Research progress of spectra and properties of ultrahard carbon materials at high pressure and high temperature

Abstract Carbon, the fourth most abundant element in the Universe, possesses numerous allotropes with diverse bonding character (sp 1-, sp 2- and sp 3-hybridized bonds) and structural motif of the constituting atoms. In particular, the carbon materials with a fully or nearly 100% sp 3-hybridized strong C-C bonds often lead to excellent mechanical properties, chemical stability, thermal and optical properties, such as crystalline diamond and diamond-like amorphous carbon (DLC). In this review, we systematically summarize the synthesis, microstructure, mechanical properties, thermal and optical properties of ultrahard carbon materials with current experimental results on nano-polycrystalline diamond (NPD), nanotwinned diamond (NTD), micro-grained polycrystalline diamond (MPD), and amorphous diamond/carbon. In addition, we discuss the difference of spectra of XRD, Raman and EELS between various nanocrystalline diamond powder and ultrahard carbon materials. Finally, we provide our insights into the future development and applications in the research of ultrahard carbon bulk materials by high-pressure and high-temperature techniques according to the current advantages, limitations and challenges in the experiment.


Introduction
Solid matter in nature can usually be divided into crystalline and non-crystalline states, depending on whether lattice periodicity in the spatial organization of the constituting atoms exists in the material [1][2][3][4]. The crystalline state generally exhibits long-range order in the atomic arrangement. However, amorphous state inherits disorder of molecular orientation and short-medium range order in the corresponding crystal, which is characterized by disorder (orientational or/and spatial) for such structure, namely it can be said as a 'glassy' state [5]. As the fourth most abundant element in the Universe, carbon possesses numerous allotropes with diverse bonding character (sp 1 -, sp 2 -and sp 3 -hybridized bonds) and structural motif of the constituting atoms, resulting in dramatically different physical and chemical properties [6][7][8][9].
Diamond, a high-pressure carbon conformation that is metastable at ambient conditions, is a distinctive crystalline phase, where the carbon atoms are characterized by sp 3 -hybridized orbitals with all four valence electrons forming tetrahedrally coordinated directional σ bonds with neighboring four carbon atoms [10][11][12][13][14]. The exceedingly strong sp 3 -hybridized C-C covalent bonds network lead to remarkable properties and applications of diamond, such as the largest bulk modulus and hardness of any solid, the largest room-temperature thermal conductivity, the highest atom density, the largest limiting electron and hole velocities of any semiconductor, the highest electron dispersion, the smallest thermal expansion coefficient, wide 5.5 eV optical band gap, high dielectric breakdown, very low friction and adhesion, chemical inertness, electronic mobility, and biocompatibility [15][16][17][18][19][20][21][22]. The carbon atoms of graphite crystalline are characterized by sp 2 -hybridized orbitals with three electrons forming strong intra-layer trigonally coordinated σ bonds with three carbon atoms in the identical plane, while the fourth valence electron forms weak, long π bonds with one or more neighboring atoms in its adjacent plane layers, resulting in the low hardness and low density of graphite [6]. Moreover, a single graphite plane is a zero band-gap semiconductor and an anisotropic metal in three dimensions [23]. Man-made single-crystal diamonds has been successfully synthesized using graphite with some catalysts/solvents as starting material near the graphite-diamond phase transformation boundary under high pressure (5-6 GPa) and high temperatures (1500-1800 °C) since 1950s [11][12][13]. Highpurity single-crystal diamonds, even of the order of 1 cm, were prepared commercially by using the temperature gradient method [24]. In addition to being used as gemstones, single crystal diamonds also have valuable for industrial use. Although single-crystal diamonds and its products have played very important role in modern manufacturing industry with excellent properties, the characteristics of their anisotropic and easy cleavage along (111) crystal plane leads to insufficient fracture toughness of diamond tools, which limit their wide application in harsh environments [25][26][27]. Sintered polycrystalline diamond (PCD) bulks show good isotropy and high toughness, and its certain performance is superior to that of single crystal diamond. But PCD bulk materials is usually fabricated under high pressure and high temperature (HPHT) with sintering binders or additives, such as Co, Ni, SiC, etc., which affect the hardness, wear resistance and thermal stability of PCD materials [25]. Therefore, it became a very challenging scientific task to synthesize high-purity polycrystalline diamond with random grain orientation, excellent mechanical and thermal properties.
Crystallinity is another significant controlling factor of material properties [7]. Under certain circumstances amorphous materials exhibit more advantageous properties than their crystalline counterparts, which make them more suitable candidates for technological applications [9]. Just a few examples, bulk metallic glasses (BMG) have physical and chemical properties combining the advantages of ordinary metals and glasses-high strength, good ductility and corrosion resistance than the corresponding crystalline metals [28,29]. Hydrogenated amorphous silicon (a-Si: H) films possessing an optical absorption edge at~1.7 eV have been the most popular photovoltaic semiconductors used in solar cells. The a-Si: H/crystalline silicon (c-Si) heterojunction-based solar cell has increased efficiency steadily to a current record value of 24.7% [30]. The sp 3 -bonded tetrahedral amorphous silicon (a-Si) and germanium (a-Ge) have been known for decades and have been popularly used in flatpanel detectors, displays, and photovoltaics [31,32]. Amorphous carbon materials show a rich variety of physical properties determined by the bonding and structure. DLC is a metastable form of amorphous carbon with significant sp 3 bonding, which is defined in long range order to be amorphous and in short range order to be diamond, also defined as amorphous diamond [33]. Although decades of research have focused on the search for purely sp 3 -bonded tetrahedral DLC for practical applications, it has not yet been observed. DLC materials with high sp 3 fractions produced by different deposition techniques from various carbon precursors exhibit terrific properties of high hardness, transparency, tunable optical bandgaps, and chemical inertness for applications as protective coatings in areas, such as micro-electromechanical devices, optical windows, magnetic storage, and biomedical components [8,30]. However, these DLC are only available as thin films with thicknesses ranging from a few to tens of nanometers, and often contain amount of hydrogenated carbon and nanocrystalline diamond that are significantly different from a fully sp 3 -bonded bulk amorphous carbon material [5,34]. Also, microstructural characterization of high-pressure amorphous carbon obtained carbon precursor with shock compression or pressure-temperature treatments was seriously limited by the tiny sample size and little information on its properties is known, resulting in controversial results reported in the literature [35][36][37][38][39][40][41][42][43]. In addition, the delamination of thick films from the substrates may be caused by the large intrinsic stresses of up to several GPa in DLC films, thus limiting the application of DLC coating [44,45]. Therefore, it has been a long-standing pursuit to synthesize large-sized amorphous carbon/diamond bulk materials with purely sp 3 -bonded network.
Based on this background, this paper reviews the recent progress in ultrahard carbon materials, such as high-purity NPD and MPD bulks and amorphous carbon/diamond bulks with sp 3 concentration close to 100%. We will systematically discuss the synthesis, microstructure, mechanical properties (hardness, fracture toughness, strength, and abrasive wear resistance), thermal and optical properties, introduce the difference of spectra of XRD, Raman, photoluminescence (PL), electron energy loss spectroscopy (EELS), Ultraviolet (UV) Raman between nanocrystalline diamond powder and ultrahard carbon materials, and provide our insights into the future development and applications in the research of ultrahard carbon bulk materials.

Ultrahard pure polycrystalline diamond materials
The direct conversion of graphite to diamond without any catalysts/solvents under static high pressure was regarded as an effective method to synthesize high-purity single-crystal and polycrystalline diamond. Bundy firstly demonstrated the feasibility of direct conversion from graphite to diamond without the use of any catalysts and obtained very fine diamond crystallites (20-50 nm) using graphite bar under a pressure of ~13 GPa and the temperature more than ~3300 K by flash heating (2 or 3 milliseconds) in a diamond anvil cell (DAC) device [11][12][13]. Since 1960s, extensive efforts have thus been made to synthesize sintered polycrystalline diamond bulks from graphite by static high pressure and shock compression, but these were failed due to the sample heating was not homogeneous and/or the reaction time was too short [14,25,27]. Direct sintering diamond powders to prepare single-phase polycrystalline diamond bulks were also unsuccessful because heterogeneous stress distribution within the recovered samples due to the extreme hardness of the raw diamond [27]. Chemical vapor deposition can also be used to produce pure polycrystalline diamond, but the recovered samples was millimetre-sized thin film, and these diamond grains were not sintered together that exhibit weak intergrain bonding strength [24]. High pressure serves as a fundamental and powerful thermodynamic parameter which can be used to tune the properties of ultrahard bulk materials [46][47][48][49][50].
With the development of high-pressure experimental techniques till 2003, high-purity nano-NPD material with grain size of 10-20 nm has been successfully synthesized by direct conversion of graphite that consisted of small (1-2 μm) grains in only a few minutes at pressures of 12-25 GPa and temperatures of 1800-2500 °C using a multi-anvil high-pressure apparatus [15][16][17][18][19]51]. As shown in Figure 1(a), the synthesized NPD sample was about ~1-1.5 mm in diameter and ~0.3-0.5 mm in thickness, and was optically transparent and colorless. So far, they have been able to synthesize NPD cylinders with diameters and heights of more than 10 mm. The NPD sample is harder and tougher than single-crystal diamonds and are therefore can be used for cutting and polishing other hard materials [20][21][22][56][57][58].
In the systematic research process of direct conversion of low-density carbon materials into diamonds, compression of fullerene C 60 under non-hydrostatic pressures to 25-30 GPa at room temperature were also a popular method to produce diamond, probably with small grain size, but the amount of synthesized material was so insufficient that it is difficult to characterize the microstructure and mechanical properties in detail [38][39][40][41][42]. Dubrovinskaia et al. reported that a bulk material of pure nanocrystalline cubic diamond with grain size of 5-12 nm was synthesized using fullerene C 60 as a starting material under 20 GPa and 2000 °C by utilizing a 5000 t multi-anvil press [59]. The recovered sample was a transparent and lightly yellow cylinders with a diameter of about 1.8 mm and a height of about 3 mm, and exhibited important and unusual properties. The recovered sample is at least as hard as usual bulk diamond. Figure 1. optical photographs of the ultrahard carbon materials from different precursors and HPHt conditions. (a) colorless nPD [15], (b) ntD [52], (c) cyan nPD [26], (d) yellow nPD [25], (e) mPD [53], (f) cFPDc [54], (g) tDc [55], (h) a-D [7], (i) am-iii [5], (j) ac-3 [9], and (k) p-D [4].
Xu et al. founded that well-crystallized graphite and high energy ball milled graphite can completely transform to NPD and at 2500 °C and 2100 °C respectively under same pressure of 16 GPa, which indicated that the ball milling process of graphite can lower the synthetic temperature of NPD bulks for about 400 °C [27]. In addition, semi-transparent NPD with a grain size of 15 nm was synthesized from ball milled and vacuum heated graphite at 16 GPa and 2300 °C by using a two-stage multi anvil apparatus based on the DS6 × 8 MN cubic press. They propose that the hardness of semi-transparent NPD sample was equal or even harder than single crystalline diamond. When heated to 2500 °C, the recovered NPD sample became black and the diamond grains exhibited an obvious growth to about 30 nm. When treated the ball milled graphite without vacuum heating at 16 GPa and 2300 °C, the sample was not sintered well, and became light yellow. Their research suggested that high energy ball milling, vacuum heating and temperature play important role in synthesizing the NPD bulks. First, high energy ball milling expanded the surface of the graphite and allow the graphite into a disordered state that make the phase conversion to occur at a relative mild condition through non-diffusion homogeneous nucleation. Second, vacuum heating would remove the adsorbed harmful substances on the surface of disordered graphite, in which no-catalyst phase transformation would happen. Third, correctly control the temperature could avoid the abnormal grain growth of diamond in the HPHT experiment [60].
It has been demonstrated experimentally that nanotwinning is an effective microstructure for obtaining a smaller size because twin boundaries have lower excess energy than grain boundaries, and, twin boundaries exhibit an identical hardening effect to those of grain boundaries for metals at nanoscale [61][62][63]. Recently nanotwinned structures have been successfully introduced in synthesizing nanotwinned cubic boron nitride (nt-cBN) bulk materials, and the nt-cBN bulk samples with a mean twin thickness of ~3.8 nm possesses a superior combination of ultra-high hardness and high thermal stability [63]. In view of this, a NTD bulk sample with an average twin thickness of ~5 nm was successfully synthesized through the direct conversion of onion carbon (~20-50 nm in diameter) under at 20 GPa and 2000 °C [52]. Onion carbon, a high-energy metastable carbon consisting of concentric graphite-like shells, contains high concentration of stacking faults and puckered layers, which provide significant role for the nucleation of NTD at HPHT. As can be seen in Figure 1(b), the transparent pure NTD sample was ~1 mm in diameter and shows unprecedented hardness and thermal stability.
Although NPD can be synthesized by direct conversion of various carbons at HPHT and the research results are also significant, the harshly synthetic condition (>2000 °C and >15 GPa) is a serious obstacle for extensive production and widely industrial application of NPD [15,25]. Reducing the synthetic pressure derives higher productivity and larger NPD bulks, which is vital for commercial applications. Tang et al. reported that high-hardness pure NPD bulks with a mean grain size of 13.4 nm was successfully fabricated using the annealing the high-disperse nano-diamond precursors with an average size of about 5 nm in proximity to industrial conditions at 1800 °C and 10 GPa [26]. The synthetic conditions were much lower than that using graphite precursors, especially the pressure was decreased significantly to 10 GPa from 15 GPa. As shown in Figure 1(c), the synthetic transparent bulk sample was ~2 mm in diameter and ~1 mm in height and shows the cyan color that probably result from the selective absorption of the grain boundaries to the optical radiation.
Defectively, the small size of the transparent NPD bulk material is a barrier for industrial applications. There are few reports on the synthesis of transparent, crack-free NPD with a size larger than 3 mm by direct conversion method [26,27,52,60]. The main reason is that increasing the recovered sample size for a fixed press devices usually sacrifices the synthetic pressure of the chamber. Wang et al. overcame the technical bottlenecks and obtained the large-sized NPD bulk materials at HPHT. The transparent high-purity NPD bulk samples have been synthesized by direct conversion of graphite with a mean grain size of 10 μm under 18 GPa and various temperatures. As can be seen in Figure 1(d), the yellow NPD bulk with a mean grain size of 20 nm was ~6 mm in diameter and ~6 mm in height, and exhibited ultrahigh hardness and excellent wear resistance and thermal stability. Moreover, green pure polycrystalline diamond with an average grain size of 35 μm can also be synthesized by direct conversion of graphite by tunning the synthetic conditions. In a word, the color and grain size of polycrystalline diamond bulks can be controlled by adjusting thermodynamic conditions. In addition, NPD bulks with larger size might be available relying on relevant high-pressure technologies [25].
Although diamond is crucial for a wide range of scientific and industrial applications, its toughness is inferior to that of many known tool materials. Improving the hardness of diamond still further (such as, providing better machining efficiency and accuracy, achieving a higher pressure in the DAC) usually leads to lower toughness, shortening the lifetime of diamond tools. Improving toughness (resistance to fracture) and hardness simultaneously is a challenging task due to the intrinsic brittleness of diamond. Fortunately, studies on various materials demonstrated that twin boundaries are preferable to large-angle grain boundaries in optimizing the mechanical properties of diamond [52,[63][64][65]. Yue et al. reported that hierarchically structured diamond composite with exceptional toughness and hardness has been synthesized from carbon onion precursors at 15 GPa and 2000 °C with a 10-MN double-stage large-volume multi-anvil system [66]. The recovered NTD composite bulks were 1-2 mm in diameter and 0.3-0.5 mm in thickness. This structural architecture is an effective strategy to promote the development of ultrahard bulk materials and engineering ceramic materials for simultaneously enhancing hardness and toughness.
Recently, with the development of ultra-high pressure and high temperature large-volume technology, ultrastrong pure polycrystalline diamond compact by direct sintering have been prepared.
Liu et al. reported that pure micro-grained polycrystalline diamond (MPD) compact has been successfully prepared using micro-sized diamond powders without any additives under at 14 GPa and 1900 °C with a DS6 × 25 MN cubic press machine [53]. Hardness and fracture toughness of MPD simultaneously enhanced due to high-pressure work hardening. As shown in Figure 1(e), the well-prepared and end-polished MPD were 11 mm in diameter and 6 mm in thickness, which are large enough for making industrial cutting/drilling tools as well as scientific research. Li et al. founded that ultra-strong catalyst-free polycrystalline diamond compact (CFPDC) materials have been synthesized using micron diamond powder with grain sizes of 8 ~ 12 µm as the starting materia at 16 GPa and 2300 °C in a twostage multi-anvil large volume high-pressure apparatus based on a DS6 × 25 MN cubic press [54]. As can be seen in Figure 1(f), the CFPDC sample shows the higher hardness, higher wear resistance, and higher thermal stability and has over 10 mm in diameter and up to 6 mm in thickness, which are sufficiently large enough and cost-effective to make parts and components for not only hydrocarbon drilling, but also a wide range of other industrial applications. Zhang et al. reported that pure transparent polycrystalline diamond ceramic (TDC) was successfully prepared from diamond powder with an average grain size of about 1 µm and without catalyst at 2400 °C and 16 GPa on a DS6 × 8 MN cubic press machine [55]. As shown in Figure 1(g), the polished TDC sample was ~3 mm in diameter and 0.5 mm in thickness and shows the high transparency, hardness and fracture toughness. The MPD (CFPDC and TDC) and NPD (NTD) are two distinguished technologies or directions with totally different sintering mechanisms. Both are important to scientific research and industry applications.

Ultrahard sp 3 -bonded amorphous carbon/ diamond materials
Glassy carbon is an amorphous carbon allotrope containing nearly 100% sp 2 bonding at ambient conditions [67]. Compressing glassy carbon to 44 GPa or higher pressure at room temperature in a DAC observed complete conversion of sp 2 bonding and formed a new carbon allotrope with a fully sp 3 -bonded amorphous structure, but the transition was reversible upon releasing pressure [6]. Used the glassy carbon ball as an indenter, its strength could reach up to 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference (>70 GPa) has never been observed in any material besides diamond, indicating this high-pressure carbon allotrope possesses the high hardness (above 40 GPa) and diamondlike strength. The extreme pressure-hardening behavior of this phase may be used as a second stage anvil or as a gasket material which hardens with pressure.
Although cold compression (pressure without temperature) is a powerful tool to induce sp 2 -sp 3 bonding transition in carbon allotropes such as graphite, carbon nanotubes, fullerenes and glassy carbon, the transition was reversible and hardly obtained superhard amorphous carbon/diamond bulks upon decompression [6,10,35,36]. Fortunately, irreversible sp 2 -sp 3 transitions in carbon materials have been extensively realized by hot compression, namely HPHT [37]. For example, the diamond bulk materials (NPD and NTD) have been synthesized from various sp 2 -bonded carbon allotropes through an irreversible sp 2 -sp 3 transition under HPHT (typically 15-25 GPa, 2100-2800 K) by using large volume presses (LVP) [15,52]. In view of this, Zeng et al. reported that a quenchable amorphous diamond (a-D) was successfully fabricated from glassy carbon at 50 GPa and approximately 1800 K by using a DAC coupled with in-situ laser heating [7]. As shown in Figure 1(h), the recovered a-D sample was optically transparent and only ~40 μm in diameter and ~15 μm in thick. The dense sample also possesses a nearly perfect sp 3 -bonded tetrahedral amorphous structure and shows ultrahigh incompressibility (bulk modulus) comparable to crystalline diamond. Their results indicated that purely sp 3 -bonded tetrahedral amorphous carbon can be synthesized and recovered to ambient conditions. It might be necessary and interesting to synthesize large-sized sp 3 -bonded amorphous carbon/diamond using various carbon allotropes as starting materials for fundamental studies and potential applications.
Fullerenes consist of the sp 2 -hybridized carbon-carbon bonds similar to those in hexagonal graphitic layer, but they are constructed into a three-dimensional spheroid through adjusting the distorted sp 2 bonds of pentagons [33]. The strongly covalent π bonding and highly symmetric truncated icosahedral structure in C 60 , lead to extremely high stability over a wide range of pressure and temperature. Solid fullerenes show numerous interesting physical and chemical properties such as a number of subtly different orientational phases, strong optical nonlinearity, unusual magnetic properties, superconductivity after doping, etc. [8]. C 60 molecules could transform reversibly to a semi-transparent strongly interacting C 60 agglomerates in the pressure range of 15-25 GPa at ambient temperature [40]. But the C 60 buckyballs get easily broken at a pressure of ~5 GPa and a heating temperature of ~800 °C and form a disordered nano clustered graphene-based hard phase with >90% elastic recovery after deformation. Even though extensive efforts in exploring the p, T phase diagram of C 60 , a pressure range above 20 GPa has not been established [42]. Moreover, the synthetic pressure significantly affects the bonding and microstructure in carbon phases produced from C 60 . Zhang et al. systematically explore the conversion behavior of fullerene C 60 at a pressure of 25 GPa and different temperatures [9]. An amorphous carbon (AM-III) bulk material with sp 3 bonding fraction of 94% was successfully synthesized from compression of fullerene C 60 at 25 GPa and 1200 °C in a large-volume multi-anvil system. As shown in Figure 1(i), the recovered AM-III bulk was yellow-transparent and ~1 mm in diameter and 1.2-1.7 mm in height. The produced AM-III carbon bulk material exhibit outstanding mechanical, optical, and thermal properties, which offer an excellent candidate for the most demanding of practical applications.
Because diamond have the very high melting point under high pressure, above 4700 K, it is hardly feasible to prepare amorphous carbon bulks by fast quenching liquid carbon [43]. Another promising way is by an amorphous-to-amorphous transition of carbon, and a valuable method to synthesize amorphous carbon bulks with sp 3 concentration close to 100% is to heat already amorphous carbon precursor created by compressing fullerene C 60 . Recently, Shang et al. successfully synthesized millimetre-sized amorphous carbon (AC-3) bulk with high sp 3 hybridization concentration (95.1%) from C 60 at 27 GPa (near fullerene C 60 cage collapse pressures) and 1000 °C in a LVP [9]. As can be seen in Figure 1(j), the recovered AC-3 bulk material was well-sintered, yellow and translucent. Meanwhile, the sample possess superior mechanical and thermal properties, optical transparency, indicate potential new applications in different fields.
According to crystallinity or whether long-range lattice periodicity exist in the material, solids can be generally classified into crystalline and amorphous states. It is now understood that amorphous carbon solids allude to materials that do not possess long-range periodicity, and their crystalline structural ordering on the atomic level are random and capricious [7]. If the degree of shortrange order in amorphous carbon solids is significantly improved or the degree of long-range order in crystalline diamond is seriously reduced, the differentiation of the two states, however, could face fundamental challenges [2,68,69]. A paracrystalline state of diamond with completely sp 3 -bonding that is distinct from either crystalline or amorphous diamond was proposed to identify this carbon state that possess only well-defined crystalline medium-range order but absence of long-range order [4,68,69]. Recently, Tang et al. successfully produced paracrystalline diamond (p-D) containing sub-nanometresized paracrystallites employing face-centred cubic C 60 as a precursor at 30 GPa and 1600 K in an ultrahigh-pressure MAP [4]. As can be seen in Figure 1(k), the transparent p-D bulk sample was ~1 mm in diameter and exhibited distinguishing physical and chemical properties. This research establishes the missing link in the length scale between amorphous and crystalline carbon states across the atomic structural level. However, the high pressures needed to synthesize these materials limit the size of the recovered samples, which presents a serious hurdle for the industrial-scale application.

Microstructure of ultrahard carbon materials
In order to better understand the atomic microstructure and chemical bonding of ultrahard carbon materials, we further systematically discuss the difference of spectra of XRD, Raman, photoluminescence (PL), and EELS between various nanocrystalline diamond powder and ultrahard carbon materials. The 10 nm diamond powder synthesizing by detonation method was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. The 25, 50, 100 nm diamond powder and single-crystal diamond (SCD) was synthesized using high pressure and high temperature. The structures of the nanocrystalline diamond powders were determined by XRD (D8 Advance, Germany) with Cu Kα radiation (λ = 1.5418 Å). Raman and UV Raman spectra of the single-crystal diamond and nanocrystalline diamond powders were measured at room temperature in a confocal Raman system with 532 nm and 325 nm laser, respectively (HORIBA Jobin Yvon, France). Figure 2 shows the XRD patterns of various nanocrystalline diamond powder and ultrahard carbon materials. The XRD measurements of various nanocrystalline diamond powder demonstrated that within the limits of XRD analysis, only cubic diamond was present. And as the grain size of nanocrystalline diamond powder reduces, the full width half maximum (FWHM) of the diffraction peak gradually broadens, which comply with the Scherrer equation [70]. The XRD measurements of colorless NPD, yellow NPD, cyan NPD and transparent NTD showed identically that this structure was pure cubic diamond phase, and the corresponding grain size or twin thickness were 10-20 nm, 20 nm, 13.4 nm and ~5 nm, respectively [15,25,26,52].

XRD analysis
As clearly seen from Figure 2, the XRD spectra of a-D, AM-III, AC-3 and p-D exhibits simultaneously broadened diffraction peaks, but a totally different structure from nanocrystalline diamond powder and NPD samples [4,5,7,9,15,25,26,52]. The first sharp diffraction peak (FSDP) of the a-D sample has a position (2.964 Å −1 ) close to that of diamond (111) peak (3.05 Å −1 ), and possess a much broad and highly diffuse FWHM (0.63 Å −1 ), which approach that of a-Si (~0.6 Å −1 ) with a random sp 3bonded network but exceed that of the nanocrystalline (mean grain size of ~2 nm) diamond (111) peak (0.32 Å −1 ) [7]. AM-III possesses a dominant broad diffraction peak at structure factor (q) of ~3.0 Å −1 , fairly close to the position of (111) reflection of diamond, and another weaker peak at ~5.3 Å −1 , which are distinctly different from previously discovered low-density AM carbon materials from compressing C 60 at lower pressures and temperatures [5]. AC-3 exhibits two intense peaks at 2.98 and 5.48 Å −1 , and the FSDP at 2.98 Å −1 is primarily associated with the presence of short-and medium-range order. A d spacing of ~2.2 Å estimated from the brightest centre of the diffraction ring is larger than the interplanar distances of crystalline diamond (2.06-2.19 Å), indicating a lattice distortion/expansion present in the amorphous sp 3 network of AC-3 [9]. Two broadened diffraction peaks of p-D at ~2.95 and 5.4 Å bear considerable resemblance to those of DLC or amorphous carbon with a high fraction of sp 3 bonds [4]. Obviously, comparing with the position of (111) diffraction peak of nanocrystalline diamond powder or various NPD samples, the FSDP of amorphous carbon/diamond is inclined to a smaller angle or structure factor due to the distortion of sp 3 bonding and the expansion lattice structure in the amorphous network. Moreover, the density of a-D, AM-III, AC-3 and p-D are estimated to be 3.3 ± 0.1, 3.30 ± 0.08, 3.2 ± 0.1 and 3.25 g/cm 3 , respectively, are slightly lower than of crystalline diamond (3.52 g/cm 3 ), which agrees with the network structure of the sp 3 -dominated amorphous carbon/diamond with expanded/distorted lattice [4,5,7,9].

Raman spectra analysis
Generally, Bragg diffraction peaks associated with lattice arrangements of atoms are obscured or absent in the spectra signals of amorphous carbon/diamond materials, which renders it notoriously difficult to precisely identify their structural organizations [4]. Fortunately, Raman spectroscopy is a particularly effective and widely employed method to characterize the phonon modes of the various allotropes and non-crystalline phases of carbon [71]. However, the Raman cross-section for sp 2 bonded structures is much greater than that for sp 3 bonding due to the resonance effects, and the scattering from sp 2 bonded structures usually dominates the Raman spectra and often conceal the signal from the sp 3 bonded fraction [72][73][74][75][76][77]. Figure 3 shows the Raman and Photoluminescence spectra of various nanocrystalline diamond powder and ultrahard carbon materials. Figure 3(a) shows the Ultraviolet (UV) Raman spectra of nanocrystalline diamond powders excited by UV laser (325 nm) compared to certain amorphous carbon film and bulk materials. As shown in Figure 3(a), the single-crystal diamond shows a sharp characteristic peak at around 1330 cm −1 , which is commonly known as D-band. As the grain size of nanocrystalline diamond powder decreases from 100 to 10 nm, the spectra broadens and the Raman peak of D-band shifts to a lower frequency from 1325.6 to 1315.5 cm −1 . But the lattice constant of nanocrystalline diamond powder calculated from the corresponding XRD dates is in good agreement with the theoretical value. From the standpoint of the above facts, one possibility for the size effect of the Raman spectra may be considered to be caused by wave scattering at the grain boundaries [80]. In addition, compared with the Raman spectra of other diamond powders, only 10 nm diamond powder shows another broad peak at around 1600 cm −1 , which is commonly known as G-band. We attribute the presence of G-band to two main factors. First, the diamond powder of 10 nm possesses a higher surface energy and is easily aggregated together, which form a grossly disordered (amorphous) intergranular phase with uncoordinated C atoms (sp 2 bonded structures) at the grain boundaries [80]. Second, when the spot size of the laser beam is set at a fixed value, the specific surface area of the nanocrystalline diamond powder increases in accordance with the decrease of the grain size, resulting in that more amorphous signals on the diamond surface can be detected. Further, we focus on the amorphous material. The spectra of ta-C(:H) films exhibit the broad G-band characteristic of sp 2 bonded carbon located at about 1600 cm −1 (Figure 3(a)) [78,79]. Comparing with that ta-C(:H) films and nanocrystalline diamond powder, the corresponding UV Raman spectra of AC-3 and AM-III all show additional broad peak appears at around 800 ~ 1400 cm −1 , which is typically recognized as T-band indicating the high fraction of sp 3 bonded carbon in this material (Figure 3(b) and (c)) [5,9]. Figure 3(d) shows the Raman spectra of nanocrystalline diamond powders, yellow NPD, and transparent p-D excited by a laser 532 nm [4,25]. The single-crystal diamond shows a sharp characteristic peak at around 1332 cm −1 , which is known as D-band. As can be seen in Figure 3(d), when the grain size of nanocrystalline diamond powder gradually decreases from 100 to 10 nm, the Raman peak intensity of D-band was decreased, and the spectra broadened and shifted to a lower frequency from 1328.5 to 1318.7 cm −1 . This variation is almost similar to those of reported diamond powder with average grain sizes of 14, 6, and 0.12 μm, suggesting that this band may not be observable when the grain size is less than a certain critical value because the detected signals were almost comparable to the noise level [78,80]. The Raman spectra of yellow NPD and colorless NPD showed that the intensity of the D-band characteristic peak at 1332 cm −1 was substantially lower than that of the synthetic single-crystal diamond, and the FWHM was greater than that of the the synthetic single-crystal  [78,79], am-iii [5] and ac-3 [9] excited by uV laser (325 nm). (b) and (c) the corresponding uV Raman spectra of ac-3 [9] and am-iii [5], respectively, with Pl background removed. (d) Raman spectra of nanocrystalline diamond powders, yellow nPD [25], and transparent p-D [4] excited by visible laser (532 nm). (e) Raman spectra of type iia diamond [15], colorless nPD [15] and transparent ac-3 [9] excited by visible laser (514 nm). (f) Pl spectra of type ia diamond, p-D [4] and am-iii [5] with an excitation wavelength of 532 nm, and ac-3 [9] with an excitation wavelength of 514 nm. diamond, especially this band was even not observed in some analysis spots for the colorless NPD (Figure 3(e)) [15]. However, no outstanding decrease of the wavenumber was observed in the yellow NPD and colorless NPD in in contrast to the result of the Raman spectra measurements of fine nanocrystalline diamond powders. This may be due to the existence of some residual compressive stress in the NPD samples synthesized at very high pressure (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), which should shift the Raman band to higher wavenumbers and counterbalance the possible decrease of the wavenumber introduced by the grain size effect. Alternatively, it is likely that an increase of local temperature of the nanocrystalline diamond powders induced by laser heating may lead to the Raman D-band toward lower wavenumbers with decreasing grain size, however, such a temperature effect is inappreciable due to high thermal conductivity of pure NPD [19][20][21][22]. At present, it is difficult to obtain high-quality Raman spectra as a trade-off must be overcame between signal to noise ratio and the risk of degradation of the fine diamond powder if the laser power is too high [22,71,78]. In addition, the Raman spectra of p-D and AC-3 exhibit no obvious peaks due to the lower power, indicating the absence of sp 2 bonds in the recovered disordered carbon [4,9].
Figure 3(f) shows PL spectra of type Ia diamond, p-D and AM-III with an excitation wavelength of 532 nm [4,5] and AC-3 with an excitation wavelength of 514 nm [9]. The bandgaps of AM-III analyzed from PL spectra and absorption spectra is 2.15 eV, suggesting the semiconducting nature [5]. The PL spectra of AC-3 possess an emission band centred at ~700 nm, different from those of diamond-like film or diamond nanocrystals [9]. The PL spectra of p-D shows a main fluorescence peak centered at around 702 nm, indicating the high background in the Raman spectra is due to a fluorescence effect [4].

EELS analysis
EELS is one of the most reliable tools for probing the bonding in carbon materials [7]. Figure 4 shows carbon K-edge EELS spectra of glassy carbon, C 60 , single-crystal diamond, 2 nm diamond powder, nanocrystalline diamond bulk, a-D, AM-III, AC-3 and p-D. The feature of C 60 at 285 eV is due to transitions of a 1 s electron to π*, corresponding to sp 2 bonding [59]. The EELS of glassy carbon show a sharp peak at ~285 eV that corresponds to π bonding, as a result of its nearly 100% sp 2 bonds [7]. This peak is not present in the EELS of the 2 nm diamond powder due to its purely sp 3 bonds. Similarly, the EELS pattern of the a-D sample has no peak at ~285 eV, implying its atoms should be fully sp 3bonded like those in crystalline diamond, which are in good agreement with the results of XRD [7]. The EELS spectra of nanocrystalline diamond bulk are also perfectly compatible with the single-crystal diamond structure and indicate that sp 3 hybridization prevails even across the numerous grain boundaries [59]. The feature of AM-III at 292 eV is due to transitions a 1 s electron to σ*, corresponding to sp 3 bonding [5]. EELS spectra of AC-3 show that the characteristic 1 s-π* peak at 284 eV representing π bonding of sp 2 carbon is absent, confirming that AC-3 is nearly completely sp 3 -hybridized, which shows a percentage of sp 3 -hybridization of 95.1 ± 1.7% using sp 2 glassy carbon as standard material by a calculation of the ratio of integrated areas under the π* and σ* peaks [9]. The p-D are completely sp 3 -bonded, demonstrating successful synthesis of non-crystalline diamond [4].

Mechanical properties
Various ultrahard carbon materials have been synthesized from different under HPHT and possessed outstanding mechanical, optical, thermal and electronic properties, etc. Therefore, it is important to define the position of the ultrahard carbon materials on the current landscape.
(H k ) of 120-140 GPa at the applied load of 4.9 N, which is equivalent to or even higher than those of natural and synthetic single-crystal diamonds (~60-130 GPa) and nearly twice as high as those of synthetic polycrystalline diamonds containing binders (~50-70 GPa) [15,16]. Microstructure observations beneath the indentations of these colorless NPD suggest that the existence of a lamellar structure and the bonding strength of the grain boundary play important roles in controlling the hardness of the colorless NPD. Moreover, the hardness remained higher than 100 GPa even at 800 °C, while the hardness of SCD sharply decreased to 60 GPa above 300 °C. The transverse rupture strength (TRS) of colorless NPD remained at about 3 GPa up to 1000 °C, above which it showed a positive temperature dependence, while the TRS of PCD decreased rapidly at about 500 °C [17][18][19]. In addition, the abrasive wear resistance of colorless NPD is comparable to that of high-pressure synthetic type Ib SCD and 10-20 times higher than that of conventional binder-containing PCDs [15][16][17][18][19][20][21][22].
Huang et al. reported that Vickers hardness (H V ) and H k of NTD at the applied load of 4.9 N were 175-203 and 168-196 GPa, respectively. And the fracture toughness values of NTD ranged from 9.7 to 14.8 MPa m 0.5 . The simultaneous improvement in hardness and fracture toughness in their nt-diamond is intimately related to the ubiquitous nanotwinning microstructure. The presence of ultrafine nanotwins introduces extra hardening, which is probably due to both the Hall-Petch and quantum confinement effects at nanoscale, while gliding of dislocations along densely distributed twin boundaries enhances fracture toughness [52]. Tang et al. reported that the cyan NPD shows high H V of 147 ± 17 GPa at load of 4.9 N. The low-density twinned structure is responsible for the improved hardness. Besides, well-sintered grain boundaries are essential to guarantee the hardness of cyan NPD [26]. Wang et al. founded that the H V of the yellow NPD at the applied load of 3 N was 130 GPa. According to China National Quality Supervision &Inspection Center for Abrasives, the abrasion ratio of yellow-transparent NPD sample is 1. 658 × 10 6 , which is 25 times higher than that of commercial Co-based PCD used on oil bits. Contrast grinding test, using yellow NPD to grind against carat-level single-crystal diamond, shows that the abrasion ratio for the yellow NPD is 2.5, which demonstrates that the abrasion performance of yellow NPD is much better than that of carat-level single-crystal diamond [25]. Yue et al. reported that the H V of the NTD composite was is 200.1 ± 8.0 GPa, the fracture toughness is 26.6 MPa m 0.5 , five times that of synthetic diamond, even greater than that of magnesium alloys [66].
Liu et al. reported that the hardness of MPD is 120.5 ± 4.2 GPa at 29.4 N, which is twice as high as traditional polycrystalline diamond composites and comparable to the reported NPD bulks [53]. The resulting fracture toughness of MPD reaches up to 18.7 MPa m 1/2 , which is superior to the reported nt-cBN (12.7 MPa m 1/2 ) and NTD (9.7 to 14.8 MPa m 1/2 ) [52,63]. Li et al. reported that the hardness of CFPDC had exceeded the Vickers hardness limit of single crystal embedded diamond (120 GPa), whereas the hardness of commercial PDC material is only about 64 GPa due to the residual cobalt binder found in commercial PDC materials [54]. Zhang et al. reported that The TDC sample had the high hardness of 113 ± 5.2 GPa, which approaches the top limit of the Vickers hardness of single crystal diamond (60-120 GPa) and much higher than all other transparent ceramics. Simultaneously, the fracture toughness of the TDC reached 15.6 MPa m 1/2 , which is about 3-fold of the single crystal diamond (3.4-5 MPa m 1/2 ) measured by indentation method [55]. Zhang [9]. The obtained Young's modulus E (937.2 ± 10.1 GPa), shear modulus G (437.9 ± 4.7 GPa) and bulk modulus K (363.3 ± 3.9 GPa) of p-D are close to those of diamond (1140 GPa, 535 GPa Figure 5. comparison of hardness for various ultrahard carbon materials. ac-3 [9], am-iii [5], p-D [4], mPD [53], single-crystal diamond (ScD) [15], yellow nPD [25], colorless nPD [15], cyan nPD [26], and ntD [52]. and 443 GPa), making it an appealing candidate for ultrahard materials [4].

Thermal properties
It is known that, in an inert atmosphere, pure SCDs are stable up to 1500-1600 °C and graphitized at higher temperatures, and the thermal stability of colorless NPD is almost equivalent to that of SCDs yellow NPD [15,22]. The nanocrystalline diamond bulk does not show any sign of graphitization even after heating at 1527 °C, and only treatment at 1627 °C results in some changes [59]. The onset oxidation temperature of NTD (1056 °C) was much higher than those of natural diamond (805 °C), synthetic diamond powders (725 °C), nanograined diamond (680 °C) and even rivalled that of ng-cBN (~1187 °C) [52,63]. The achieved average twin thickness of ~0.5 nm would certainly delay the graphitization of NTD and would result in a higher oxidation temperature. The onset oxidation temperature of the cyan NPD was 642 °C [26]. The onset oxidation temperature of the yellow NPD was 970 °C, which is almost the same as that of carat-level single-crystal diamond [25]. CFPDC material exhibits the high thermal stability and oxidation resistance of up to a temperature of 1200 °C, which is much higher than that of natural diamond (~800 °C), NTD (~1056 °C) and commercial PDC (~600 °C) [54]. The oxidation onset temperatures of AM-III were determined at 734 °C, which is comparable to that of single crystalline diamond [5]. AC-3 exhibits a very high thermal conductivity of 26.0 ± 1.3 W m −1 K −1 , much higher than for any other amorphous material reported [9]. The onset oxidation temperature of p-D is 677 °C [4].

Conclusion
In summary, we overviewed the synthesis, microstructure, mechanical, thermal and optical properties of ultrahard carbon materials with current experimental results on NPD, NTD, MPD, and amorphous diamond/carbon, and discuss the difference of spectra of XRD, Raman, PL and EELS, between nanocrystalline diamond powder and ultrahard carbon materials.
Large-sized high-purity transparent NPD with grain size of 10-20 nm have been successfully synthesized by direct conversion of graphite under pressures of 15-25 GPa and temperatures of 2300-2500 °C and shown high hardness, TRS, abrasive wear resistance and thermal stability, etc., which has outstanding potential applications regarding as the ''third-generation'' anvils for LVPs, optical window under extremely severe conditions, next-generation high-performance cutting tools and wear-resistant tools [81,82]. Both NTD and NTD composite show superior hardness, fracture toughness and thermal stability than NPD, however, the small bulk size limits their wide applications. This structural architecture strategy could promote the development of superhard materials and engineering ceramics for simultaneously enhanced hardness and toughness, with great potential for cost reduction and lifetime extension, as well as future technical innovation. Centimeter-sized ultra-strong catalyst-free MPD have been prepared by directly sintering diamond powder at HPHT and displayed excellent hardness, fracture toughness, wear resistance and thermal stability, which are large enough for making scientific research as well as cutting/drilling tools in the oil and gas industry.
The spectra result of XRD, Raman, PL and EELS experiments on the synthesized a-D, AM-III, AC-3 and p-D samples confirm their tetrahedral amorphous structure and nearly 100% sp 3 bond. The four disordered sp 3 -carbon materials combine exceptional mechanical properties, chemical stability, thermal and optical properties, suggesting potential new applications in different fields and may open new applications for amorphous solids. However, the high pressures needed to synthesize these materials limit the size of the bulk samples that only can be produced to the millimetre range, and this presents a serious hurdle for the development of industrial-scale processes. Nevertheless, the findings of the amorphous materials contribute to our knowledge about advanced amorphous materials and the synthesis of bulk amorphous materials by high-pressure and high-temperature techniques, but also add to our understanding of how to use such disorder to tune physical and chemical properties [83].

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the National Natural Science Foundation of China (grant number 52073254).