1-1 Diamond
The diamond crystal is formed by SP3-bonding structured carbon atoms. There are two face-centered cubic lattices in the diamond lattice shifted by a vector (a/4, a/4, a/4). The lattice parameter a is 3.56683Å . Diamond has the greatest number density, i.e. atoms per unit volume, of any known substance. This combined with the strength of the carbon-carbon bond, giving rise to an extraordinarily high bond energy density.
Therefore, diamond exhibits extremely mechanical properties. Diamond is known as the hardest natural materials scoring 10 on old Mohs scale of mineral hardness [1.1].
The high bonding energy density also leads to good chemical stability for resisting with a wide band gap of 5.45eV. Due to the wide band gap, diamond has high optical transmissivity around >95% form deep UV to far IR. The wide band gap grants diamond a very low or negative electron affinity; hence, it is then able to emit
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electrons from its surface with very little applied voltage.
1-2 Synthesize of diamond
1-2-1 HTHP (high temperature and high pressure) diamond
In 1954, Hall and his co-workers in General Electric (GE) achieved their first commercially successful synthesis of diamond under HPHT (high pressure and high temperature) where diamond is a more stable form than graphite [1.4, 1.5]. Their breakthrough was to use a belt press which was capable to produce the pressure over 10 GPa and temperature above 2000oC. Because of the large activation energy required for breaking carbon-carbon bonds, graphite is not necessarily transformed into diamond, even under high pressure and temperature [1.6, 1.7]. The molten iron, nickel or cobalt which acted as solvent-catalyst was dissolved in graphite and accelerated its conversion into diamond. In addition, the morphology of the grown crystals is determined by the pressure and temperature. Furthermore, the growth rate of diamond depends on the solubility of carbon on graphite-metal and metal-graphite interface. Therefore the concerns for the features of the stable HPHT conditions were kept to grow high-quality diamonds. The mass-production and high-quality diamond crystals make the HPHT process a more suitable choice for industrial applications.
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1-2-2 CVD (chemical vapor deposition) diamond
In the 1950s, a domain of research performed in the Soviet Union and U.S. focused on pyrolysis of hydrocarbon gases at the relatively low temperature at 800oC. This low-pressure process is known as chemical vapor deposition (CVD). Since the early 1980s, this method has been the subject of intensive worldwide research. The advantages of CVD diamond growth include the ability to produce diamond over large areas and on various substrates, and its well control over chemical impurities.
The CVD process does not require pressure as high as HPHT.
To activate the carbon-containing precursor molecules in the CVD process, the gas typically must reach a temperature exceeding 2000oC. To achieve this target, several techniques can be employed, including hot filament, plasma-assisted (DC or microwave), combustion flame, and so on activation. Because hot-filament CVD is a simple growth method, it has been widely accepted among diamond researchers in the early stages of research [1.8, 1.9].
The crucial fact for the growth rate and crystal quality is not only the gas temperature, but also the gas composition, which typically use the mixture of CH4 and H2. During the diamond growth which is under low pressure, the graphite phases are formed on the growth surface in a certain amount which is in accordance with the diamond phases. Consequently, the graphite must be removed effectively to continue
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the diamond CVD. For keeping the diamond phase stable, the atomic hydrogen has gotten the function to etch the graphite phases selectively. The carbon source gas must therefore be highly diluted with hydrogen. The hydrogen atoms which are characteristic of diamond CVD under low pressure are believed to play crucial roles in the process.
Depending on the types of substrate, there are two principal synthesis varieties: the growth of diamond on diamond (homoepitaxy) and on non-diamond substrate (heteroepiyaxy). Homoepitaxy growth results in monocrystalline layers with superior properties. However, the growth rate is slow due to the low surface chemical activity of diamond. Additionally, the diamond substrate is expensive and limited in size. In the growth of heteroepiyaxy, films may consist of oriented or non-oriented grains.
They contain numerous defects, like grain boundaries and amorphous carbon, but can be grown to large sizes.
1-2-3 Detonation of explosive
The explosive detonation is the other method of diamond synthesis. The technology was based on the detonation transformation of carbon-containing explosives with negative oxygen balance. The product is a mixture of different kinds of carbon, carbon black, etc; the major product among them is ultra-dispersed diamond (UDD) powder which consists of highly defective structure and an active surface with special
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absorption ability and high reacting capacity [1.10]. Nevertheless, it is not able to control the production more precisely in diamond size.
1-2-4 Ultrasound cavitation
Micron-sized diamond crystals can be synthesized from a suspension of graphite, which is in the organic liquid at atmospheric pressure and room temperature, by using ultrasonic cavitation. The diamond yield is about 10% of the initial graphite weight.
The estimated cost of diamond produced by this method is comparable to the HPHT method; the crystalline perfection of the product is significantly worse to the ultrasonic synthesis. This technique requires relatively simple equipments and procedures, however, it has only been reported by two research groups, and has no industrial use as of 2009 [1.11]. Numerous process parameters are not yet optimized, such as, the preparation of the initial graphite powder, the choice of ultrasonic power, synthesis time and the solvent. However, it does leave a route for potential improvement of the efficiency and reduction of the cost of the ultrasonic synthesis.
1-3 Impurities in diamond
Pure diamond crystal is colorless. Colored diamonds contain crystallographic defects, including intrinsic defects and impurity-related defects, which cause the
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coloration. Intrinsic defects include vacancies, interstitials and extended structural defects. Small foreign elements, such as nitrogen and boron, can appear as single atoms at lattice sites, while large impurity atoms tend to form complexes with other impurities or vacancies. Nevertheless, more than 300 optical centers due to the element H, He, Li, B, N, NE, S, Si, P, Ti, and so on in diamond are documented [1.12].
1-3-1 Classification of diamond
Natural diamond can be typically classified into four types, i.e. Ia, Ib. IIa and IIb, according to the dominant type of defect present. More than 95% of natural diamonds belong to the type Ia. There was the highest concentration of nitrogen (up to 3000 ppm) in type Ia diamond. The aggregative nitrogen present in two major structures of which are the A-center and B-center. The A-center consists of a pair of substitutional nitrogen atoms in nearest neighborhood, while the B-center occurs as a complex of four substitutional nitrogen atoms surrounding a lattice vacancy. The concentration of nitrogen which is ranging between 150 to 600 ppm is still considerable in type Ib diamond. Nitrogen forms dominantly in single-substitutional structure. The single-substitutional nitrogen atoms act as donor centers in diamond. The optical absorption of type I diamonds give rise to slightly yellow color [1.13].
The concentration of nitrogen impurities lower than IR absorption sensitivity (~
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1ppm) was defined as type II diamond. The type IIa diamond refers to the purest crystals. Boron is one of the most dominant impurities in type IIb diamond. The only boron related defect is single substitutional acceptor with an energy level 0.37eV above the valence band. Therefore, the type IIb diamond shows P-type conductivity at room temperature. Optical absorption which gives type IIb diamond the characteristic blue color was produced by ionization of the acceptors.
The CVD diamond can contain different impurities, depending on the substrate and mixing reaction gases. This classification is not used in CVD diamond, yet, is used for HPHT synthesis diamond. Nitrogen is the most common contamination in the HPHT synthesis diamond. Most diamond synthesized through the growing process by HPHT is the type Ib. If boron was added into the growth chamber to reduce the concentration of nitrogen, the type IIb diamond can be produced. The high temperature annealing which lead to aggregation of single-substitutional nitrogen converse type Ib into type Ia [1.14].
1-3-2 Analysis of defects in diamond
The Raman spectrum of diamond consists of a sharp peak at 1332cm-1, while graphite gives a rise in the boarder peaks characterized by the D-band at 1350cm-1 and the G-band at 1580cm-1. The width of 1332cm-1 peak line reveals that how much random stress exists in the diamond [1.15]. The figure 1-1(a) shows the Raman
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spectra of three different CVD diamond films. The quality of diamond is clearly distinguishable. The excitation wavelength influenced the results of Raman spectra.
The figure 1-1(b) shows the different spectra of various wavelength of laser excitation of diamond. While IR laser was used in Raman measurement, non-diamond components emerge in the spectra due to that the IR laser is more sensitive for SP2-bond carbon scattering. If the Raman spectra were measured by UV laser, the fairly good quality can be obtained [1.15].
The surface morphology and structure properties also influenced the results of the Raman spectra. In figure 1-2, Raman spectra of nanodiamonds of different size are presented. For the size of 5-50nm nanodiamond, characteristic D-band and G-band were dominated. For the size of 100-500nm, the intense diamond peak at 1332cm-1 exhibited with traceable graphitic or amorphous signature [1.16]. However, for the size larger than 50nm, the structure is usually explained as diamond polycrystal structure with graphitic structure on surface. While the nanodiamond with size 100nm and larger, the Raman spectra have similar characters of bulk diamond which has strong and sharp 1332cm-1 peak. For the smaller nanodiamond, the graphitic or amorphous structure dominated the spectra. We observed that nanodiamonds with sizes 100 nm and larger, the Raman spectra have similar characters with sharp and intense diamond peak; while the spectra for 5–50 nm nanodiamonds appear differently.
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The same difference was observed in photoluminescence spectra.
Luminescence can be excited by either an electron beam or light. The methods are called cathodoluminescence and photoluminescence, respectively. The Raman scattering is often measured together with photoluminescence. Additionally, electron beam excites different optical centers at the same time, which leads to complications in interpretation of the spectra. By contrast, selective excitation is allowed in PL by choosing proper excitation wavelengths. Figure 1-3 (a) and (b) depicts the photoluminescence spectra of different sizes nanodiamonds obtained by using 488 and 532 nm excitation wavelength, respectively [1.16].
Electron spin resonance (ESR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons. Unpaired electrons in solids have a non-vanishing spin which is associated with a magnetic moment. Consequently, the spin states have different energies in a magnetic field. Natural and synthetic diamond has been extensively studied by ESR, now, this method is gaining new attention due to the possibility of using the spin states in the defect center to implement a solid state quantum bit at room-temperature [1.17].
1-3-3 Color center of diamond
If the excited and ground states of a defect are both located within the band gap and then an optical dipole transition is allowed between them, this defect is luminescent
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under excitation. Those defects give the crystal a characteristic color, and are thus called color centers. A huge variety of optical color centers which maintain more than 100 different color centers can be found in diamond owing to the wide band gap of diamond [1.18].
A very important transition occurs between the levels M=0 and n=0, it is the zero-phonon line. In absorption, the phonon-assisted transition takes place at energies which is higher than ZPL; in emission, however, it is at lower energies, which leads to a mirror symmetry relative to the ZPL. In addition, the charge states are another important property of the color centers. Generally, different charged states give rise to the complete spectra. In the semiconductors with shallow donors and acceptors, the equilibrium charge state of defects is determined by the position of the Fermi-level.
Nitrogen is the most prominently known impurity, which forms the nitrogen vacancy defect in diamond and it consists of a substitution nitrogen atom with a next nearest neighbor vacancy. According to the impurities which are close to the surrounding area acting as electron donor or acceptor, the defects can form two types of states: neutral nitrogen vacancy center [(N-V)0] and negative-charged nitrogen vacancy center [(N-V)-] [1.19-1.22].
The [(N-V)-] defect in diamond which consists of a substitution nitrogen atom with a next nearest neighbor vacancy is the most dominant defect in irradiated and
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annealed type-Ib diamond. The resulting N-V pair has the expected C3v symmetry. It is established that the ground state is spin triplet and the ZPL absorption corresponds to 3A3E transition. The excited orbital doublet state produces the characteristic zero-phonon line at 637nm (1.945 eV) with a radiative decay time of 13ns [1.23]. The center also produces a paramagnetic resonance signal, which are the properties confirming the C3v symmetry.
The [(N-V)0] center has nominal C3V symmetry and zero-phonon E-A optical transition at 575nm (2.158 eV). The remarkable point of the 575nm center is the absorption that is to be very weak to detect, especially in the case of highly nitrogen contained type-Ib diamond. Its lifetime is 6ns at room temperature [1.24].
It has been reported that several other color centers emitting at the wavelength of 768, 746, 749, 764, 756, and 772nm was ascribed to defects containing Ni, Si and Cr atoms [1.25-1.29]. These centers were found in diamond nanocrystals grown by CVD method or were made in bulk diamond by ion implantation of the corresponding species. Even the atomic structure of these centers has not been established yet.
However, the 768nm center has been tentatively ascribed to a complex containing Ni and Si atoms: the 746, 749, 764, 756nm centers have been ascribed to Cr-related defects, while the 772nm center night is a defect incorporating Cr and Ni atoms. A great advantage of these centers is the short luminescence lifetime. For example, the
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luminescence lifetime of the 749nm Cr-related center is about 1 ns, which makes it the most efficient single-photon emitter working in bulk diamond.
1-4 Application of diamond
The research efforts in diamond synthesis are rewarded by its unique properties, which are then useful for many applications. Diamond has various advantages for electronics in the semiconductor. Due to the band gap, it is inherently suitable to emit or detect UV light. The low electron affinity allows electron emission at low temperatures. Devices based on diamond work readily at harsh conditions, e.g. under radiation or chemical corrosion. UV-sensors[1.30], light emitting diodes[1.31], cold cathode,[1.32] metal-semiconductor FETs,[1.33] and electro-chemical electrodes[1.34]
have all, meanwhile, been implemented using diamond.
1-4-1 Nanodiamond as bio-marks
Over the past few years, nanomaterials, a branch of nano-biotechnology with emphasis on the views of the biological, have attracted much attention [1.35]. The study of nanomaterials involves biological applications and fabrication of bio-functionalized devices. Quantum dots have specifically been applied in fluorescent probes in recent years, which were immediately turned into a brightness
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owing to the quantum yield can be compared to fluorescent dyes and have less photobleaching [1.36]. They were widely employed for the vitro imaging of pre-labeled cells due to those advantages. The ability to image single-cell migration in real time is expected to be important to several research areas, such as embryogenesis, cancer metastasis, stem-cell therapeutics, and lymphocyte immunology. However, the bio-cytotoxicity of quantum dots was concerned on medical applications. The semiconductor quantum dots can be dissolved, in a process as known as photolysis, to release toxic ions into the culture medium. The quantum dots must be carefully examined before its bio-cytotoxicity can be approved for clinical use.
In comparison, nanodiamonds which consist of carbon atoms possess remarkable features of low bio-cytotoxicity [1.37, 1.38]. As a result, the nanodiamonds is a relative safe nanomaterials based on its non-bio-cytotoxicity and biocompatibility.
Nanodiamonds are promising nano-materials owning to good chemical stability, various nanosizes, biocompatibility, good surface modification and good optical property for bio-applications [1.39-1.42]. The surface of nanodiamonds can also be a unique platform for the conjugation of chemicals and biomolecules after functional modifications. The surface of nanodiamond can carry a variety of oxygen containing functional groups immediately after purification in strong oxidative acids or oxidation in air. The acid-treated oxidative nanodiamonds show high affinity for biomolecules.
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Surface carboxylated group of nanodiamond are typical ligands used for covalent coupling of these biomolecules to nanodiamonds through amide linkage.
The Raman[1.15] and photoluminescence[1.19-1.12] properties of nanodiamonds have been intensively studied. Some intrinsic Raman signals can be used as detection markers or can be employed in biological objects. This diamond Raman peak is strong and isolated, so it can be used as an indicator for allocating nanodiamonds.
1-4-2 CVD diamond for bio-chip applications
CVD diamond can be mono- or polycrystalline. Whereas monocrystalline diamond requires a diamond substrate, polycrystalline diamond can be grown on different materials, such as silicon (Si) and quartz (SiO2), and is dominated by columnar growth. For grain sizes below 500 nm, the CVD film is called nanocrystalline diamond (NCD); after a certain thickness, the grain size near the surface exceeds 500 nm and the film then becomes microcrystalline diamond (μ-CD). In addition, a
slightly different material is ultra-nanocrystalline diamond (UNCD) which the grain size and roughness are not dependent on the film thickness, because these films do not show columnar growth. For optimization of detection techniques and sensitivities, for realization of highly integrated sensor arrays and for bio-interfaces, materials like Si, SiO2, gold, glassy carbon, SnO2, and ZnO2, do not possess desired chemical stability.
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CVD diamond is an outstanding material for bioelectronics with good electronic and chemical properties [1.43, 1.44]. Additionally, ultra-hard diamond (50–150 GaP) is promising with respect to mechanical stability of nanostructure.
1-4-3 Diamond for Single-photon source
The development of reliable devices for the generation of single photons is crucial for many applications such as, for example, quantum cryptography[1.45], optical quantum computation[1.46] as well as experiments on the foundations of quantum optics [1.47, 1.48]. Single-photon emission was first observed from single atoms and ions in traps and from molecules. More recently, semiconductor quantum dots and photoactive point defects, such as nitrogen-vacancy (N-V) and silicon-vacancy (Si-V) in diamond have been used in SPE experiments [1.49, 1.50]. Among the array of luminescent nanomaterials, color centers in diamond seem to be the most-promising single-photon source for quantum-physics applications, such as optics, information processing and cryptography. This is due to the remarkable photoemission properties of the defects, such as their extraordinary stability at room temperature and their high quantum efficiency (typically >0.1), which are combined with the unique chemical and mechanical properties of the diamond material itself. Moreover, the association of a spin structure in the ground level resulted in a long coherence time at room temperature with spin-dependent optical transitions allows it to be developed for
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quantum-state preparation by optical pumping and single-spin quantum state readout [1.51].
1-5 Research motivation and object
Nanodiamond carry the following features, such as, good photostability, easy
Nanodiamond carry the following features, such as, good photostability, easy