that nanofibers of any material which can withstand plasma-enhanced deposition conditions are seeded with diamond nanoparticles, seeded nanofibers are then mixed in a sacrificial material, the resulting mixture of the sacrificial material with seeded nanofibers is then applied on a substrate and dried to form a film of a solid composite of seeded nanofibers and sacrificial material. The resulting solid composite film is then subjected to plasma enhanced chemical vapour deposition of diamond under conditions where the sacrificial material is decomposed.

The sacrificial material is a material decomposed in H and O plasma, more specific decomposed in H rich plasma used for PECVD diamond deposition, advantageously an organic polymer.

Nanofibers can be manufactured from any material which can withstand PECVD deposition conditions (e.g. metal, carbon, silicon, SiO₂, TiO₂, Al₂O₃) in any shape (e.g. whiskers, nano-rods, nano-pillars, nano-tubes, straight or curled nanofibers).

Nanofibers are seeded with diamond and subsequently dried. Several methods exist to seed substrates for diamond thin film growth, here we preferably use seeding using nanoparticles of diamond colloid [O.A. Williams, Chem. Phys. Lett. 445 (2007) 255].

Dry seeded nanofibers are then mixed in the sacrificial material, preferably in an organic polymer, e.g. viscous solution of a polymer or its precursor.

The resulting mixture of seeded nanofibers in polymer is then applied in the form of a thin film on a substrate and solidified, e.g. dried, to form a solid composite film consisting of a polymer matrix loaded with nanofibers. The thickness of the solid composite film depends of the coating method used, normally is a few µm, generally between 1 and 100 µm. The substrate can be metal, glass, ceramic or another material which can withstand PECVD deposition conditions.

This composite film is then subjected to plasma enhanced chemical vapour deposition under conditions of diamond deposition and at the same time polymer decomposition. The polymer as a sacrificial material is gradually decomposed upon contact with said plasma simultaneously during diamond growth. Seeded nanofibers withstand the deposition conditions and form a reinforcing scaffold.

The diamond can be intentionally doped with admixtures for obtaining conductivity, if required. Boron is known to be good dopant. A resistivity can drop to few tens mOhm.cm in case of SiO₂ nanofibers. Replacing SiO₂ nanofibers by carbon nanofibers or metallic whiskers, a porous conducting diamond layers with high conductivity by factor > 10 (for multiwall carbon nanotubes) to >1000 (for whiskers of metals with good conductivity) can be obtained.

After completing the steps of the invented method (i.e. coating of the substrate with the polymer/nanofiber mixture, it drying, and the PECVD diamond deposition) as described above, another one or more layers of porous diamond can be created by analogous way always on the preceding one. For this purpose, the process steps of forming the solid seeded nanofibers/sacrificial material composite film and subjecting it to the plasma enhanced chemical vapour deposition of the diamond can be repeated for forming a nanofibers reinforced porous diamond layers or bodies of desired thickness, which thickness can be from a few µm to a few mm, generally from 4 µm to 10 mm. The steps of forming the solid nanofibers/sacrificial material composite coating and subjecting it to the plasma enhanced chemical vapour deposition of the diamond are preferably at least another one, more preferably at least another five times (for obtaining thickness about 40 µm) repeated. For obtaining very thick layers, said steps are at least twelve times (thickness about 100 µm) or even more times repeated.

The term "layer" as used here is more general than the term "thick layer" and covers also the term "film". The term "body" as used here means a thick freestanding layer.

The result of the inventive process is a three dimensional porous structure of nanofibers homogeneously coated and interlocked with diamond.

Nanofiber supported thick porous diamond layer according the invention consists of two or more layers of randomly laid chopped nanofibers fully covered with diamond. The term "randomly laid" is meant in a sense that the orientation of the fibers is a result of the step applying the mixture of the sacrificial material with seeded nanofibers on the substrate. In accordance with it, the length of the nanofibers in each layer can be larger than thickness of respective layer.

Individual nanofibers are joined in mutual crossing points and touching points inside each layer by the diamond deposited on respective fibers. There is no clear interface between two neighboring layers, rather the fibers belonging to one layer are in crossing points and touching points with the fibers belonging to neighboring layer are joined by the diamond.

Thick porous diamond layer according the invention has uniform porosity in all the thickness. The porosity, expressed as a surface to volume ratio, is at least 6000 cm^- ¹, in a preferred embodiment at least 16,000 cm^-1.

A thickness of the thick porous diamond layer according the invention is at least 4 µm. Nevertheless, advantages of the invented method of manufacturing a thick porous diamond layer are best utilized for manufacturing thicker layers or bodies, e.g. 50 µm to 10 mm thick.

For some applications, freestanding thick porous diamond layers, i.e. freestanding porous diamond bodies, are required. For this purpose, the thick diamond layers are separated from the substrate by dissolution or etching the substrate. The invention provides freestanding, mechanically stable, nanofiber supported porous diamond bodies several hundred micrometer thick, e.g. plates even a few mm thick.

The three dimensional nanofiber supported porous structure according to the invention has controllable mechanical and electrical properties.

In appended drawings are depicted

a porous diamond layer deposited on a diamond coated substrate,

a diamond coated individual nanofiber,

a thick porous diamond layer having thickness many times larger than that depicted in fig. 1, and

freestanding porous diamond body.

Examples

After performing the process of plasma enhanced chemical vapour deposition of the diamond simultaneously with decomposition of the sacrificial material by the method according the invention a porous diamond layer 1 is obtained on a substrate 2. Fig. 1 illustrates a specific embodiment, where the method according the invention includes a step of pretreating the substrate 2 by diamond coating. In consequence of this process step, the substrate 2 is provided by a diamond film 5. The porous diamond layer 1 is present on this film 5 and consists of randomly oriented nanofibers 3 fully covered with diamond 4.

In the illustrated embodiment, nanofibers 3 are randomly inclined nanorods distributed crisscross over the substrate surface. The nanofibers 3 constitute a three dimensional supporting structure homogeneously coated and interlocked with diamond (only schematically depicted, the diamond is not indicated in figs. 1, 3 and 4). Is to be mentioned, that the images are not in scale. True proportions of the fibers and the layer or layers are stated in following detailed description.

In Fig. 2 an individual, very much enlarged, nanofiber 3 is schematically depicted, which is present in the porous diamond layer 1, according the invention. Nanofibers 3 are fully covered with diamond 4 deposited according to the invention by means of plasma enhanced chemical vapour deposition of the diamond simultaneously with decomposition of the sacrificial material.

In Fig. 3 a thick porous diamond layer 6 supported by nanofibers created by the method according the invention is schematically depicted. In this embodiment, steps of forming the solid nanofibers-polymer composite coating and subjecting it to the plasma enhanced chemical vapour deposition of diamond were executed 6-times (1st porous diamond layer 1 lies on the

thin diamond film

5, and 2^nd to 6^th

layers

1 are deposited always on the preceding layer 1) for forming a nanofiber supported thick porous diamond layer 6 having a thickness nearly 6 times larger than the porous diamond layer 1 as depicted in Fig. 1.

For special applications the thick porous diamond layer 1 can be subsequently coated by diamond thin film 5. Fig. 3 illustrates this embodiment with a diamond thin films 5 on both sides of the thick porous diamond layer 1.

In the embodiment as illustrated in Fig. 4, there is no diamond thin film and the substrate has been separated from the thick diamond layer. A freestanding mechanically stable porous diamond body 7 is then obtained.

Detailed description of the examples

Example 1

SiO₂ nano-fibers from Elmarco s.r.o. Company, CZ, are sonicated in DI water at 400 W with a duty cycle of 50 % for 10 min using a UP400S Ultrasonic Processor with a Ti sonotrode H22 from Hielscher-Ultrasound Technology to breakup and disperse the nano-fibers (50-500 nm in diameter and 5-20 micrometer long. The suspension of dispersed chopped nano-fibers is dried in air at high temperature (> 100 °C) to evaporate the water. The resulting powder is mixed with an aqueous colloid of diamond nano-particles (0.2 g/l), sonicated, and dried to obtain nanofibers coated with diamond seeds (same conditions as previous treatment). The dried seeded SiO₂ nano-fibers are mixed with a polymer solution, ma-P 1210 (positive tone photoresist solution based on polymethylmethacrylate from Micro resist Technology GmbH, DE), with a concentration ca. 80 mg fibers per milliliter) to form a stable “nanofibers in polymer solution” suspension. The suspension is spin-coated onto a glass or silicon substrate 2, first coated with a thin diamond film 5, at 3000 rpm for 30 seconds. Coated samples with the “nanofibers in polymer solution” suspension are then annealed on a hot plate at 110 °C for 90 seconds to form an homogeneous composite polymer/nanofibers thin film (thickness of ca 4 micrometers). The sample is loaded into a plasma enhanced chemical vapour deposition system (an ASTeX 5010 from Seki Technotron, Japan) for diamond deposition. The diamond coating is deposited using the following conditions: pressure: 50 mbar, microwave generator power: 1150 W, gas composition: 99.3 % H₂, 0.5 % CH₄ and 0.2 % trimethylboron (B(CH₃)₃) for a total flow of 500 standard cm³ per minute and a deposition temperature of ca 700 °C. The composite nanofibers/polymer coating and the diamond deposition are consecutively repeated to achieve the desired diamond layer thickness. By performing this coating process 6 times, a thick, with nanofibers 3 reinforced, porous and conductive boron doped diamond layer 1 of ca 25 micrometers in thickness, with resistivity ca 60 mΩ.cm (miliohm.centimetr) and a surface to volume ratio of 16,000 cm^-1 is produced.

Example 2

Reproducing in example 1 described process but increasing the number of steps, one can obtain several hundred micrometer freestanding, with nanofibers 3 reinforced, thick and mechanically stable porous diamond bodies 7, by removal of the substrate 2 by wet chemical etching (e.g. using HF to etch glass substrate, or HF + HNO₃ mixture to etch silicon substrate).

Example 3

Reproducing in example 1 described process but replacing SiO₂ nanofibers by multiwall carbon nanofibers one can obtain a porous conducting diamond layers 1, 6 with resistivity about 2 mW.cm. Replacing SiO₂ nanofibers by metallic whiskers, one can obtain the resistivity as low as 0.02 mW.cm.

Example 4

Reproducing in example 1 described process but by deposition on a diamond coated substrate 2, one obtains a fully corrosion protected material with enhanced surface area which is ideal for long term operation as an electrochemical electrode (in the case the material of the fibers 1 and/or the substrate 2 is electrically conducting).

Example 5

Reproducing in example 1 described process by deposition on diamond coated substrate 2 and increasing the diamond deposition time of the last step, one obtains a closed surface and a porous diamond layer 6 sandwiched between two closed and flat diamond thin films 5. This material with controllable mechanical properties can be used for microelectronical (MEMS) applications (e.g. sensors).

Layers 1, 6 and/or bodies 7 produced according to present invention can be used, thanks their controllable mechanical properties, such as chemically inert electrodes for advanced electrochemical sensing or processing devices. A foil comprising at least one layer can serve for microelectronical (MEMS) applications where chemical stability is required. Particularly, porous conductive boron-doped diamond foils can be employed for supercapacitor application.

Porous diamond layers can also serve as a filters to separate organic electrochemical substances from water.

The present invention is not limited to the specific applications disclosed herein but may be utilized to different purposes.

A method according to present invention provides cheap and efficient solution for manufacturing the above described new porous diamond layers 1, 6 or bodies 7.

List of reference numerals

1 porous diamond layer

2 substrate

3 nanofiber

4 diamond

5 thin diamond film

6 thick diamond layer

7 porous diamond body

Claims

A method of manufacturing a porous diamond layer by means of plasma-enhanced chemical vapour deposition of the diamond simultaneously with decomposition of a sacrificial material, the method comprises the step of:
nanofibers of any material which can withstand plasma-enhanced deposition conditions are seeded with diamond nanoparticles,
seeded nanofibers are then mixed in a sacrificial material,
the resulting mixture of the sacrificial material with seeded nanofibers is then applied on a substrate and dried to form a solid seeded nanofibers/sacrificial material composite film,
resulting solid composite film is then subjected to plasma enhanced chemical vapour deposition of the diamond under conditions where the sacrificial material is decomposed.
The method according to claim 1, wherein applying the mixture of the sacrificial material with nanofibers is performed by spin coating or dip coating or spray coating, advantageously by spin coating.
The method according to claim 1 or 2, wherein said steps of forming the solid seeded nanofiber/sacrificial material composite film and subjecting it to the plasma enhanced chemical vapour deposition of the diamond are at least another one repeated for forming a nanofibers supported porous diamond layer of a desired thickness.
The method according to any one of the preceding claims, wherein the sacrificial material is an organic polymer decomposed in H rich plasma.
The method according to any one of the preceding claims, wherein the diamond is doped by an admixture, preferably by boron.
The method according to any one of the preceding claims, wherein the nanofibers are carbon nanofibers or metallic whiskers.
The method according to any one of the preceding claims, wherein the method includes a step of pretreating the substrate by diamond coating.
The method according to any one of the preceding claims, wherein the method includes a subsequent step of coating the porous diamond layer by diamond.
The method according to any one of the preceding claims, wherein the method includes a step of separating the diamond layer from the substrate by dissolution or decomposing the substrate.
A nanofiber supported thick porous diamond layer, characterized by that
it comprises at least two layers (1) of randomly laid nanofibers (3) fully covered with diamond (4),
wherein an average length of the nanofibers in each layer (1) is larger than thickness of the layer (1),
wherein individual nanofibers (3) in mutual crossing points and touching points inside said layers (1) as well between said layers (1) are joined by the diamond (4), and
wherein a thickness of the thick porous diamond layer (6) is at least 4 mm and a surface to volume ratio of the thick porous diamond layer (6) is at least 6000 cm^-1.
The nanofiber supported thick porous diamond layer according to claim 10, wherein its thickness is at least 16 mm and its surface to volume ratio is at least 1200 cm^- ¹.
The nanofiber supported thick porous diamond layer according to claim 10, wherein it consists of at least ten layers (1) of randomly laid nanofibers (3) fully covered with diamond (4), and wherein its thickness is at least 50 mm.
The nanofiber supported thick porous diamond layer according to claim 10 or 11, wherein the diamond (4) is doped by an admixture, preferably by boron.
The nanofiber supported thick porous diamond layer according to claim 13, having resistivity under 40 mW.cm, wherein the nanofibers (3) are carbon nanofibers.
The nanofiber supported thick porous diamond layer according to claim 13, having resistivity under 0.4 mW.cm, wherein the nanofibers (3) are metallic whiskers.
The nanofiber supported thick porous diamond layer according to any one of the claims 10 to 15, wherein it is carried on a substrate (2).
The nanofiber supported thick porous diamond layer according to claim 16, wherein the substrate (2) and/or a surface of the thick porous diamond layer is coated by a thin diamond film (5).
The nanofiber supported thick porous diamond layer according to any one of the claims 10 to 15, wherein it is freestanding body (7).
The nanofiber supported thick porous diamond layer according to claim 18, wherein at least one of its surfaces is coated by a thin diamond film (5).