A single-crystalline diamond X-ray detector based on direct sp3-to-sp2 conversed graphene electrodes

Abstract Diamond is an ultrawide bandgap semiconductor with excellent electronic and photonic properties, which has great potential applications in microelectronic and optoelectronic devices. As an allotrope of diamond, graphene also has many fantastic properties like diamond, which caught much attention in combing them together. In this work, a direct sp3-to-sp2 conversion method was proposed to fabricate graphene layers on single crystal diamond by thermal treatment with Ni film catalyst. By optimizing the conversing conditions, a thin graphene layer with low sheet resistance was obtained on diamond. Based on this, an all-carbon sandwich structural graphene-diamond-graphene (GDG) detector was fabricated, which shows low dark current of 0.45 nA at 0.5 V μm−1 applied electric field. The maximum sensitivity of this detector is obtained when the incident X-ray is 12 keV, with the value of 2.88 × 10−8 C Gy−1. Moreover, the rise time and delay time of the GDG detector is about 1.2 and 22.8 ns, respectively, which are very close to that of diamond detector with Ti/Au electrode. The realization of the direct in-situ sp3-to-sp2 conversion on diamond shows a promising approach for fabricating diamond-based all-carbon electronic devices.


Introduction
Diamond is an ultrawide bandgap semiconductor showing great potential applications in radiation detectors and particle physical applications, due to its fantastic properties, such as ultrawide bandgap (5.47 eV) [1,2], high carrier mobility (4,500 and 3,800 cm 2 V −1 s −1 for electrons and holes, respectively) [3], high breakdown field (≈10 7 V cm −1 ) [4,5] and high radiation hardness [6,7]. Compared to other semiconductors, like Si, Ge and GaAs, diamond devices can operate under a temperature and radiation hardness high up to 500 °C and 10 15 ions cm −2 , respectively, where devices made of traditional semiconductors do not have a long-term durability [8,9]. In the early 1950s, natural diamond was selected to fabricate X-ray, electron, proton, and neutron detectors [10,11]. However, the non-producibility, uncontrollable defects and impurity concentrations of natural diamond restrict its further applications in electronic devices [12,13]. In recent years, with the mature of synthesis technology of artificial diamond through chemical vapor deposition (CVD) and high-pressure high-temperature (HPHT) method, single crystalline diamond with large size and low impurities can be synthesized both in laboratory and industry [14]. Especially, diamond synthesized by microwave plasma chemical vapor deposition (MPCVD) method can have an ultrahigh purity with impurity concentration <5 ppb, which further pushes the investigation of diamond semiconductors in electronic and optoelectronic devices.
High purity CVD diamond is regarded as the ideal material for fabricating highly energetic radiation detectors owing to the wide band gap and high radiation hardness. For the demand of sensitivity and time responsivity, diamond radiation detectors are usually fabricated in sandwich structure with metals as electrodes, such as Ti/ Au, and Cr/Au [15,16]. Graphene, as an allotrope of diamond, has many fantastic properties like diamond, such as high thermal conductivity (≈5,300 W m −1 K −1 ) [17], high carrier mobility (≈20,000 cm 2 V −1 s −1 ) [18] and high radiation hardness [19]. Recently, it has caught wide attention to be used as the electrode materials in electronic devices [20][21][22]. Compared to conventional metal electrode, graphene has low atomic number and resulting low absorption and beam scattering of UV and X-ray radiations, which make it an excellent choice of electrode materials for a transmissive UV and X-ray detectors. For example, Wei et al. [23] fabricated a diamond UV detector with graphene as front electrode, the introduction of the graphene electrode achieved high responsivity and gain factor in solar-blind sensing. Besides, both graphene and diamond have high thermal conductivity and high radiation harness, which promise great potential of their applications working in high temperature and harsh environment with high radiation dose. Due to the above excellent properties, the combination of graphene and diamond would promote the synergy of them in graphene-diamond heterostructure devices.
Usually, for most graphene electronic and optoelectronic devices, graphene films were grown on metal substrate via CVD method and then transferred to the target substrate with the assistance of PMMA [24]. However, impurities and damages cannot be avoided during the transfer process, leading to the degradation of graphene properties [25]. In order to overcome this, carbon-containing substrate SiC is annealed under high temperature (>1300 °C) to induce the formation of graphene on SiC surface [26]. Inspired by this, Tokuda et al. achieved graphene layers on diamond surface through direct vacuum annealing at a temperature over 1100 °C [27]. However, the obtained graphene layers on diamond are uncontrollable during this graphitization process. To obtain controllable graphene layers on diamond, transition metal catalysis, like Cu and Ni, are employed to induce the formation of graphene layers on the diamond surface. For the diamond samples coated a thin Cu [28] or Ni film [29][30][31], the carbon atoms on diamond surface transform from sp 3 -hybridization to sp 2 -hybridization, leading to the formation of graphene on diamond surface under a treatment temperature among 800-1000 °C. The direct conversion of graphene on diamond promises the fabrication of diamond devices with graphene as the electrodes.
In this work, a sandwich structural diamond X-ray detector was fabricated with direct sp 3 -to-sp 2 conversed graphene layers as electrodes. The graphene layers were fabricated by thermal treatment of Ni coated diamond, in which the carbon atoms on diamond surface transferred from sp 3 -hybridization to sp 2 -hybridization. The fabricated graphene-diamond-graphene (GDG) all-carbon detector shows a low dark current of 0.45 nA at 0.5 V μm −1 applied electric field, and good response to 3-20 keV incident X-ray photons. Moreover, the maximum sensitivity of the diamond detector was obtained at 12 keV incident photons, with a responsivity of 2.88 × 10 −8 C Gy −1 at 0.5 V μm −1 . Moreover, the rise and delay time of the GDG detector is about 1.2 and 22.8 ns, respectively, which are very close to that of diamond detector with Ti/Au electrode. The successful fabrication of direct sp 3 -to-sp 2 conversion of graphene on diamond surface provide a possible method for the development of diamond based all-carbon radiation detectors.

The graphene growth
Before thermal treatment, diamond substrate was immersed in the mixture of H 2 SO 4 and H 2 O 2 solution (2: 1 in volume) at 80 °C for 4 h to remove the surface impurities. A 15-nm-thick Ni film was then deposited on the both sides of the diamond at a deposition rate of 0.5 Å s −1 in an e-beam deposition system (MUE-ECO, Japan), at a background pressure of 2.0 × 10 −3 Pa. To covert sp3-boned carbon on the diamond surface into sp 2 -hybridized atoms, the Ni-coated diamonds were then treated at 950-1050 °C for a certain time in a tube furnace system (BTF-1200C-II-SL, Anhui BEQ Tech., China) with an 8 sccm H 2 flow and 100 sccm Ar flow. After thermal treatment, the Ni residual on the diamond surface was removed by an etching solution (CuSO 4 : HCl = 10 g: 50 mL in 50 mL deionized water). At last, another thermal treatment can be performed under 600 °C in air for only 10 min to remove the unexpected graphite formed on the side edge of the above diamond sample, if necessary.

Characterizations
The quality of diamond and graphene was characterized by Raman spectrometer (Renishaw plc, Wotton-under-Edge, UK) with 532 nm excitation wavelength of He-Ne laser. The ultraviolet visible (UV-Vis) transmittance of the diamond sample was characterized by a UV-Vis spectroscopy (Lambda 950, Perkin-Elmer, USA). Field emission scanning electron microscopy (SEM, QUANTA 250 FEG, FEI, USA) and transmission electron microscopy (TEM, JEOL JEM-2100, Japan) were used to investigate the surface morphology and microstructure of the diamond-graphene interface. 60 kV X-ray source (MAGPRO, Moxtek, USA) with an output power of 12 Watts was employed to study the performance of diamond-graphene heterostructure detector, and the electrical response under X-ray irradiation was measured by a precision source/measure unit (B2902A, Agilent Technologies, USA). During time response test, the X-ray source used here is Golden XRS-3 (Golden Engineering Inc, USA) portable X-ray source, and the signal was captured by an oscilloscope (4456E, Ceyear Technologies Co., Ltd, China).

Sp 3 -to-sp 2 conversion and characterization
The properties of the CVD diamond before catalytically thermal treatment were characterized in Figure 1. On the Raman spectra in Figure 1a, only the intrinsic peak of diamond at 1332.4 cm −1 can be observed, with the full width at half maximum (FWHM) of about 1.9 cm −1 . Moreover, the transmittance of the CVD diamond is about 71.8% in the visible region and 68% at 238 nm as shown in Figure 1b, corresponding to the theoretical values of the pure diamond. The above Raman spectra and the UV-Vis transmittance indicate the good crystallinity and high purity of CVD diamond. The carbon sp 3 -to sp 2 -conversion process using a thin Ni layer as catalyst was schematically illustrated in Figure 1c. After thermal treatment, the carbon atoms on the diamond surface transform from sp 3 to sp 2 hybridization with the assistance of Ni catalyst. The specie and content of the carbon bonds on diamond surface before and after thermal treatment were characterized by XPS analysis, as shown in Figure 1d. According to the specific C 1 s spectra, it can be clearly observed that there are three carbon peaks (C-C, C-O and C = O) on the diamond surface. The high C-C content (74.4%) indicates that besides the oxygen containing groups, pristine diamond surface is composed of all sp 3 bond. After thermal treatment, the content of C-C bond nearly halved, while the proportion of C = C bond increased to 44.4%, indicating the carbon sp 3 -to-sp 2 conversion occurred on diamond surface [32]. Meanwhile, as the content of oxygen containing groups nearly halved from 20.9% to 11.6% after thermal treatment, the water contact angle on diamond surface increased from only 42° to about 114° after sp 3to-sp 2 conversion process, as shown in Figure 1e, which was caused by the hydrophobicity of the diamondgraphene surface.

Characterizations of graphene formed on diamond
In order to invstigate the influence of conversion parameters on the quality and layer numbers of graphene formed on diamond substrate, the thickness of Ni films (10, 15, 25, 50 and 100 nm), treatment temperature (950, 1000, and 1050 °C) and treatment time (10,15, and 20 min) are systematically investigated. As shown in Figure 2a-c and Figure S1, the typical peaks of graphene can be found at 1585 (G-band) and 2712 cm −1 (2 D-band), respectively. The appearance of G-band originates from the sp 2 -hexagonal hybridization of carbon atoms, and the 2 D-band is caused by second-order two phonon scattering and sensitive to the layer number of graphene stacking [33]. According to Figure 2a, b, when the thickness of Ni films is smaller than 15 nm, a sharp peak can also be founded at ≈1332 cm −1 , which is the typical peak of single crystalline diamond [34]. Similarly, the peak of diamond can be also found when the treatment temperature is lower than 1000 °C (Figure 2a-c) and the treatment time is smaller than 15 min ( Figure S1a, b). The observation of the strong diamond peak indicates the formed graphene on diamond surface is thin enough that the laser can transmit the graphene layers to the diamond substrate. With the increase of Ni film thickness, treatment temperature and treatment time, the peak of diamond gradually weakened and finally disappeared, while the D-band (at ≈1350 cm −1 ) of graphene became obvious, as shown in Figure 2a-c. The absence of the diamond peak and appearance of D-band can be attribute to that with the increase of the thickness of Ni film, the graphene layers formed on the diamond became from incomplete to full coverage, even with some defects [29].
The relation of the I 2D /I G ratio and the full width at half maximum (FWHM) of 2 D-band of the graphene on diamond surface as a function of the conversion parameters were represented in Figure 2d-c. At each temperature, the I 2D /I G ratio (FWHM of 2 D-band) firstly increases (decreases) with an increase of Ni thickness and reached the maximum (minimum) value when Ni thickness is 15 nm, then it decreases (increases) with a further increase of Ni thickness. The change tendency of the curves of I 2D /I G ratios and FWHM of 2 D-band can be explained that the coverage of graphene on diamond become from incomplete (Ni: 10 nm) to complete (Ni: 15 nm), and further, to thicker with the increase of Ni film thickness. Typically, for each thickness ( Figure  2d, e), the I 2D /I G ratios and FWHM of 2 D-band always have the maximum and the minimum value when the temperature is 1000 °C, respectively. As a result, the optimized sp 3 -to-sp 2 conversion condition was finally obtained with 15 nm-thick Ni film, 1000 °C treatment temperature and 15 min treatment time (in Figure 2d, e and Figure S2). Under this optimized condition, the I 2D /I G ratios and 2 D-band FWHM reach the values of ≈1.3 and ≈43 cm −1 , respectively, corresponding to 2-3 layers of graphene [35]. The layers of the graphene formed on diamond surface at optimized condition was also characterized by TEM images, in which the fewlayer graphene can be clearly observed, as shown in Figure 2f.

Comparison and simulation of different graphene-diamond interface
The interface properties of direct graphene on diamond and on hydrogen-terminated diamond (H-diamond) structure were computed based on density-functional theory (DFT). When a CVD grown graphene layer is transferred onto the diamond surface, a hydrogen termination layer may exist at the graphene-diamond interface, leading to the formation of graphene-H-diamond heterostructure [36]. However, in this work, the graphene-diamond heterostructure was fabricated by direct sp 3 -to-sp 2 conversion, and the graphene is in direct contact with the diamond substrate, without a hydrogen layer inserted between them. To compare them, the DFT is applied in this work to calculate the interface properties of graphene on diamond and graphene on H-diamond structures, with the calculation models shown in Figure 3a, b, respectively. In each model, there are three graphene layers on the diamond (100) and H-diamond (100) surface, and the bulk diamond is composed of 12 atomic layers of carbon atom. After the optimization of graphene/diamond structure, the corresponding lattice constant mismatch is 1.29%, and vacuum buffer layer is greater than 15 Å in each supercell. The nearest neighboring graphene carbon layer shows a lattice deformation, no longer maintaining a 2 D structure on the diamond surface, as shown in Figure 3a, such surface reconstruction initiates a stepby-step conversion of the topmost diamond layer to graphene due to the structural and bonding changes under high vacuum conditions at high temperature [37]. The calculation results reveal that the interlayer distance is 1.64 Å between the neighboring diamond and graphene layers in Figure 3a, which is much smaller than that between the individual graphene layers (3.38 Å). However, when a hydrogen termination layer exists on the diamond surface, the H-diamond exhibits weak Van der Waals interactions with the graphene layers, resulting in an equilibrium spacing of 2.34 Å for the graphene/H-diamond interface, as shown in Figure 3b. Meanwhile, the graphene layer on the H-diamond surface can still maintain the 2 D structure due to the weaker van der Waals force. However, when graphene and diamond contact directly, a reconstitution of the nearest graphene layer on diamond occurred. The total electron density distributions of the graphene on diamond and H-diamond heterostructure are plotted in Figure 3c, d. The electrons, accumulated at the graphene on diamond interface as shown in Figure 3c, suggests the formation of covalent bonds between the interface based on large orbital overlapping, thus leading to a strong interfacial interaction. This is contrary to the lack of an electron accumulation at the graphene/H-diamond interface, as shown in Figure 3d, which contacts by van der Waals interactions. As a result, direct graphene on diamond contact can be expected to have a much higher electron injection efficiency than the case of the graphene on H-diamond contact. Besides, according to the flat band structure shown in Figure 3e, f, the Dirac point and band structure of graphene (blue lines) have few changes in both structures due to the slight orbital hybridization between both graphene and diamond/H-diamond surface. Strong band hybridization is found in clean diamond in Figure 3e, which induces to the band of diamond crossing the Fermi level. However, when hydrogen exists on diamond surface in Figure 3f, there is still a large band gap of over 3.0 eV for diamond.

Fabrication of GDG devices and its response to X-ray irradiations
A graphene-diamond-graphene (GDG) sandwich structural device was obtained by forming graphene layers on the double sides of diamond through the above direct sp 3 -to-sp 2 conversion process. A GDG X-ray detector was then fabricated using the sp 3to-sp 2 conversed graphene layers (area: ≈20 mm 2 ) as the front and back electrodes, as schematically illustrated in Figure 4a. The dark current of the devices was about 0.45 nA at a forward bias of 150 V (at 0.5 V μm −1 ), corresponding to a current density of 2.22 nA cm −2 at the bias of 0.5 V μm −1 , as shown in Figure 4b. Due to the high resistance of intrinsic diamond, a space-charge effect occur on diamond surface, leading to the nonlinearity on the IV curve in the -10 − 10 V region, which could be avoid by forming a thin diamond like layer (DLC) on the diamond and graphene interface [38]. According to the theory calculation, the direct conversed graphene has strong interface interaction with diamond. However, the calculated barrier height through thermionic emission equation is about 0.953 eV ( Figure S3), which is close to the literature reports [23]. Besides interfacial defects that leading to a strong space-charge effect, the high barrier height was also caused by the surface state of the diamond [39], which can be also demonstrated by the specific contact resistance measurement of different electrode on diamond substrate (as shown in Figure S4).
When exposed to high energy (hv) X-ray photons, excess electron-hole pairs will generate in the diamond bulk and then be separated by the applied electric field, and finally collected by graphene electrodes. The performance of the proposed GDG detector under X-ray radiation with different photon energies (3,5,7,9,12, and 20 keV) was investigated in dark at room temperature in the atmospheric environment. As shown in Figure 4c, the sensitivity of the detector depends on the applied voltage and incident X-ray energies. In the higher voltage region (60-150 V), the sensitivity of the device showed positive relation to the applied bias voltage. Meanwhile, according to Figure 4d, the sensitivity increased with the incident photon energy increasing and reached to a maximum value (2.88 × 10 −8 C Gy −1 at 150 V) at 12 keV followed by a falling tendency with the incident photon energy keeping increasing, which is close to the result of 50-140 nC Gy −1 mm −3 for a commercial natural diamond-based X-ray detectors [40]. The response of the GDG detector to X-rays indicates the possibility of fabricating diamond electronic devices with direct sp 3 -to-sp 2 graphene layers as electrode materials.

Comparison of the time response
The time response of the graphene-diamond-graphene detector was tested and compared to the diamond detector with Ti/Au electrode, as shown in Figure 5. The X-ray source is Golden XRS-3, which emits two X-ray pulse at one time, the response of the diamond detector to X-ray is measured by an oscilloscope. When excited by 30 keV X-ray at 100 V bias, the two peaks of the double X-ray pulse can be clearly observed, indicating both the diamond detectors with graphene and Ti/Au electrodes have fast time response to X-rays. Detailly, the rise time of the GDG detector was as low as 1.2 ns, similar to that of diamond detector with Ti/Au (20/60 nm) electrodes (1.1 ns), as shown in Figure 5. In term of the delay time, the value (~ 22.8 ns) of GDG detector is lower than that of diamond detector with Ti/Au electrodes of ~ 50 ns. The fast time response of the two diamond detectors was attributed to the high quality of the diamond. The diamond detectors with graphene and Ti/Au electrodes show similar time response to X-ray irradiations, indicating graphene can also be used as electrode materials as well as conventional metal electrodes.

Conclusion
In this work, a graphene-diamond-graphene (GDG) detector was fabricated with Ni-catalytically conversed graphene layers as electrodes. Based on the direct sp 3to-sp 2 conversion, the carbon-carbon bond on the diamond surface transfer from sp 3 -to sp 2 -hybridization with the assistance of Ni thin film. The layers of graphene can be limited into 2-3 layers with 15 nm-thick Ni film, under 1000 °C and 15 min treatment. Based on the first principles simulation, the direct formation of graphene on diamond surface has a strong interface interaction than CVD transferred graphene on H-diamond interface. The fabricated GDG detector shows a low dark current of 0.45 nA at 0.5 V μm −1 , and a maximum response of 2.88 × 10 −8 C Gy −1 at 0.5 V μm −1 at a 12 keV incident photons. Compared to conventional Ti/Au electrodes, the GDG detector shows a similar time response like detector using Ti/Au electrode. The one-step prepared all-carbon detector shows a high promise for the application of X-ray detectors and a potential method to fabricate carbon-based electronics. However, the difference of the the contact and interface between graphene and diamond need more investigation, together with the operation temperature and radiation toleration of using graphene as electrodes in diamond devices.

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