ČESKÉ VYSOKÉ UČENÍ TECHNICKÉ V PRAZE

Transkript

1 ČESKÉ VYSOKÉ UČENÍ TECHNICKÉ V PRAZE TEZE K DISERTAČNÍ PRÁCI

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3 České vysoké učení technické v Praze Fakulta jaderná a fyzikálně inženýrská Katedra inženýrství pevných látek Oleg Babchenko PŘÍPRAVA A CHARAKTERIZACE VYBRANÝCH UHLÍKOVÝCH NANOSTRUKTUR Doktorský studijní program: Aplikace přírodních věd Studijní obor: Fyzikální inženýrství Teze disertace k získání akademického titulu "doktor", ve zkratce "Ph.D." Praha, červen 2014

4 Disertační práce byla vypracována v prezenční a kombinované formě doktorského studia na Katedře inženýrství pevných látek Fakulty jaderné a fyzikálně inženýrské ČVUT v Praze. Uchazeč: Mgr. Oleg Babchenko Fyzikální ústav AV ČR, v. v. i. Cukrovarnická 10/112, Praha 6 Školitel: Prof. Ing. Zdeněk Bryknar, CSc. Katedra inženýrství pevných látek Fakulta jaderná a fyzikálně inženýrská ČVUT Břehová 7, Praha 1 Školitel-specialista: Ing. Alexander Kromka, PhD. Fyzikální ústav AV ČR, v. v. i. Cukrovarnická 10/112, Praha 6 Oponenti: Doc. Ing. Marián Veselý, CSc. Doc. Ing. Andrey Shukurov, Ph.D. Doc. Ing. Jiří Houška, Ph.D. Teze byly rozeslány dne:... Obhajoba disertace se koná dne v 11:00 hod. před komisí pro obhajobu disertační práce ve studijním oboru Fyzikální inženýrství v zasedací místnosti č 386 Fakulty jaderné a fyzikálně inženýrské ČVUT v Praze, Trojanova 13, Praha 2. S disertací je možno se seznámit na děkanátě Fakulty jaderné a fyzikálně inženýrské ČVUT v Praze, na oddělení pro vědeckou a výzkumnou činnost, Břehová 7, Praha 1. Prof. Ing. Stanislav Vratislav, CSc. předseda komise pro obhajobu disertační práce ve studijním oboru Fyzikální inženýrství Fakulta jaderná a fyzikálně inženýrská ČVUT, Břehová 7, Praha 1 2

5 Disertační práce na téma: Příprava a charakterizace vybraných uhlíkových nanostruktur Abstrakt Doktorská práce se věnuje studiu přípravy a charakterizaci dvou alotropních forem uhlíku, jmenovitě diamantových vrstev a uhlíkových nanotrubek. V oblasti studia tenkých diamantových vrstev byly uskutečněny studie zaměřené na způsoby nukleace diamantu, procesy růstu diamantových vrstev pomocí depozice z plynné fáze v mikrovlnném výboji a přípravou struktur metodami shora-dolů a zdola-nahoru. Studium uhlíkových nanotrubek bylo zaměřeno na přípravu katalytických nanočástic a syntézu uhlíkových nanotrubek ve dvou odlišných depozičních reaktorech. Hlavní analytické metody použité pro charakterizaci zkoumaných materiálů jsou rastrovací elektronová mikroskopie, mikroskopie atomárních sil a Ramanová spektroskopie. Práce také obsahuje příklady a diskuze syntézy nebo modifikací povrchu diamantu a uhlíkových nanotrubek pro pokročilé studie, např. infračervenou spektroskopií nebo pěstování buněk. The thesis on: Fabrication and characterization of selected carbon-based nano-structures Abstract In the doctoral thesis the synthesis and characterization of the two carbon allotropes, namely the diamond thin films and carbon nanotubes are studied. In the frame of diamond thin film direction the studies on the diamond nucleation by several strategies, growth process using the microwave plasma assisted chemical vapour deposition and structures fabrication by the top-down and bottom-up methods were realized. In the carbon nanotubes section the fabrication of catalytic nanoparticles and carbon nanotubes synthesis in the two various deposition reactors were studied. The main analytic techniques used for material characterization in the studies were scanning electron microscopy, atomic force microscopy and Raman spectroscopy. In addition, the examples of diamond and carbon nanotubes synthesis or surface modification for advanced studies, e.g. infrared spectroscopy or cells cultivation, are discussed in the work. 3

6 Preface The growth and basic characterization of nanostructured carbon allotropes was chosen as the subject of my PhD research work. In brief the state-of-the-art of selected subject is given in the Introduction. It is followed by the list of research targets that given in the Thesis Objectives. The equipment employed in the work is mentioned in the Experimental part. The Results and Discussions part includes the most significant outcomes of the thesis. The Conclusions gives the results summarization. The list of publications is enclosed. I. Introduction 1. Carbon and carbon allotropes Carbon (C) is a non-metallic chemical element with four electrons that could be involved into strong chemical (covalent) bonds formation [1-3]. Carbon is the unique and versatile element which allotropic forms (polymorphs) reveal very dissimilar properties. For instance, from the ancient time people knew it in such distinct forms as graphite (coil) and diamond (gem stones) supposing their nature is different. Whilst nowadays it is known that these materials are of the same basis (i.e. carbon atom). The soot, petroleum, natural gas, the majority of all chemical compounds and all known life forms on our planet are having Carbon as common in their structures. Generally, the diversity of carbon allotropes and their properties could be attributed to the different carbon atom hybridization and thus lattice arrangement (bonding in solid state). Depending on the number of involved electron orbitals distinguish sp 3, sp 2, and sp carbon s atom hybridization [1-3]. At present, within the most known carbon allotropes are sp 3 based: diamond in cubic or hexagonal arrangement, and sp 2 based: graphite with related allotropic forms, bulk graphite, fullerenes, carbon nanotubes, graphene, etc. The sp carbon hybridization appears in such carbon form as carbine and also in polymer compounds [1-4]. Due to combination of various outstanding properties, intrinsic for certain carbon polymorphs, presently, carbon allotropes attract interest of researchers all over the world. Moreover, the carbon allotropes are often appear in the nanostructured forms that, as it is known, reveal properties different to the bulk materials. Next, the diamond (particularly diamond thin films) and carbon nanotubes as representatives of sp 3 and sp 2 hybridized allotropes, respectively, are described in more details Diamond Diamond, which is known by civilization from the ancient time, has a face-centred cubic structure, also called a diamond lattice, where carbon atoms are bonded with nm long, sp 3 hybrid bonds. Each atom is surrounded by four equidistant nearest neighbours, lies at the corners of imaginary tetrahedron, in the way shown in Fig. 1 [1, 5-8]. Diamond is known as the hardest and the strongest natural material (10 at Mohs scale hardness) very resistant to chemical corrosion. Besides this, diamond is known as good electric insulator (resistivity at room temperature up to Ω cm) and natural material with the highest bulk, i.e. the same in all directions, thermal conductivity (2000 W/mK) [5-7]. In spite of diamond amazing properties the use of natural diamonds is very expensive. Thus, the practical interest for scientists has the synthetic diamonds produced either in the form of nano- or microcrystalline films or in the form of diamond monocrystals or gemstones. The important features in the case of diamond film synthesis are efficient nucleation (required for dense and homogeneous diamond film growth) and growth process itself (its parameters affect the final film properties) [5-7]. In the field of diamond nucleation the following techniques are known: substrates abrasion by diamond paste, bias enhanced nucleation, ultrasound nucleation, seeds distribution by spin-coating, etc. In the most cases, the diamond growth is realized by the hot filament or plasma assisted chemical vapor deposition from the carbon-containing gas mixture or in the acetylene/oxygen flame. The deposition conditions during such, usually, high temperature, processes are shifted towards the diamond lattice formation [5-7]. The wide range of diamond thin films applications, based on its properties, is further enlarged by the diamond structuring. The structuring of diamond films is realized by using the after growth etching (topdown) or before-growth structuring also known as selected area deposition (bottom-up) Carbon nanotubes Fig. 1: Atoms conformation in the diamond lattice, according [8]. On the contrast to well-known diamond the carbon nanotubes (CNTs) were firstly reported only in 1991 by S. Iijima [9]. CNTs could be presented as the graphene sheets (plain sp 2 hybridized material with thickness of 1 carbon 4

7 atom) rolled into form of hollow tube. These tubes have diameters in the range of few nanometres and their lengths are up to several micrometers. Each carbon nanotube is a single molecule consisted of sp 2 hybridized carbon atoms covalently bonded into a hexagonal network. The end of the tube (cap) is formed by appearing pentagonal sites in the graphene sheet [2, 3, 10]. Depending on the way the graphene sheet is rolled up (define the angle between tube axis and C-C bond in separate hexagon) distinguished zigzag, armchair, and chiral carbon nanotubes (Fig. 2). The carbon nanotube chirality inherently defines some nanotubes properties (e.g. electrical conductivity or chemical reactivity). It is known that all armchair nanotubes (and some of chiral) reveal very high conductivity (up to 10 6 S/m), thus, such nanotubes are called metallic. The other nanotubes (i.e. with higher resistivity) are called semiconductor. In general, all the nanotubes are very elastic, flexible and highly absorbent (due to large surface area), and as sp 2 form of carbon have high thermal conductivity in direction of nanotube axis (up to 6600 W/mK) [2, 3, 11, 12]. However, the number of impurities or defects in the sheets strongly affects the CNTs properties and deteriorates its practical applications. The CNTs are generally catalytically synthesized by the high temperature processes from the carbon-containing gas (or evaporates) mixture [2, 3]. As the catalyst materials the iron, cobalt, and nickel (or their derivatives) are mostly used. Depending on the number of graphene sheets forming a nanotube distinguish single-walled and multi-walled carbon nanotubes. The single-walled carbon nanotube (SWCNT) is formed by a single graphene sheet, while multiwalled carbon nanotube (MWCNT) consists of several graphene sheets coaxially inserted one into another (straight nanotubes). When the separate walls are not parallel to each other the bamboo-structured MWCNTs are formed [11, 12]. 2. Analytic techniques Fig. 2: Three types of carbon nanotubes conformation, image adopted from [10]. Thanks to developing and improvement of various analytic techniques it is became possible clearly distinguish (or discover) the mentioned variety of carbon allotropes. Numerous analytic techniques are used for the carbon allotropes characterization but the most popular from the basic are Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). By these techniques, used in a standard configuration, it is possible to distinguish different carbon forms from each other as well as find out about fundamental materials properties. Typically, measurements that give information about physical form of material and its purity are required. Thus, the combining of several techniques for material characterization is reasonable. The mentioned analytic techniques could be used for the characterization of material quality, e.g. purity or composition (Raman spectroscopy), general morphological features, i.e. visualize the studied material (SEM), topography, surface roughness, mechanical or adhesive properties (AFM) and so on [6, 13, 14]. II. Thesis Objectives From the variety of carbon forms the diamond thin films and carbon nanotubes were chosen for the PhD thesis research as the representatives of two different carbon hybridization families. The separated topics and the assigned tasks of the research were indicated on the base of the detailed literature review and generally associated with the key features of the selected carbon allotropes synthesis. Thus, the research work devoted to diamond thin films was separated to three main topics with 11 specified tasks altogether. The work dealing with carbon nanotubes is as well divided into three scientific topics with 9 specified tasks. In general, the experimental work was oriented onto the fabrication of nanostructured carbon materials, investigation and understanding of the process parameters influence on the fabricated structures, and technologies optimization. The basic characterization of fabricated carbon allotropes was realized for each study in respect to its purpose. The data observed using the analytic techniques (SEM, AFM, Raman spectroscopy) were used for results interpretation, growth and/or modification phenomena understanding, the synthesis model proposing and so on. Moreover, the fabricated carbon materials were often used in advanced studies, e.g. investigation of biocompatibility, electrical, optical properties, etc. This was realized in the form of collaboration with other teams. Overall, the topics and tasks of the PhD research work are given below. 5

8 Scientific topics and tasks related to the diamond thin films: 1) Nucleation technologies: a) basic study on the nucleation influence on diamond film growth b) comparative study on different seeding strategies c) seeding of geometrically complicated objects and/or soft objects 2) Diamond film growth: a) analysing of process parameters influence on large area diamond growth b) understanding of diamond growth phenomenon c) growth of diamond films with controllable morphologies d) growth of diamond thin films for advanced studies 3) Diamond structures fabrication: a) developing of effective procedure for the reactive ion etching b) fabrication of diamond micro- and nanostructures by top-down strategy c) developing of effective procedure for patterned diamond thin film growth (bottom-up strategy) d) fabrication of diamond structures for advanced studies Scientific topics and tasks related to the carbon nanotubes: 1) Catalyst particles preparation: a) study on process parameters influence on catalyst nanoparticles b) fabrication of catalyst nanoparticles with different sizes 2) Carbon nanotubes growth in the focused microwave plasma system: a) defining of appropriate catalyst particles size for carbon nanotubes growth b) investigation of gas chemistry for carbon nanotubes growth c) testing of substrate bias influence on carbon nanotubes formation 3) Carbon nanotubes growth in the linear antenna pulsed microwave plasma system: a) defining of plasma discharge type for carbon nanotubes growth b) characterization of gas chemistry influence on carbon nanotubes growth c) study on plasma volume relation to CNTs growth d) CNTs synthesis and modification for advanced studies III. Experimental part The experimental work of the PhD research was realized at the Institute of Physics of the AS CR, v.v.i. Due to the complexity of the research to solve each particular task it was necessary to set up its own experimental scheme which will be described further. In general, the employed processing equipment, technologies and analytic equipment used in the different studies are described as follows. 3. Equipment and technological procedures applied in the experiments The main processing equipment applied at the different experimental stages is separated to chemical vapour deposition (CVD), physical vapour deposition (PVD) and reactive ion etching (RIE) systems. The standard technological procedures (lithography and metal pattern fabrication) employed in the experiments are given as well. In addition to main processing facilities the different supporting devices and other equipment were used in the research work. Thus, experiments with non aggressive chemistry, e.g. powders, solutions, were performed in the flowbox system (SCR engineering) with constant filtered air flow. The ultrasound bath (TRANNSONIC DIGITAL S or TRANNSONIC T570/H from Elma GmbH) were used for the ultrasound seeding in deionised water-based nanodiamond suspension. Different spin-coater set ups were applied to spin-off the rests of water from the substrates and/or to uniformly distribute polymer over the substrates. The polymer and/or water drying were performed in the automatic ovens. The experiments with ultraviolet (UV) sensitive polymers were performed in the clean room area with filtered air and under yellow light. The stage for the UV light irradiation applied for the optical lithography was either JuB 2 (Veb Elektromat) or optical mask aligner MJB 3 (Karl Suss). The inductively coupled RF plasma system (214 VT, Tesla) was employed for oxygen plasma treatment of carbonaceous materials and for polymer residues removing Equipment for chemical vapour deposition The principle schemes of the main facilities used for material synthesis are shown in Fig. 3. Both reactors used for microwave plasma enhanced CVD and operated at 2.45 GHz. The one of the reactors employed in the research work 6

9 is the ellipsoidal cavity based resonator P6, AIXTRON AG (i.e. focused microwave plasma system). This system (Fig. 3 a) has a removable quartz bell jar (process chamber) placed on the water cooled substrate stage with vertical movement. During the deposition the bell jar on the substrate stage is inserted into the metallic resonator cavity. The geometry of this cavity is a rotational ellipsoid in one focal point of which the microwave antenna is mounted and in the other a plasma discharge appears. Such system, originally designed for diamond film synthesis, allows homogeneous deposition within 2-3 inch substrate diameter. The computer controlled set up is able to maintain stable plasma discharge within several days. The process pressure could be as high as 250 mbar (25 kpa) and the effective power is up to 6 kw. The substrate temperature is measured using two colour pyrometer (IRC type), i.e. without contact with substrate. Another CVD reactor employed in the research was the large area linear antenna pulsed microwave plasma system (Fig. 3 b), videlicet, modification of the commercially available apparatus used for semiconductor and solar cell technologies AK 400, Roth&Rau MicroSystems. The process chamber of this system is water cooled, metallic, rectangular cuboid, one side (front) of which is used as a chamber door. Inside the chamber, the two parallel antennas (linear conductors), inserted into the quartz tubes, are used to ignite the microwave (MW) discharge (along their axis). The discharge is maintained by two generators (one from each side) working at pulse-frequency of up to 500 Hz and power up to 4.4 kw in a pulse. The substrate holder stage, placed under antennas, can be resistively heated up to 700 C and moved up/down to control the distance to the antenna. Moreover, it can be radiofrequency (RF) biased (13.56 MHz, 600 W/500 V) to control energy of ions impinging on the substrate surface. The temperature of heater is controlled by thermocouple while the substrate holder temperature is measured in non-contact mode. The pressure range employed for CVD is from 0.04 mbar (4 Pa) to 2 mbar (200 Pa). Prior to any deposition the chambers of deposition equipment are evacuated to the base pressure conditions of 5x10-6 mbar (5x10-4 Pa) for P6 or to 5x10-4 mbar (5x10-2 Pa) for AK 400. After the samples loading the process gases are injected to the chamber and the plasma discharge is ignited and maintained to the end of the deposition. The whole deposition process is monitored by temperature and reflected power measurements. 7 a) b) Fig. 3: The principle scheme of a) focused microwave plasma system and b) the linear antenna pulsed microwave plasma system System for physical vapour deposition The home-made system for metal evaporation works in the low pressure (in order of several hundred Pa) conditions and has four resistively heated tungsten boats with rotational shielding (Fig. 4 a). Such arrangement allows consistent deposition of up to four different materials in one samples load. The samples in this system are placed top-side down in the holder that, by rotation, positioned above one of the boats. After the chamber evacuated, metal evaporation is performed from one direction (source). The distance to the target (substrates) is so that evaporated metal forms quite uniform layer. The thickness of the deposited layer is monitored by the quartz crystal balance monitor with angstroms precision. All metals applied for PVD (nickel gold, titanium, etc.) are of at least 99.9% purity Reactive ion etching system The applied in the research work a reactive ion etching system was Phantom III (LT), Trion Technology, Inc. It is the commercially available, capacitively coupled plasma reactor with water cooled substrate stage. The samples are loaded top-side up onto the table in diameter of 30 cm (Fig. 4 b). The base pressure of evacuated chamber is 30 mtorr (around 4 Pa), while the system is used in pressures range from 60 mtorr (around 8 Pa) to 300 mtorr (around 40 Pa). The gas inlet is made in a form of ring above the edge of substrate stage supporting homogeneity of process gas

10 distribution. The plasma is generated above the table by 600 W, RF generator operated at MHz. As a counter electrode at the RIE process the whole chamber is used. a) b) Fig. 4: The photo of processing equipment: a) deposition chamber of home-made system for metal evaporation, b) the chamber view of reactive ion etching system Phantom III (LT), Trion Technology, Inc Lithography and metal pattern fabrication procedures The principle scheme for lithography procedure is shown in Fig. 5 a. In this case it means following. Firstly, by the spin coating, clean (i.e. without surface contaminations) samples are covered by the adhesive agent to diminish influence of any surface adhered water. In my case, the hexamethyldisilane was obviously used and the spin coating conditions were 3000 rpm (rotations per minute) for 15 s. Immediately after this, samples are covered by the photosensitive polymer resin (i.e. photoresist), by the same spin coating at 3000 rpm for 60 s. In my experiments usually positive tone photoresist ma-p 1215 was used. Next, samples covered by photoresists are immersed into oven for 45 min baking (drying) at 100 C. After this, through the appropriate shadow mask, samples are irradiated by the ultraviolet UV light 10 s or 15 s (time of irradiation generally depends on the used resin and its thickness) from the side of photoresist. Further, samples are treated by the developer solution which removes the irradiated parts of polymer resins (in case of applying negative photoresist the non-irradiated parts will be removed). Finally, samples are purified by distilled water flow (1 min) and dried by nitrogen gun. The similar procedure is employed in case of e-beam lithography. Here, instead of UV sensitive polymer resin, after the hexamethyldisilane, samples are covered by the e-beam sensitive polymethylmethacrylate. Consequently, the polymer is not exposed to the UV light through the shadow mask but irradiated by the electron beam that is deflected by the scanning coils over the samples surface. a) b) Fig. 5: The scheme of a) lithography procedure, b) metal pattern preparation procedures. Two ways for metal pattern fabrication (Fig. 5 b) were utilized in my work. In both cases for metal PVD the home-made system for metal evaporation was employed. In case of employing direct evaporation through the physical mask its means that metal is deposited directly onto uncovered by mask places, i.e. openings (Fig. 5 b). The physical mask is in close mechanical contact with substrate but not stick to it, i.e. less precise borders are formed. After the metal evaporated, the mask is simply taken away. In case of pattern preparation employing the lithography the process includes next steps. Firstly, on the substrates the polymer resin structures are fabricated, by the lithography (Fig. 5 a). If it is necessary the polymer residues 8

11 at opened areas are removed by oxygen plasma treatment (using 214 VT, Tesla). Then, prepared structures are covered with metal by thermal evaporation. Next, samples are processed in the polymer resin remover which dissolves polymer resin and thus removes metal that was deposited on it. The metal remains on the substrate in the place of openings in the polymer layer (Fig. 5 b). This procedure is often called lift-off. Generally, for its efficiency the greater thickness of polymer than thickness of evaporated metal (could be even several times greater) is required. 4. Equipment applied for material characterization The following analytic equipment was applied in the research for basic material characterization (i.e. surface morphology, topography and chemical composition). The techniques that were used are scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The utilized equipment is given in the next sections. In addition to the main characterization equipment the supporting equipment and devices were applied for investigation. For example, the profilometer Dektak 150, Veeco was applied for the investigation of diamond structures height, either etched or grown. The IR-Plan Advantage Microscope, Spectra Tech and commercially available Film Wizard SCI software were applied for the reflection spectra measuring and film layer thickness evaluation, respectively. The investigation of surface wettability (by a standard sessile water droplet method) was performed applying the Surface Energy Evaluation system with related software for contact angle calculation (Advex Instruments, s.r.o.) Scanning electron microscopy The scanning electron microscopy measurements were performed applying two different systems: the e_line system writer, Raith (Fig. 6 a) and the scanning electron microscope (SEM) Mira 3, Tescan (Fig. 6 b). The e_line system writer (Raith) is a combination of scanning electron microscope with magnification up to and electron beam lithography (EBL) workstation. The SEM system has two standard electron detectors: the backscattered electron detector and the secondary electron detector. This system placed in the clean room area (clean level for laboratory and 100 at EBL workstation place) was operated by the K. Hruška. a) b) Fig. 6: The photo of scanning electron microscope: a) e_line system (Raith), b) Mira 3 (Tescan). The scanning electron microscope Mira 3 (Tescan) is an experimental setup consist of scanning electron microscope with 3 axis movable substrate stage and micro-manipulators for electrical measurements. The SEM set up is equipped by the backscattered electron detector and the secondary electron detector. The Mira 3 was used by author of the thesis mainly for monitoring measurements, i.e. measurements of testing samples Raman spectroscopy The invia Reflex Raman microscope, Renishaw (Fig. 7) was employed for material investigation by Raman 9 Fig. 7: The photo of Reflex Raman microscope invia, Renishaw.

12 spectroscopy. This system is equipped by HeCd laser with optical system fitted to wavelengths of 325 nm (ultraviolet line) and 442 nm (blue line) and focused on the sample via optical microscope. The corresponding powers are 3 mw and 30 mw, respectively. The microscope lateral chemical resolution is up to 1 μm. By the Raman microscope the character of deposited materials was studied. The system was operated by M. Ledinský or by T. Ižák. Next, the collected data were processed employing commercially available Origin 8.1 software Atomic force microscopy Atomic force microscopy measurements were performed either by scanning probe microscope Solver PRO NT- MDT, Ntegra (Fig. 8 a) or by AFM Microscope Dimension 3100, Veeco (Fig. 8 b). Both these set ups were used in the commercial configuration for surface topography investigation. For measurements the silicon AFM cantilevers with a typical tip radius of 10 nm, force constants between 0.02 and 40 N/m and resonance frequency of khz were employed. The measurements and results interpretations were supervised by B. Rezek. a) b) Fig. 8: The photo of atomic force microscope: a) Solver PRO NT-MDT (Ntegra), b) AFM Dimension 3100 (Veeco). IV. Results and Discussions The most significant results and related discussions of the research or those that affect all further work are summarized in this part. As was already mention, the performed studies are thematically separated in two directions, i.e. diamond thin films and carbon nanotubes. 5. Diamond thin films The studies of the diamond thin films direction related to nucleation strategies (including several studies), structures fabrication (by two different methods) and diamond growth process (in the large area linear antenna microwave plasma system) are described below Nucleation strategies Generally it is claimed that diamond deposition process at ordinary clean surfaces caused self-formation of crystals that do not exceed density of particles per cm 2, i.e. isolated diamond crystals [15]. As the origins of these crystals are usually assumed either local inhomogeneities that induce diamond lattice formation or random appearing of growth sites (nucleation centres) that are stabilize and grow with the time. Therefore, the key feature for growth of continuous diamond thin film is the preparation of seeding (nucleation) layer on the substrate [5-7]. Among the known and popular seeding strategies are polishing with diamond paste [16, 17], bias enhanced nucleation (BEN) [18-20] and ultrasound agitation in a diamond powder suspension [20-21]. Each of them has its own advantages and disadvantages and is useful for specific applications [6, 7]. The important factor for any nucleation (seeding) procedure is the nucleation density, i.e. the number of growth sites (e.g. diamond seeds) per unit area. The typical densities that result into growth of homogeneous diamond films are in the range of particles per cm 2 [19-22]. My experimental work related to the substrates seeding for planar diamond film growth starts from seeding by ultrasound bath. The influence of diamond powder suspension and time of ultrasound treatment were investigated in this study. Were tested three different suspensions (S1, S2, S3) with dispersed diamond particles of sizes (count by 10

13 number): 900 nm in S1, mixture of 10 nm and 460 nm in S2, and mixture of 13 nm and 77 nm in S3. The time of seeding was ranged from 1 min to 40 min. All work was performed under the guiding of A. Kromka and Š. Potocký. The nucleation effectiveness was investigated by atomic force microscopy using AFM Microscope Dimension 3100, (Veeco), as managed by B. Rezek. The resulted AFM images are shown in Fig. 9. As it is seen the 1 minute treatment resulted in: for S1 suspension the smallest grains with poor seeding density (Fig. 9 a), for S2 suspension relatively dense seeding (Fig. 9 b), for S3 suspension quite dense and homogeneous coverage of substrate by diamond seeds (Fig. 9 c). The prolonging of ultrasound treatment improves the surface coverage by diamond seeds and after the 40 minutes treatment resulted to: for S1 poor seeding density (Fig. 9 d), for S2 yet not sufficient homogeneity (Fig. 9 e), for S3 improved seeding homogeneity and seeding density around cm -2 (Fig. 9 f). Thus, we conclude that S3 suspension is more appropriate for ultrasound nucleation at chosen conditions. Although, the optimization for S1 and S2 suspensions employing is possible, e.g. by plasma pre-treatment [23], using of S3 is simpler. Moreover, as has been reported by some authors [21] the using of aqueous solution with dispersed diamond nanoparticles in size of 4-5 nm could result to growth of continuous diamond films with thickness <100 nm. Finally, it should be noted that developed and tested nucleation by S3 suspension became standardly used in our laboratory for diamond thin films growth. a) b) c) d) e) f) Fig. 9: The AFM images of Si substrates seeded: a) 1 min in S1, b) 1 min in S2, c) 1 min in S3, d) 40 min in S1, e) 40 min in S2, f) 40 min in S3. Scan area is 1x1 µm 2, z-scale 50 nm. Partially published in [95]. An example of grown diamond film characterization (used nucleation in S3, thickness up to 300 nm) is given in the (Fig. 10). The shown by SEM film morphology is represented by randomly oriented densely packed diamond crystals with grain size up to 250 nm (Fig. 10 a). The measured Raman spectrum (Fig. 10 b, excitation wavelength was 442 nm) reveals peaks and bands typical for diamond thin films. The characteristic diamond peak, centred at 1332 cm -1, disordered (D-) and graphitic (G-) bands typical for sp 2 bonded carbon (i.e. non-diamond or amorphous) and centred at 11 a) b) Fig. 10: Typical a) SEM image of diamond film and b) measured Raman spectrum with fitting curves.

14 1350 cm -1 and 1580 cm -1 respectively, and band attributed to transpolyacetylene segments at grain boundaries (Ta shoulder) centred at 1180 cm -1 are observed (Fig. 10 b) [24-26]. The intensity of collected signal is given in arbitrary units (a.u.). The experimental work with seeding strategies is continued in the studies of nucleation via using the diamond seeds embedded into water soluble polymer matrix [B1]. The transformation of polymers to diamond was reported by several authors [27-30]. However, such processes result in single crystals (or porous layer at the best conditions) formation rather than in continuous films. The employed in my studies a water soluble polyvinylalcohol (PVA) was chosen as polymer that is characterized by good dispersive properties. The preliminary experiments with PVA were already done by my colleagues Š. Potocký and H. Kozak (not published). While the comparative experiments with the diamond films growth were realized during my research. In the growth experiments we found that the employed polymer deteriorates the seeding efficiency (more evident at higher deposition temperature) and resulted to voids in the grown diamond layer. It was attributed to oxygen content in the polymer compound that is released during the deposition process [31]. At higher temperatures the release rate is higher which causes the more evident seeding layer damage. Overcoming of this issue is possible when the higher concentration of diamond seeds in polymer matrix is used [32]. On the other hand it increases the danger of diamond particles clustering and thus less homogeneous layer growth. The comparative study on the use of different seeding strategies for soft 3D structures was realized using ultrasound bath seeding, PVA (different concentration) polymer matrix with nanodiamonds, and drop-and-dry of S3 suspension, i.e. ultra dispersed diamond (UDD) particles in water [B2]. As the substrates in this case the Si wafers with fabricated by lithography stripes on it were used for seeding. It was shown that on the contrast to PVA polymer the employed lithography polymer (ma-p 1215) has no negative influence onto diamond growth. Moreover, due to formation of lifted from the background level areas in the form of initial stripes it was assumed that lithography polymer during the deposition process serve as the additional source for diamond growth, similar to [27, 30]. The support of this deduction was observed in the case of PVA (high concentration) employing. The voids were found on the areas with initial polymer stripes while almost free standing diamond crystals were observed out of stripes. In addition, we found that the use of nucleation by ultrasound bath caused damage of the initial polymer edges while UDD drop-and-dry technique keeps the edges undamaged. Finally, use of low concentration PVA with UDD could be used for porous diamond layer formation Diamond structures fabrication by top-down method Due to diamond s chemical resistance (stability) the convenient in the semiconductor industry structuring methods (e.g. by wet chemistry) are not always applicable to diamond films. Nevertheless, the diamond structure fabrication is possible for example by the post-growth etching method [33], i.e. top-down technique. Basically, the postgrowth etching means that from the continuous diamond film the selected (spare) part is removed (etched). The theoretical research shows that quite high etching rates with efficient selectivity can be achieved by employing the reactive ion etching in CF 4 /O 2 based gas mixture [34]. The other important issue in the post-growth diamond structures fabrication is utilizing of proper masking material [35, 36]. The nickel was shown in the testing study as useful masking material while the process conditions were established on the base of the reports about diamond structuring [34, 36, 37]. Detailed study of the influence of different process parameters on the etching rate and the further optimization of the etching process [B19] was revealed in PhD study of T. Ižák (STU, Bratislava, SR). The top-down method of structures fabrication was primarily employed for diamond surface modification via the self-assembling mask strategy, i.e. fabrication of nanostructures. In this case as mask I use nickel or gold nanoparticles formed as result of thin layer annealing or diamond particles distributed on the surface of grown film by ultrasound seeding. The etching was realized applying the RIE system Phantom LT (Trion ink) at power 100 W and pressure 150 mtorr (approximately 20 Pa) in oxygen based gas mixture with 4% of CF 4. After the etching metallic masks were removed by samples washing in the appropriate acid solution by Z. Polačková (the chemist). The diamond seeds were removed during the plasma etching. The diamond film morphologies were investigated by SEM e_line system (Raith) with technical support of K. Hruška. The morphologies of as-grown and etched diamond films are shown in Fig. 11. The initial diamond layer is composed of dense randomly oriented diamond crystal in size up to 400 nm (Fig. 11 a). Depending on the employed masking material the three different morphologies were observed. Applying of gold nanoparticles resulted to formation of structures that have cone-like shape and are densely packed on the sample surface copying the initial diamond grains (Fig. 11 b). Applying of Ni nanoparticles as masking material resulted to formation of diamond nanorods (Fig. 11 c). Finally, using of diamond seeds for masking resulted in formation of isolated grit-like (or cone-like) structures randomly distributed on the diamond grains (Fig. 11 d). The differentiation of resulted structures was explained by ability of masking material withstand the etching process [38-40] and/or their sizes. Thus, the Ni nanoparticles were able to withstand the whole etching process while Au nanoparticles probably were damaged by the ion bombardment. The 12

15 diamond seeds were removed during the etching. The results of this study were published in Babchenko et al., Central European Journal of Physics [B4] as well as used in the further investigations Babchenko et al., physica status solidi a [B28]. a) b) c) d) Fig. 11: The skew SEM images of diamond films: a) as-grown, b) structured via gold mask, c) structured via nickel mask, d) structured via diamond seeds mask. Published in [B5]. In addition, the fabricated diamond films with different morphologies were tested as the artificial substrates for cells cultivation. It was found that morphology of diamond films strongly influences the cells adhesion, proliferation and differentiation. The most significant influence was observed for nanorods and for grit-like structures. The nano-rods caused formation of small focal adhesion complexes localized at the edges of well-spread fibroblast-like cells. While the grit-like structures are characterized as suitable for large focal adhesion complexes localized over the entire cell base, i.e. the osteo-specific cells differentiation, see Babchenko et al., physica status solidi a [B5] and [B6]. Besides, the nanorods, or as they often called nanowires, were utilized for surface area enlargement and employed as a part of detector unit in gas sensing application [B7], which was reviewed in PhD thesis of M. Davydova (CTU in Prague, CR). The observed sensitivity at room temperature of phosgene detector employing nano-rods modified surface was more than 600 times higher in compare to flat diamond. As the example of microstructures fabricated by the mentioned top-down method could be shown the photonic crystals (Fig. 12) that were studied by L. Ondič (CU in Prague, CR) in his PhD work [B12, B13, B26]. In this case the masking material was patterned by employing the electron beam lithography and high optical quality diamond films were used for structures fabrication. The fabricated structures are found as very promising for optics and presently the similar structures are tested as moulds for advanced photonic crystal fabrication employing Si nano-particles. The other study, where this strategy was utilized, was connected with fabrication of piezoresistive strain sensors from the boron doped diamond films which was done in collaboration with P. Kulha (CTU in Prague, CR) and group of K. Haenen (IMO IMEC, Belgium) [B8, B21]. a) b) Fig. 12: The SEM images of: a) Ni mask on the diamond films, b) diamond nanocolumns after etching and mask removing. 13

16 5.3. Diamond structures fabrication by bottom-up method Another method for diamond structures fabrication utilizes the technology of diamond growth on the initially patterned areas, i.e. bottom-up, and is called selected area deposition (SAD). The effectiveness of this bottom-up method is primarily based on the efficiency of seeding layer patterning [41, 42]. Thus, the parameters that influenced growth selectivity are the amount of parasitic diamond crystals (i.e. not wished growth) and seeding density (important for continuous layer formation). Among the known diamond SAD concepts are selective bias enhanced seeding (e.g. on SiN or Pt pre-coatings) [43-45], in-print or ink-jet technologies applying [46], growth in the moulds [47, 48], combination of BEN and moulds [49], employing of diamond seeds in polymer matrix directly used for the lithography [50], etc. The several concepts and procedures were tested in my PhD research work with the aim to define appropriate technology for selected area diamond deposition. The first experiments with the selective growth of diamond nanostructures were performed applying two concepts. The first one is based on the suppressing of diamond growth on the defined areas and the second one employs direct seeding layer modification, i.e. removing of diamond seeds from the areas where no growth should appear. The schemes for both concepts are shown in Fig. 13. a) b) Fig. 13: Schemes for employed selected area deposition concepts: a) growth suppressing, b) seeding layer modification. In the growth suppressing concept seeded parts of not wished growth were covered by 100 nm thick Au layer (Fig. 13 a), while in the case of seeding layer modification uncovered part of SiO 2 layer with staked diamond seeds were removed by buffer oxide etchant (Fig. 13 b). As final step in both concepts the diamond growth in the AIXTRON P6 was realized by our technician J. Potměšil. We found that golden mask employed in the growth suppressing concept was not able to withstand the CVD process and random, not negligible growth of diamond was observed on Au protected areas. On the contrast, in case when the seeding layer modification concept was applied we observed highly selective growth of diamond structures. On the areas with not wished growth the AFM scanning revealed only random single diamond crystal. Density of these residual crystals was less than 10 5 particles per cm 2. This issue was explained by the properties of gold that even in the case of thick film have tendency to draw [51]. However, the similar concepts could be used if the other, more stable, masking materials are employed [52, 53]. On the other hand, in the mentioned cases such drawbacks as complications with lift-off due to thick mask [52] or mask delaminating at high temperatures [53] were observed. The second concept reveals selectivity with residual crystal density as low as 10 5 particles per cm 2 which is a technological limit for natural occurring of diamond on Si defects [15]. The similar procedures with as well good selectivity were reported by other authors too [42, 53]. In our case, the employed procedure was not only successfully used for microstructures fabrication [B9], but also optimized for nanodimensions. In the collaboration with Department of Spintronics and Nanoelectronics (FZU), particularly K. Hruška, we improve the developed concept and fabricate nanostructures in width of only 250 nm (Fig. 14) that were electrically active, Babchenko et al., Vacuum [B18]. 14

17 a) b) Fig. 14: Representative a) scanning electron microscopy image and b) atomic force microscopy map (scan area 5x2.5 µm 2, Z colour scale 250 nm) of as-grown 250 nm wide diamond channel. Published in [B18]. Even though the efficient bottom-up structuring technology was developed it has such a drawback as need of specific type of substrate with sacrificial (i.e. that should be partially or completely removed) oxide layer. Thus, it is not applicable to such substrates as ceramics, ordinary Si, metallic or glass substrates (due to danger of damage), etc. The further developing of selected area deposition method resulted to the studies on the procedure that would be effective on different type of substrates and would not lead to their damaging. This research named developing of universal structuring technology (Babchenko et al., physica status solidi b [B10]) is based on the seeding layer modification concept and combines all knowledge accumulated in previous studies (i.e. seeding strategies, diamond growth on polymer, etching). It resulted to the strategy which principal scheme is shown in the Fig. 15 a. In this case the diamond nucleation was realized in such way that the seeding layer became packed between two photolithography polymers, i.e. sandwich-like organization. During the UV lithography the sandwich-like structure is processed and packed diamond seeds are washed out. Next, the possible diamond residues are removed by the reactive ion etching in CF 4 /O 2 gas mixture. The effectiveness of the developed procedure was similar to the observed in direct seeding layer modification and is characterized by residual diamond crystals density about 10 5 cm -2, i.e. approaching to the technological limit [15]. An example of developed technology is shown in Fig. 15 b, where the diamond structures were formed on the rough Al 2 O 3 ceramic. The SEM image is captured by using Mira 3 microscope, Tescan. We propose that the developed procedure is especially useful in the case of applications that require efficient selective seeding on such substrates as SiO 2, or any other with etchable oxide layer, metals, etc. 15 a) b) Fig. 15: Representative: a) scheme for developed SAD technology and b) SEM image of the edge region between wished and not wished diamond growth areas on Al 2 O 3 ceramic.

18 5.4. Diamond growth mechanism The investigation of diamond growth mechanism realized using large area linear antenna microwave plasma system is a complex of studies oriented onto investigation of gas composition (methane, carbon dioxide, hydrogen concentration), pressure influence and distance to antenna (plasma volume), etc. The main differences of this system to traditional reactors used for diamond synthesis are possibility to perform deposition at lower plasma load (which, however, slow down the deposition rate) and effect of pressure onto plasma volume. The total gas pressure during first experiments on methane concentration was such that the plasma was either localized close to the antenna tubes (2 mbar, Fig. 16 a) or distributed in whole chamber volume (0.1 mbar, Fig. 16 b). a) b) Fig. 16: The plasma discharge in the linear antenna microwave plasma system at gas pressure a) 2 mbar and b) 0.1 mbar. The microwave power and the substrate stage heating were set onto the 2500 W and 650 C respectively. The time of deposition, due to lower deposition rate in comparison with focused plasma system, was 15 hours. In this way the experiments with CH 4 in H 2 concentration ranging from 0.5 to 20% were performed. The starting point of experiments was utilizing of higher methane concentrations as an effort to compensate low deposition rate. The results observed in this study (not shown) reveal low deposition rates typical for this system [22] even at high methane concentrations. Moreover, the morphology of grown films has an ultra nanocrystalline (UNCD) character (i.e. with a lot of graphitized carbon). We propose that the observed character of grown films could be attributed to high secondary nucleation yield reported at low pressures [54, 55]. The low deposition rate (less than 6 nm/hour) observed for pressure 2 mbar even at high methane concentrations was explained by far distance from antennas with plasma discharge (Fig. 16 a), i.e. source of growth species, to the substrate surface [54] that caused slow film formation. Moreover, the saturation of surface reactions (rather low by-products etching than lack of growth species) [56, 57] should be responsible for similar growth rates at different methane concentrations. When the pressure was reduced to 0.1 mbar and plasma expanded from antenna to the substrate stage (Fig. 16 b) the UNCD film morphology remains. On the other hand, the deposition rate in this case was higher and increases with the methane concentration increasing (up to 29 nm/hour for 10% of methane concentration). We propose that pressure reducing caused the prolonging of plasma species mean free path [55] and thus predefine conditions for faster film growth. However, the lack of hydrogen in close vicinity to substrate surface prohibits large diamond crystals formation [54, 57], thus, the UNCD films are formed. The results of the study were published in [B20]. The next set of experiments was realized by adding of carbon dioxide into growth mixture. The proposal was based on the issue that oxygen used in small amount during the diamond deposition in other systems improves etching of non-diamond phases and enhances the growth rate [58, 59]. Thus the experiments were realized at pressure 0.1 mbar and 2.5% of methane in hydrogen (as optimal combination of purity and growth rate from previous study). As previous, the microwave power was 2500 W and the substrate stage heating 650 C. The CO 2 was added into CH 4 /H 2 process gas mixture in amount of 0%, 5% and 10% of H 2 flow. After the deposition grown films were investigated by the SEM measurements using e_line system writer, Raith (provided by K. Hruška) and Raman spectroscopy using invia Reflex Raman microscope, Renishaw (provided by T. Ižák). The resulting SEM images and Raman spectra of deposited films are summarized in the Fig. 17. The SEM images (Fig. 17 a-c) clearly show changing of surface morphology as influence of CO 2 adding. Similar to previous case, at 0% of CO 2 in process gas mixture the film with UNCD morphology (grain size around nm) and clustering was observed (Fig. 17 a). The 5% of CO 2 in growth mixture resulted in film morphology modification (Fig. 17 b). We proposed to identify observed features as irregular crystals of estimated size up to 400 nm. Finally, the 10% of CO 2 in process gas mixture resulted in nanocrystalline diamond (NCD) film formation with crystals size up to 500 nm (Fig. 17 c). Moreover, the CO 2 in growth mixture significantly improves growth rate, from 10 nm/hour (growth without CO 2 ) to 35 nm/hour (5% of CO 2 ) and 38 nm/hour (10% of CO 2 in mixture). 16

19 a) b) The changes of grown films composition were monitored by the Raman spectroscopy (Fig. 17 d). The peaks and bands detected in the spectra were the G-band, centred at cm -1, D-band, centred at cm -1, diamond peak, centred at 1332 cm -1, and transpolyacetylene residues at grain boundaries ( cm -1 ) [98-100]. According to diamond peak intensity the amount of sp 3 related carbon it the layer increases with CO 2 adding. On the contrast, at pressure 2 mbar the CO 2 adding increase the growth rate but does not change the UNCD morphology and only slightly sp 3 phase amount. We assumed that the high re-nucleation observed in the case of CH 4 /H 2 process gas mixture [54, 55] is suppressed by the presence of oxygen [59]. On the other hand, such parameter as pressure affects the growth species distribution [54, 57], thus, surface reactions important for film formation [56, 57]. Hence, when the diamond deposition was realized at pressure 0.1 mbar the crystals size increasing was observed, while at pressure 2 mbar the UNCD morphology of grown films remains. Besides, the diamond growth using oxygen contained process gas mixture in the linear antenna plasma system should take into account plasma-nucleation as well [60]. Altogether these parameters are affecting the growth rate and film morphology. Therefore, we proposed that in the case of deposition at pressure 0.1 mbar the lateral diamond growth is dominating process that resulted to NCD films. While at the of pressure 2 mbar, due to possible lack of growth species and slower by-products etching, the dominating process is the re-nucleation which resulted to UNCD film morphology. The results of the study were published in the [B20]. The effect of pressure (plasma volume) and table position was investigated in the other study that confirms our previous assumption. Moreover, it confirms the principal role of pressure for the films morphology formation. This influence is dominant due to change of mean free path (affected by pressure) of growth species [55] which varies their concentration near the substrate surface even at the same gas composition. Next, on contrast to standard deposition systems [54, 55, 61] the table position has less significant influence onto morphology of grown diamond films affecting rather the growth rate and the homogeneity of deposition within the process area. The temperature load of samples placed closer to antennas (hot plasma) is increased as well. In our case the system design and safety limitations prohibit samples position at close vicinity to antennas due to lower plasma discharge stability. However, we assume that further approaching of the samples towards the antennas (hot plasma) will approximates the process conditions to focused plasma systems, i.e. the case of Bachman s diagram and no diamond growth at chosen conditions due to very high etching rate [5, 7]. 17 c) d) Fig. 17: The SEM images of films grown at 0.1 mbar pressure and a) 0% of CO 2 in growth mixture, b) 5% of CO 2, c) 10% of CO 2 and d) Raman spectra measurements.

20 Further increasing of oxygen content in the growth mixture caused decreasing of growth rate, and has negligible influence onto diamond film purity which is in agreement with work of Y. Liou et al. [68]. However, via increasing the methane concentration in the growth mixture it is possible to shift the limit of maximal carbon dioxide amount in the growth mixture to higher values [16, 61]. Another set of studies was devoted to the change of hydrogen amount in the growth mixture [B24]. Similarly to the focused plasma system (or any other standard deposition system) in the linear plasma system the hydrogen plays a dominant role in the crystals facets formation [54, 56, 57]. However, in our case high H 2 concentration in the gas mixture caused significant etching of carbon phase [5-7] and due to low pressure and temperature slow down the film growth [56, 61]. With reducing of hydrogen amount the relative concentration of carbon (and oxygen) in growth mixture increases and the diamond growth became possible. The further H 2 amount decreasing caused the lack of hydrogen and worse facets formation, i.e. grown films became UNCDlike but growth rate remain high [5, 57]. Further H 2 reducing increases influence of CO 2 component on the diamond deposition [59]. This means that the high CH 4 relative concentrations that forced fast film formation, is compensated by etching caused by CO 2 [58]. Therefore, the voids formation in the deposited layer is stimulated as result of different etching efficiency of sp 3 and sp 2 carbon phase. Finally, this process resulted to wire-like character of Fig. 18: The SEM image of wire-like diamond grown at the non-growth region according Bachman s diagram. nanodiamond agglomerates and porous diamond films are formed (Fig. 18). The observed purity (i.e. diamond character) of grown layers in this case is explained by efficient etching rate of oxygen related species [58, 59]. These deductions were further confirmed by the optical emission spectroscopy measurements carried out by Š. Potocký [B24]. Finally it should be noted that the observed results of diamond deposition are located out of known diamond growth region stated by the Bachman s diagram [7]. According the observed results [B23, B24, B29] we proposed that diamond growth in the linear antenna plasma system differs from the standard conditions which was also shown in similar systems by researchers from Japan National Institute of Advanced Industrial Science and Technology [22, 60]. The summarized knowledge about the diamond growth in the linear antenna plasma system allows fabrication of optical elements suitable for infrared spectroscopy (Fig. 19), see Babchenko et al., physica status solidi b [B14, B15, B30]. The advancing features of such elements are, for example, resistance to scratching and surface that could be functionalized for homogeneous distribution of studied material. The fabricated diamond coated mirrors (Fig. 19 a), were recognized as utility model [C3] while diamond coated Si prism (Fig. 19 b) was successfully employed, for example, in the investigation of functionalized diamond nanoparticles [B27]. a) b) Fig. 19: The photos of optical elements coated by diamond films in the linear antenna plasma system: a) gold mirrors (for grazing angle reflectance), b) Si prism (for attenuated total reflectance). 6. Carbon nanotubes The results of research work related to carbon nanotubes (CNTs) synthesis are summarized shown below. The realized experimental work dealt with catalyst particles preparation and CNTs synthesis and understanding of growth processes in two types of deposition equipment. 18

21 6.1. Catalyst fabrication As it was shown by the literature survey, one of the important items in an efficient carbon nanotubes synthesis is the preparation of appropriate catalyst [11, 12, 62]. The way of catalytic particles formation depends on the technique used for further CNTs deposition. In my case I used the strategy based on the catalyst particles preparation from the thin metallic layer as the result of plasma treatment. As it is shown in the literature, the size of the resulted catalyst particles depends on the thickness of initial metallic layer [11, 12, 63, 64]. On the other hand, the influence of the used equipment for metal deposition and annealing as well as applied treatment procedure should also be considered [63, 65-67]. Therefore, calibration of our equipment and testing experiments to define appropriate catalyst film thickness were required. Basing, on the chosen technology the systematic investigation of thin metallic (nickel) layer hydrogen plasma treatment was realised. By the home-made system for metal evaporation the Ni layers with thicknesses from 0.9 nm to 12.5 nm were deposited on SiO 2 substrates side. Samples, prepared in such way, were annealed by hydrogen plasma in the focused microwave plasma system AIXTRON, P6, realized by J. Potměšil. The plasma discharge was ignited at the pressure 30 mbar, microwave generator power 1300 W, and H 2 flow 300 sccm. The time of annealing was 5 min at substrate temperature up to 750 C. Next, samples were examined by scanning electron microscopy (SEM) on e_line system (Raith) with support of K. Hruška (shown partially). As observed by SEM (Fig. 20), after the 5 min treatment at the high temperature continuous metal layers are changed into nanoparticles and/or island-like features due to reducing of surface strain [51, 64-68]. Depending on the thickness of primary Ni layer the two different types of formed features were distinguished. The initial Ni layers with thickness less than 5.9 nm resulted in a formation of circle-like (or island-like) nanoparticles in diameter ranging from nm (primary Ni layer 0.9 nm) up to about 120 nm (primary Ni layer 5.9 nm). The representative SEM image of formed nanoparticles is shown for initial Ni layer 1.5 nm (Fig. 20 a). a) b) c) Fig. 20: The SEM images of Ni particles formed from layer with initial thickness of: a) 1.5 nm, b) 7.4 nm, c) 12.5 nm. For Ni layer thickness of 7.4 nm, a transition (break) point, where circle-like features are changed to rather rectangle-like, was observed (Fig. 20 b). The area of these rectangle-like features increases from 100x200 nm 2 up to 200x300 nm 2 as the primary layer thickness increases from 7.4 nm to 10.6 nm. In addition, small and isolated nanoparticles with diameters of nm were observed over the remained area. The similar particles size dissipation was for example reported for cobalt films [65]. Thus, we assume it is regular. For the film thickness of 12.5 nm, the tendency in clustering of Ni features was detected. Formed clusters were of complicated geometry, but in general they can be defined as the rectangular structures in size around 300x500 nm 2 (Fig. 20 c). The particles of smaller sizes were not observed in this case. The sizes of resulted catalytic particles as the function of primary Ni layer are plotted in the Fig. 21. The observed increasing of catalyst particle size is typically attributed to increase of initial layer thickness [63-66]. In the prolonged treatment (up to 30 min) we observed that Ni films thinner than 6 nm has tendency to catalyst particles enlarging. The increasing of resulted particles size is attributed to ability of small metal particles to migrate over the substrate surface at high temperatures (Brownian-like motion) [68]. Thus, large particles would be formed as the result of coalescence. On the other hand, Ni film with initial thickness of 12.5 nm reveals tendency to transformation to the rectangular-like features into clustered features consisted of nanoparticles with 19 Fig. 21: The sizes of resulted catalytic particles as the function of primary Ni layer thickness.

22 different size, but generally smaller than for 5 min annealing. It has been supposed H 2 plasma etching of nickel [69] similar to effect of NH 3 plasma [70]. On the other hand, any clear evidence of this process was not given yet. Therefore, we propose that in our case the hydrogen plasma, probably, does not significantly etch Ni but rather forced it stabilization (i.e. separation as result of bombardment). The sick SiO 2 layer should prevent Ni to diffusion the Si [64]. Nevertheless, it needs to be taken into account in the case of sensibly longer processes and/or processes with higher temperature when the oxygen from SiO 2 layer would be reduced by free hydrogen CNTs synthesis in the focused plasma system Concerning the nanotubes synthesis in the focused plasma system, the first experimental effort was to synthesize carbon nanotubes directly by the MW plasma as it is reported by some authors [65, 71]. Although we make assumption that particles formed from Ni film less than 6 nm should be used for CNTs synthesis the role of equipment was not clear [63, 71-73]. Due to the fact that our focused plasma system is primarily targeted on the diamond deposition it was necessary to define the influence of particles size onto CNTs synthesis, e.g. particles stability during the deposition as similar to treatment time dependence. The set of samples with different Ni layer thicknesses (duplicate of employed in previous study) was utilized for first experiments with CNTs growth in focused plasma system. The samples were treated for 5 min by hydrogen plasma as described previously. Then, the methane was added to hydrogen in amount of 40% of H 2 flow. The growth step was running for 10 min. with temperature around 720 C, microwave power 1300 W and pressure 30 mbar. After the deposition samples were analyzed by scanning electron microscopy with support of K. Hruška. The representative SEM images of processed samples are shown in Fig. 22. The poor growth of nano-structures was noticed in case of 0.9 nm Ni film while for substrates with initial Ni film from 1.5 nm to 5.9 nm growth of short, randomly oriented nanostructures was observed (representative image Fig. 22 a). In this case the Ni particle at the end of each nanostructure is indicating tip-type of growth and curled nature of the bamboo-like multi-walled nanotubes [12, 71]. Besides, the structures similar to CNTs with multi tips [74] were observed. Very few in amount, short and tiny, randomly grown nanostructures were found on the substrates with initial Ni film from 7.4 to 12.5 nm (representative images Fig. 22 b, c). The formed structures are looks rather as Ni particles encapsulated in the carbon. On the other hand, some authors reported about large CNT-like formation synthesis on thick catalyst by the direct current plasma while the typical thermal CVD failed [73]. Therefore, we propose that for our system in used configuration and process parameters Ni film with thickness less than 6 nm is preferable. The minimizing of particles size, in compare to just after annealing, was attributed to reducing of surface strain [68] that could be as well influenced by presence of CH 4. a) b) c) Fig. 22: The SEM images of CNTs grown using Ni particles formed from layer with initial thickness of: a) 1.5 nm, b) 7.4 nm, c) 12.5 nm. In the further experiments with growth condition optimization I use gas mixtures with different concentration of CH 4 since it affects the catalyst activity [75]. The observed difficulties in CNTs synthesis in previous study could be attributed to high etching rate of hydrogen in plasma [67]. Unfortunately, the set of experiment with different CH 4 concentrations show that high amount of methane probably contaminates the deposition chamber. Thus, the attempt to compensate high hydrogen etching rate by methane concentration increases the danger of saturated surface reactions [76]. Therefore, any similar to reported [72, 75] CNTs growth cannot be realized. Basing on these experiments we propose that for our system the methane concentration lower that 40% are desirable, similar to [71]. Finally, we assume that in our case, besides methane concentration, the other limitations for CNTs synthesis are present. While some authors report about successful carbon nanostructures growth in the MW plasma only [71, 72], other insist on the employing of substrate bias [77, 78]. The most authors agree that employing of negative substrate bias promote the growth and alignment of carbon nanotubes [77-79]. Therefore, to promote the CNTs formation, in the following experiments the negative bias potential was applied to substrate stage. As in the previous studies firstly the catalyst particles were formed by the Ni film annealing. Then the CNTs growth process was realized at CH 4 20

23 concentration in H 2 of 10% and 40%. The negative bias potential applied to samples was in magnitude of 200 V. After the deposition the samples were analyzed by SEM as provided by K. Hruška. a) b) Fig. 23: The SEM images of carbon nanotubes grown with bias at methane in hydrogen concentration of a) 10% and b) 40%. The growth of nanotubes with diameter in range from 20 to 100 nm was observed in both cases (Fig. 23). However, CH 4 concentration of 10% results to better, i.e. more dense and longer, structures (Fig. 23 a). The higher methane concentration (40%) results in smaller amount of nanotubes that were shorter in length (Fig. 23 b). Thus the issue of saturated surface reactions at higher methane concentration that suppress nanotube structure assembling, or as in case of [71] caused lower nanotubes density is evident. On the other hand, the dense bundles of curled nanotubes formed at CH 4 concentration of 10%, weren t expected. It let us to assume that growth of aligned nanotubes in the our CVD set is quite challenging and moreover requires major changes of deposition equipment, thus, not reasonable CNTs synthesis in the linear plasma system The first experiments with CNTs synthesis in linear plasma system were performed while taking into account information from the literature survey and results observed with focused plasma system. After the catalyst annealing process (similar to realized by focused plasma system) the CNTs deposition was done at the moderate CH 4 concentration. The nanotubes were synthesized for 30 min from methane-in-hydrogen gas mixture (30% of CH 4 in H 2 ) maintained at pressure 0.2 mbar. The plasma discharge was initiated either by radiofrequency (RF) generator at power 600 W and bias 500 V or by MW generator at power 2000 W with on/off pulse cycle 6/3 ms or by their combining (MW+RF). The heater temperature during the process was 600 C. After the deposition the samples were analyzed by scanning electron microscopy (e_line system writer, Raith) as provided by K. Hruška. The SEM images of grown structures, captured at the angle of 45 (for better visualization), are shown in the Fig. 24. It was found that even if the microwave plasma discharge effectively decomposes working gases to growth species it is quite far from the substrate to catalytic reactions could be initiated. Therefore, the CNTs deposition in the remote microwave plasma [67] can t be realized (Fig. 24 a). Next, on the contrast to reported success with CNTs growth by RF plasma [11, 12] in our case the temperature was probably not high enough for efficient carbon species decomposition. Thus, only short randomly oriented carbon nanotubes formations were observed (Fig. 24 b). Finally, in the case of combined RF and MW plasma we observed densely packed and vertically oriented nanotubes with diameter in the range of nm (Fig. 24 c). We propose that in the combined arrangement of RF and MW powering the microwave plasma creates the growth species in enough quantities and RF helps to transport these species to the substrate a) b) c) Fig. 24: The SEM images of carbon nanotubes grown in the AK 400 employing: a) microwave plasma, b) radio frequency plasma, c) combination of microwave and radio frequency plasmas. 21

24 surface where the synthesis reactions take place. The similar effect, i.e. necessity of two different plasma types for aligned CNTs growth was also observed by other authors [78, 80]. Therefore, next experiments with the carbon nanotubes growth in the linear antenna plasma system were performed applying together radio frequency and microwave ignited plasmas. In the set of experiments with different methane concentrations were tested 10, 20, 30, 40 and 60% methane in hydrogen amounts. After the deposition samples were analyzed by K. Hruška employing SEM under the angle of 45. To evaluate the CNTs height the nanotubes forest was scratched (peeled) from the substrate by the tweezers and SEM image was captured from the edge of the scratch. The Raman shift measurements were realized by T. Ižák using Renishaw invia Reflex Raman microscope. In these experiments the dense forest of tip-type nanotubes was found on all samples. The representative image of formed structures is shown in Fig. 25 a. Concerning the methane influence I observed that although the shapes of formed structures remain the same their length decrease with increasing of CH 4 concentration. At CH 4 concentration in gas mixture of 10% CNTs length was more than 300 nm, however, when the CH 4 concentration was as high as 60%, the CNTs length decreases to 200 nm. The shorter length is probably observed due to reactions saturation on the catalyst particles, i.e. when particle become fully covered by carbon the growth is slowing down [76]. Thus, we concluded that for our set up higher methane concentration negatively influencing the nanotubes growth [71]. a) b) c) Fig. 25: The SEM images of a) dense nanotubes forest, b) edge of the scratch in the CNTs forest and c) Raman shift measurements. The nanotube width in all cases decreases with the time within separate deposition, thus forming the conical structures (Fig. 25 b). The initial (i.e. at the bottom) CNTs width was the same in all cases and was defined by the size of catalyst particles [62-65]. Due to conical shape of the grown nanostructures we can speculate about the growth of either multi-walled CNTs in the form of cones [74] or carbon nanofibres ( herringbone nanostructures) [3, 12]. The cone-like multi-walled nanotubes are observed in the case when the initially cylindrical MW CNTs are affected by the high level plasma destruction [74]. In case of carbon nanofibres, first of all, it should be noted that the mechanism of their formation is quite similar to nanotubes [77, 80]. Moreover, the characteristic Raman spectra of these carbon-based features could be distinguished only by subtle difference [80, 81]. The typical Raman spectrum (measured using 442 nm excitation wavelength) in case of our structures is shown in the Fig. 25 c. The observed positions of D-band at 1354 cm -1 and G- band around 1600 cm -1 confirm the graphitic nature of grown structures and let us to assume that the grown structures are most probably multi-walled CNTs with high number of defects [81]. The weak signal is attributed to quite short nanotubes and to lower sensitivity of our Raman set up to sp 2 carbon phase. In the study related to the plasma volume influence onto nanotubes formation the CNTs were grown at different pressures and different substrate table position. We found that at low pressures the low plasma density caused formation of few in number and less aligned bamboo-like [12, 71] structures. While at high pressures the lack of growth species affects the structures formation and poor CNTs growth is observed. Basing on these experiments and on the observed in section related to diamond we can claim that in our plasma system the growth species are localized generally closer to antennas. It is become more evident at higher pressures, i.e. low plasma volume. Therefore, we propose that generally for this type of CVD system (i.e. with linear plasma) the influence of pressure onto grown structures is more significant than distance to hot (microwave) plasma region. By effective changing of plasma volume the pressure affects the grown structures morphology while distance to antenna rather influences the growth rate due to changes in the growth species concentration. Finally, the linear plasma system was used for single walled CNTs surface modification, namely oxidation, to increase their wettability (Fig. 26). The study shows that 5 min oxidation does not cause significant damage of SWCNTs and improves nanotubes biocompatibility [B16]. Moreover, such treatment could be employed for improving of CNTs dispersion for concrete fabrication [C2]. 22

25 V. Conclusions The summarization of the all realized studies is given below. The results obtained in the diamond films direction were as follows. First of all, during the studies on nucleation technologies experimentally was shown that for the diamond seeding by ultrasound agitation the size of used particles and treatment time play an important role in seeding layer formation. Achieved seeding density using 40 min treatment in the water based diamond powder (4-5 nm) suspension was sufficient for continuous layer formation even in case of thin (100 nm) diamond films growth. Thus, the seeding procedure that at present time predominantly used in our laboratory was established. The formation of porous diamond layer was achieved using shorter nucleation time and/or suspension with larger (up to 900 nm) particles. The next outcome was an approving of nucleation technology that has a potential in the fabrication of transportable seeding layers and/or fabrication of porous diamond layers. This technology is based on nucleation employing diamond nanoparticles embedded in polyvinilalcohol matrix. The one of this technology advances is employing of non-aggressive water-mixable components. Therefore, the realized composite of polyvinilalcohol with diamond nanoparticles is simple in fabrication and avoid environment pollution. Finally, the comparative investigation of different technologies for soft 3D polymer structures seeding was realized. Their influence on grown structures topography, formation of porous diamond layers or diamond air bridges was discussed. In general, the formation of porous and continuous diamond films employing different nucleation strategies was achieved. The porosity increases diamond films active surface area and thus overall chemical activity of the diamond surface. Therefore, porous diamond films could be effectively used in the studies related to gas and/or liquid sensor devices. Nevertheless, the more attention in my work was devoted to continuous diamond thin films as bulky uniform material. Generally, the continuous diamond films were utilized for fabrication of diamond structures and further studies. The diamond structures fabrication by the top-down method (i.e. from continuous diamond film) was realized using the reactive ion etching technology. During the experiments related to these studies first of all the efficient etching procedure for diamond structures fabrication was established and appropriate masking material was defined. Next, the diamond nanostructures were realized by etching of continuous diamond layer using self-assembled metal nanoparticles as mask. The formed diamond nanostructures were suitable for surface modification and application as biosubstrates in advanced studies (i.e. for cells growth and tissue engineering in perspective). Moreover, the enhanced (due to structuring) diamond surface area was found attractive for sensing applications studies (e.g. used for phosgene gas sensors). Finally, different diamond microstructures realized by the top-down method were used in various advanced studies (e.g. as piezoresistive sensors or photonic crystals). In the studies related to patterned diamond film growth (bottom-up method of structures fabrication) several technological procedures that utilize two concepts (growth suppressing and seeds removing) were systematically studied. The best results were achieved by the concept that is based on the seeds removing. The procedures utilizing acid-assisted removing of diamond seeds staked to sacrificial SiO 2 layer or seeds removing employing combination of lithography with plasma etching were the most effective. These procedures reveal the values of parasitic diamond crystals as low as technological limit for natural occurring of diamond on Si defects. The nano-sized (250 nm wide) electrically active structures were realized employing acid-assisted seeds removing. The combination of lithography with plasma etching was shown as the nondestructive method for selected area diamond deposition applicable to various substrates. The series of studies oriented on understanding of diamond films growth phenomenon were realized in the linear antenna pulsed microwave plasma system. The experiments were mainly focused on the investigation of process chemistry and plasma volume influence onto morphology and purity (sp 3 content) of grown diamond films. In these studies the continuous nanocrystalline and ultra nanocrystalline diamond films growth was achieved as well as porous diamond layers. The systematic studies of growth trends and diamond morphologies at various gas compositions were helpful for understanding of diamond growth processes (e.g. in the oxygen presence). The tendencies in diamond film depositions at different pressures (affecting the plasma volume), among the other results, reveal the importance of growth 23 a) b) Fig. 26: Water contact angle of: a) as-received SWCNTs, b) SWCNTs treated by 5 min oxygen plasma.