Morphological transformations of BNCO nanomaterials: Role of intermediates

Applied Surface Science 442 (2018) 682–692

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Morphological transformations of BNCO nanomaterials: Role of intermediates B.B. Wang a,⇑, X.L. Qu b, M.K. Zhu c, I. Levchenko d,e, O. Baranov f, X.X. Zhong g, S. Xu d, K. Ostrikov e,h a

College of Chemistry and Chemical Engineering, Chongqing University of Technology, 69 Hongguang Rd, Lijiatuo, Banan District, Chongqing 400054, PR China Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, PR China c College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China d Plasma Sources and Applications Centre, National Institute of Education, Nanyang Technological University, 1 Nanyang Walks, 637616 Singapore, Singapore e School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane QLD 4000, Australia f National Aerospace University, Kharkov, Ukraine g Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, PR China h CSIRO-QUT Joint Sustainable Processes and Devices Laboratory, P.O. Box 218, Lindfield NSW 2070, Australia b

a r t i c l e

i n f o

Article history: Received 11 December 2017 Accepted 19 February 2018 Available online 22 February 2018 Keywords: Boron and nitrogen co-doped tube-like carbon nanorods Carbon and oxygen co-doped boron nitride nanosheets Chemical vapor deposition Behaviors of intermediate product B2O3 Photoluminescence

a b s t r a c t Highly controllable structural transformation of various doped carbon and boron nitride nanomaterials have been achieved with the perspective of their application in microelectronics, optoelectronics, energy devices and catalytic reactions. Specifically, the syntheses of one-dimensional (1D) boron and nitrogen co-doped tube-like carbon nanorods and 2D vertical carbon and oxygen co-doped boron nitride nanosheets on silicon coated with gold films in N2-H2 plasma was demonstrated. During the synthesis of nanomaterials, boron carbide was used as carbon and boron sources. The results of characterizations by scanning and transmission electron microscopes, as well as micro-Raman and X-ray photoelectron spectroscopes indicate that the formation of different nanomaterials relates to the growth temperature and quantity of boron carbide. Specifically, 1D tube-like carbon nanorods doped with boron and nitrogen are formed at
910 °C using a small quantity of boron carbide, while 2D vertical boron nitride nanosheets doped with carbon and oxygen are grown at
870 °C using a large quantity of boron carbide. These studies indicate that the behaviors of a reactive intermediate product B2O3 on surfaces of Au nanoparticles play an important role in the formation of different nanomaterials, i.e., whether the B2O3 molecules deposited on Au nanoparticles are desorbed mainly determines the formation of different nanomaterials. The formation of 2D vertical carbon and oxygen co-doped boron nitride nanosheets is related to the high growth rate of edges of nanosheets. Furthermore, the photoluminescence (PL) properties of 1D boron and nitrogen co-doped tube-like carbon nanorods and 2D vertical carbon and oxygen co-doped boron nitride nanosheets were studied at room temperature. The PL results show that all the nanomaterials generate the ultraviolet, blue, green and red PL bands, but the 2D vertical carbon and oxygen co-doped boron nitride nanosheets emit more and stronger PL bands than the 1D boron and nitrogen co-doped tubelike carbon nanorods. The significant differences in the PL properties can be attributed to different carbon structures in these nanomaterials. These achievements can be used to synthesize and control the structures of nanomaterials and contribute to the development of the next generation optoelectronic nanodevices based on 1D and 2D nanomaterials. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Low dimensional carbon nanomaterials exhibit many properties different from those of bulk carbon materials. In particular, the heteroatom (e.g., B and N atoms) doped low dimensional ⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Chongqing University of Technology, PR China. E-mail address: [email protected] (B.B. Wang). https://doi.org/10.1016/j.apsusc.2018.02.195 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

carbon materials show excellent physical and chemical properties such as high capacitance, superior oxygen reduction reaction, near room temperature ferromagnetism and so on [1]. Due to these features, the heteroatom doped carbon nanomaterials have extensive applications in the areas of energy sources, microelectronics and optoelectronics [1–6], and they are the possible reasons that the heteroatom doped carbon nanomaterials have been extensively studied in last years.

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Hexagonal boron nitride (h-BN) is 2 dimensional (2D) structure, which is isostructural and isoelectronic to graphene [7]. However, the van der Walls force between BN layers is weaker than that in graphene, and the electronic structure of intrinsic h-BN makes it an electrical insulator due to its wide bandgap of 4–6 eV [8]. As a result, it is difficult to apply h-BN in microelectronics and optoelectronics. However, it was found that h-BN can be easily doped with heteroatoms such as carbon and oxygen atoms, which can tune the electronic structure of h-BN. As a result, the bandgap of h-BN could be reduced through carbon and oxygen doping so that the carbon and oxygen co-doped h-BN exhibits outstanding optical properties [9,10], and the carbon and oxygen co-doped h-BN has wide applications in microelectronics, optoelectronics and other applications such as biology and water cleaning [9,11]. Thus, the doped h-BN nanoflakes have attracted much attention in recent years. The 1D and 2D nanomaterials including carbon nanotubes, nanorods, nanocones and graphene nanoflakes were successfully and effectively synthesized by plasma-enhanced chemical vapor deposition [12–16]. Furthermore, we have successfully used a plasma-enhanced hot filament chemical vapor deposition (PEHFCVD) system to synthesize the C and O co-doped h-BN nanosheets and other nanomaterials in N2-H2 plasma using B4C as the boron and carbon sources [17,18]. However, control of nanomaterial morphology and controllable transformation between different morphologies still remain a problem. In the early works, Pakdel et al. synthesized differently structured BN nanomaterials by CVD, such as BN nanotubes, BN nanosheets and oriented BN nanosheets through altering the growth temperature and the composition of source materials [19–21]. These motivated us to synthesize the differently doped BN nanomaterials by PEHFCVD through changing the growth conditions. Moreover, while complex C and O co-doped BN nanomaterials (i.e., quaternary BNCO nanomaterials) have already been synthesized [9,10,17,18], the ability to control elemental composition, chemical structure and morphological features by simple means in a single process is quite limited. In particular, the control of reaction parameters such as the quantity of precursors (e.g., boron and carbon) and growth temperature remains limited. Since the BNCO nanomaters are synthesized by multi-precursors [9,10,17,18], some intermediate products produced by the reactions of multi-precursors take important roles in the formation of different BNCO nanostructures, but the role of intermediate products in predetermining the structural and morphological features of BNCO nanostructures remains largely unexplored. Thus, we aim here to study the roles of intermediate products in the synthesis of different BNCO nanomaterials. In our previous works we have described the carbon and oxygen co-doped h-BN nanosheets synthesized by PEHFCVD using B4C as the boron and carbon sources, and have found that the intermediate product B2O3 is an important precursor for the formation of carbon and oxygen co-doped h-BN nanosheets [17,18]. However, it is not clear how the quantity and states of B2O3 influence the structure of BNCO nanomaterials, and how does B2O3 affect the formation of different BNCO nanomaterials? In this work, we altered the quantity and states of intermediate product B2O3 through adjusting the growth temperature and the quantity of B4C. Specifically, the B and N co-doped tube-like carbon nanorods and vertical C and O co-doped h-BN nanosheets were synthesized on silicon substrates covered with Au films. The critical evaporation temperature of B2O3 was estimated in our experimental conditions by Clausius-Clapeyron equation. The studies indicate that the absorption or desorption of B2O3 molecules deposited on the Au NPs mainly determines the formation of different BNCO nanomaterials. Furthermore, the studies also indicate that the C and O co-doped hBN nanosheets feature high growth rate of edges of nanosheets. Considering the optoelectronic applications, the photolumines-

683

cence (PL) properties of B and N co-doped tube-like carbon nanorods and vertical C and O co-doped h-BN nanosheets were further studied at room temperature. It is found the all the nanomaterials emit the ultraviolet (UV), blue, green and red PL bands, but there are significant differences in the number and intensity of PL bands. The studies show that the significant differences in the PL properties relate to the existing forms of carbon in these nanomaterials. 2. Experimental details The synthesis of B and N co-doped tube-like carbon nanorods and vertical C and O co-doped h-BN nanosheets was carried out in PEHFCVD system described in Ref. [17] and shown in Fig. 1. Before synthesis, the silicon substrates were ultrasonically cleaned in the methylbenzene, acetone and alcohol solutions to remove the residual organics, and then the substrates were boiled in the mixed solution of NH3H2O, H2O2 and deionized water to remove the inorganics. Au film of about 15 nm was deposited onto the clear silicon substrate by magnetron sputtering. To synthesize the B and N co-doped tube-like carbon nanorods and vertical C and O co-doped h-BN nanosheets by PEHFCVD, the silicon substrate deposited with Au film was placed on the substrate holder, where the B4C sheets pressed by B4C powder were arranged around the silicon substrate. In the CVD chamber, N2 and H2 were supplied at the same flow rates of 50 sccm after the CVD chamber was evacuated to a background pressure lower than 2 Pa. Once the pressure in the CVD chamber was stabilized at about 2  103 Pa through controlling the vacuum valve, the tungsten filaments in the CVD chamber were heated to
1800 °C. At the same time, the substrate was heated to the growth temperature by the thermal radiation of hot filaments due to the short distance of
8 mm between the filaments and substrate. During the syntheses of nanomaterials, N2-H2 plasma was produced by a DC power supply of which the positive and negative electrodes were connected to the filaments and substrate through a Mo holder, respectively. In this work, four samples A-D were prepared and the growth conditions were shown in Table 1. The morphology, structure and composition of the synthesized nanomaterials were characterized by S-4800 field scanning electron microscopy (FESEM), Titan G2 transmission electron

Fig. 1. Schematic of the experimental plasma-enhanced hot filament chemical vapour deposition system used to synthesize graphene nanoflake/BNCO composition using B4C precursor. N2 and H2 were used as reactive gases. The bias current in the electric bias system was gradually increased till a blue glow appeared near the substrate, and then it was set to 160 mA to grow the nanowalls.. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Reprinted with permission from B. B. Wang et al., J. Mater. Chem. C 4, 9788-9797 (2016)

684

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Table 1 Gas flow rate, growth temperature T, bias current I and growth time t. Sample

N2 (sccm)

H2 (sccm)

T (°C)

I (mA)

t (min)

A B C D

50 50 50 50

50 50 50 50


910
910
870
870

160 160 160 160

25 30 25 30

microscope (TEM), HR 800 micro-Raman spectroscope using 532 nm line of semiconductor laser, and ESCALAB 250 X-ray photoelectron spectroscope (XPS) using Al Ka X-ray source, respectively. The room temperature PL properties of the synthesized nanomaterials were measured in Horiba Scientific Labram HR evolution Raman spectrometer under the excitation of 325 nm line of He-Cd laser, where the power of laser is set to 6.8 mW. 3. Results and discussion 3.1. Structure of B and N co-doped tube-like carbon nanorods and vertical C and O co-doped h-BN nanosheets Fig. 2 is the FESEM images of samples. As show in this figure, samples A and B are composed of nanorods while samples C and D are composed of vertical nanosheets. From the areas circulated by red dot circles in Fig. 2(a) and (b) one can apparently see that gold nanoparticles (NPs) are located on the tops of nanorods, indicating that the nanorods are grown in a vapor-liquid-solid model.

In Fig. 2(a), the bowl-like areas marked with red arrows indicate that the surface of substrate is wetted by Au NPs. Furthermore, figure Fig. 2(d) indicates that some short nanowires are formed from the nanosheets. Fig. 3 is the Raman spectra of samples A-D. The peaks at about 1365–1378 cm1 correspond to the E2g mode of h-BN, and the peaks at about 1586–1612 cm1 are associated to carbon phases [22,23]. These peaks imply that the synthesized nanomaterials are the mixture of carbon and BN phases. As shown in Fig. 3(b), the Raman peaks related to h-BN are located about 1375 and 1378 cm1, which approach to the Raman peak of bulk h-BN material (
1377 cm1) [23]. This means that samples A and B contain small h-BN grains, while samples C and D are composed of thick h-BN nanosheets doped with carbon. To further study the fine structure of synthesized nanomaterials, the samples were characterized by TEM and XPS, respectively. Figs. 4 and 5 are the TEM images of samples A and B, which indicate that the nanorods are ‘‘bamboo” type tube-like structure. Furthermore, Fig. 4(b) and (c) indicate that the walls and bamboos of

Fig. 2. FESEM images of samples A-D which indicate that samples A and B are composed of nanorods, while samples C and D are composed of nanosheets. The gold nanoparticles circulated by red rings indicate that the nanotips grow via a vapor-liquid-solid model. The bowl-like areas marked with red arrows indicate the surface of substrate is wetted by the molten Au NPs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

685

Fig. 3. Raman spectra of samples A - D. The peaks at about 1365–1378 cm1 relate to the h-BN phases, and the peaks at about 1586–1612 cm1 are associated to carbon phases.

Fig. 4. TEM image of sample A. (a) indicates the nanorod is tube-like structure. (b) and (c) show that the walls and bamboos of nanorod are amorphous structure.

Fig. 5. TEM image of sample B. (a) Indicates that the Au NPs are located on the tops of nanorods. (b) Demonstrates that the nanorods are amorphous structure. (c) is the mapping image of Au in the nanorpds, which indicates that the Au NPs are in the interior of nanorods.

nanorod are amorphous structure. From Fig. 5(a) one can apparently see that the Au NPs are located at on the top of nanorods, indicating that the nanorods are indeed grown in the vaporliquid-solid model. Fig. 5(b) indicates that the nanorods are amorphous structure. Fig. 5(c) is the mapping image of Au NPs in the nanorods, which demonstrates that Au NPs indeed locate on the

tops of nanorods. Figs. 6 and 7 are the TEM images of samples C and D, respectively. From Figs. 6 and 7 one can see that that the samples C and D are composed of nanosheets. The high resolution (HR) TEM images shown in Fig. 6(b) and 7(b) indicate that the nanosheets are crystalline structure mixed with some amorphous phases. We used Digital Micrograph software to measure the space of 5 layer nanoshoots and found that they are about 1.706 and 1.678 nm for the samples C and D, respectively. According to these data, the interspace of two nanosheets is about 0.34 nm, which approaches the interspace of two h-BN nanosheets [21]. To confirm the relationship between boron and nitrogen, we used the energy dispersive X-ray spectroscopy coupled with TEM to map the sample D and the results are shown in Fig. 8. In Fig. 8 (b)–(e) are the mapping images of C, B, N and O elements, respectively. From Fig. 8 one can see that the distribution of B atoms is similar to the distribution of N atoms, while the distributions of C and O atoms are non-uniform. Furthermore, the atomic concentrations of elements are obtained by the mapping results and they are 47.24%, 38.83%, 10.33%, 1.98% and 1.62% for B, N, C, O and Au elements, respectively. The similar distributions of B and N atoms and the ratio (
1.2) of B to N atoms indicate that the nanosheets are h-BN structure doped with carbon and oxygen. Fig. 9 is the XPS spectra of samples A-D. In Fig. 9, the XPS spectra of all the samples show the C 1s, N 1s and O 1s XPS peaks at about 285, 398 and 533 eV, and the XPS spectra of C and D show the B 1 s XPS peaks at about 191 eV, respectively. In the XPS spectra of sample A and B, the peaks at about 103.5 and 153.0 eV are associated to SiO2, respectively [24]. From the XPS results, the atomic concentration of B, N, C and O elements is shown in Table 2. According to the data listed in Table 2, the main component of samples A and B is carbon. The high concentration of oxygen in samples A and B may be related to the native oxygen of SiO2 on silicon surface. For samples C and D, the ratio of B to N atoms is about 1.06 and 0.97, which approaches to the stoichiometric ratio of h-BN. Thus the main component of samples C and D is h-BN. Here we should note that the difference in the ratios of B to N atoms between the XPS and mapping results originates from the carbon film on the Cu microgrid supporting the TEM sample. Fig. 10 is the amplified XPS spectra of B 1s, N 1s, C 1s and O 1s of samples A-D. To determine binding states of elements, the peaks were fitted using the standard XPS fitting software. The fitted peaks are shown in Fig. 10 and their positions are summarized in Table 3. As shown in Fig. 10, the B 1s XPS peak is fitted by three peaks for sample B, while it is decomposed into the two peaks for other samples. The B1, B2 and B3 peaks are related to NAB, NABAO and BAOAH bonds, respectively [8,17,24]. The N 1s and

686

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Fig. 6. TEM image of sample C. (a) Indicates that the sample C is composed of nanosheets. (b) is the HR TEM image, which demonstrates the sample C is crystalline structure mixed with some amorphous phases and the interspace of tow nanosheets is about 0.34 nm.

Fig. 7. TEM image of sample D. (a) Demonstrates that the sample D is composed of nanosheets. (b) is the HR TEM image of nanosheets, which indicates the nanosheets are crystalline structure mixed with some amorphous phases and the interspace of tow nanosheets is about 0.34 nm.

C 1s peaks of all samples are fitted by two peaks. Among these fitted peaks, the N1 and N2 peaks originate from the BAN and CAN bonds, and the C1 and C2 peaks are associated to sp2 and sp3 CAC bonds, respectively [17,24]. For the O 1s XPS peak of sample B, it is fitted by one peak, while the O 1s XPS peak of other samples is fitted by two peaks, respectively. The O1 and O2 peaks are related to the OAC@O and CAOAC radicals, respectively [17]. The above results indicate that all the samples are composed of B, N, C and O elements, but the main component of samples A and B is carbon and the main component of samples C and D is h-BN. Thus, samples A and B are composed of carbon materials doped with B and N, while sample C and D are constituted of h-BN doped with C and O

3.2. Formation and growth of B and N co-doped tube-like carbon nanorods and C and vertical O co-doped h-BN nanosheets From Table 1, it is seen that the growth conditions are similar for samples A-D, why there are great differences in their structure and composition? In the synthesis process of samples A-D, the number of B4C sheets around the substrates and the growth temperature are different. Compared with the number of B4C sheets

and growth temperature during synthesizing samples C and D, the number of B4C sheets and growth temperature in synthesizing samples A and B were reduced and raised, respectively. What leads to the formation of different nanomaterials due to the changes in the quantity of B4C and the growth temperature? In this section, starting from melting of gold, we analyze the mechanisms of different nanomaterials formation. When the substrate is heated, Au film on the silicon substrate melts and form the Au NPs [25]. The results of Rath et al. indicate that there are mutual diffusion between gold and silicon at above 850 °C with and without SiO2 on silicon surface to form the metastable gold silicate [26], thus the silicon surface is wetted by gold in our growth conditions, which is confirmed by Fig. 2(a). During heating the substrate, the B4C sheets near the substrate is simultaneously evaporated [17],

B4 C ! 4B þ C:

ð1Þ

After N2-H2 plasma is produced, N2 and H2 are ionized into a number of ions including N+, H+, and NH+3, etc. [27]. Due to the ion bombardment, the B4C sheets are sputtered to form the carbon and boron atoms. Thus, the evaporation and sputtering of B4C sheets simultaneously occur in the growth process of nanomateri-

687

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Fig. 8. Mapping images of sample D. The similar distributions of B and N atoms shown in (c) and (d) indicate that the nanosheets are mainly composed of h-BN.

als. When the growth temperature is
910 °C, it corresponds to energy of
0.1 eV, which is much lower than the energy of ions (several ten eV) [28]. Therefore, the formation of carbon and boron atoms mainly results from the sputtering. Furthermore, the residual oxygen in the CVD chamber can lead to the occurrence of following reactions [17]:

Fig. 9. XPS spectra of samples A–D. The spectra show the B 1s, C 1s, N 1s and O 1s XPS peaks, indicating that samples are composed of B, N, C and O elements.

4B þ 3O2 ! 2B2 O3 ;

ð2Þ

C þ O2 ! CO2 ;

ð3Þ

B2 O3 þ 2NHþ3 þ 2e ! 2BN þ 3H2 O:

ð4Þ

In a B4C molecule, the ratio of boron to carbon atoms is 4:1, which implies that there are more boron atoms than carbon atoms in the gas environment. In other words, the probability of reaction (3) is lower than the reaction (2), thus a lot of carbon atoms diffuse toward the molten gold NPs and they are dissolved in the Au NPs [29]. At the same time, some N+ ions are implanted into the Au

Table 2 The atomic concentration of elements obtained from XPS results. Sample

B (at.%)

N (at.%)

C (at.%)

O (at.%)

Si (at.%)

A B C D

2.96 2.54 36.20 35.83

17.38 9.18 33.91 36.85

24.07 17.82 18.84 13.01

26.71 37.75 10.14 12.96

28.89 32.71 0.91 1.35

688

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Fig. 10. B 1s, N 1s, C 1s and O 1s XPS spectra of samples B and D. The B1, B2 and B3 peaks relate to NAB, NABAO and BAOAH bonds, the N1 and N2 peaks originate from the BAN and CAN bonds, the C1 and C2 peaks are associated to sp2 and sp3 CAC bonds, and the O1 and O2 peaks are related to the OAC@O and CAOAC radicals, respectively.

Table 3 The positions of fitted peaks in Fig. 10. Sample

B1 (eV)

B2 (eV)

B3 (eV)

N1(eV)

N2 (eV)

C1 (eV)

C2 (eV)

O1 (eV)

O2 (eV)

A B C D

190.5 190.6 190.2 190.3

191.5 191.6 191.0 191.1

– 192.7 – –

397.5 397.7 397.8 387.8

398.2 398.4 398.4 398.5

284.7 284.7 284.7 284.7

285.6 285.7 285.9 285.4

531.8 – 531.3 531.8

532.5 532.5 532.8 532.9

NPs [30]. For the boron atoms, they mainly react with oxygen to form boron oxide, thus only a small number of boron atoms are dissolved in the Au NPs [31]. Simultaneously, the B2O3 molecules deposit on the surfaces of Au NPs, but they can exhibit absorption or desorption depending on the growth temperature. B2O3 exists in the form of single molecule at above 1000 °C [32], i.e., it easily evaporates at this temperature. Thus this temperature could be considered as the critical evaporation temperature of B2O3 at the environmental pressure. However, the evaporation temperature lowers with decrease of pressure according to the ClausiusClapeyron equation: g

ln

DH l 1 p2 1 ¼ ð  Þ; p1 R T2 T1

ð5Þ

where DHgl is the evaporation heat, R is the gas constant, and T1 and T2 are the evaporation temperature at the pressures p1 and p2,

respectively [33]. According to Eq. (5), the critical evaporation temperature of B2O3 is about 873 °C under our experimental conditions; here DHgl = 374 kJ/mol [32]. When samples A and B are synthesized, the growth temperature is
910 °C, thus the B2O3 molecules deposited on the surfaces of Au NPs easily desorb. As a result, the probability of reaction (4) on the surfaces of Au NPs is very low, so the surfaces of Au NPs cannot be covered by B2O3 molecules. Thus the dissolution of carbon and nitrogen can continuously occur on the surfaces of Au NPs. Due to fast diffusion on the surface of a grain compared to its interior [34], the atoms dissolved in the surface layers of Au NPs can fast diffuse to the bottoms of Au NPs. However, the solubility of carbon in gold in liquid state is limited (4.7 at.%) [29], thus the carbon dissolved in Au NPs quickly reaches saturation. Similarly, the dissolution of nitrogen in the Au NPs is quickly saturated because there is very low solubility of nitrogen in gold in liquid state [35].

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

As a result, carbon and nitrogen can fast precipitates from the bottoms of Au NPs along their surface layers to form the carbon layers containing nitrogen between the silicon substrate and Au NPs. Here we should note that the nitrogen in liquid gold mainly originates from the implantation of some nitrogen ions with high energy, i.e., the quantity of precipitated nitrogen is very small. In addition, boron and liquid gold exist in eutectic state [31], which implies that boron difficultly separates from liquid gold. For a carbon layer between the silicon substrate and a gold nanoparticle, it is similar to a wedge between the silicon substrate and the gold nanoparticle. Thus, the interaction of Au NPs with silicon substrate is weakened by the precipitation of carbon because the gold silicate is metastable [26]. The continuous precipitation of carbon lifts the Au NPs to form droplet-like NPs, which is confirmed by Fig. 2. After the Au NPs separate from the silicon substrate, some carbon locates the surface layer where the gold NPs interact with the silicon substrate, or some carbon reacts to silicon to form silicon carbide; thus, Fig. 2 shows some bowl-like traces in interface zones between the nanorods and silicon substrate (the zones pointed by red arrows). In this case, the tube-like nanorods are gradually formed [36]. The rise of droplet-like Au NPs implies that the Au NPs are close to the filaments, which leads to the rise of temperature of Au NPs and the diffusion improvement in the Au NPs. Furthermore, the dissolution of nitrogen enhances the activity of carbon in the gold NPs to further promote the diffusion of carbon in the interior of Au NPs [37], thus some carbon atoms in the interior of Au NPs can diffuse to the bottoms of Au NPs and they precipitate from the Au NPs. As a consequence, some bending carbon sheets can form in the tube-like structure. It is the reason why the nanorods shown in Figs. 4(a) and 5(a) are the ‘‘bamboo” type tubelike structure. After the tube-like nanorods are formed, the reactions (2)–(4) can occur on the surfaces of nanorods. In addition, the B2O3 and BN molecules simultaneously deposit on the surfaces of tube-like nanorods. These are the reasons why the nanorods are doped with B and N atoms. When the samples C and D grow, the number of B4C sheets around the silicon substrate increases and the growth temperature reduces to
870 °C. The increase of boron carbide quantity results in the acceleration of the reaction (1), thus the partial pressure of carbon and boron is enhanced. The chemical potential li of the ith gas in the mixed gases is related to the partial pressure pi of the ith gas, which is expressed by

li ¼ lhi ðgÞ þ RT ln

pi ; ph

ð6Þ

where lhi is the chemical potential of the ith gas at the standard pressure, and ph is the at the standard pressure [38]. According to Eq. (6), the chemical potentials of carbon and boron rise. As a result, the dissolution of carbon and boron is improved. However, Eq. (1) indicates that the partial pressure of boron is 4 times than the partial pressure of carbon, which implies that the dissolution rate of boron is higher than carbon. Due to higher solubility (>5%) of boron in gold in liquid state than carbon [39], the dissolution of carbon in Au NPs is relatively reduced to restrain the precipitation of carbon from the Au NPs. At the same time, the low growth temperature compared to the nanorods, the diffusion of carbon toward the bottom of gold NPs is weakened. As a result, the wetting of silicon substrate by gold is improved [40], i.e., the interaction between Au NPs and silicon substrate is improved and it is difficult for Au NPs to separate from the silicon substrate. Because the oxidation reaction of boron occurs
450 °C [32], the reactions (2) is accelerated to enhance the quantity of B2O3 molecules. After the formed B2O3 molecules through the reaction (2) deposit onto the surfaces of Au NPs, their desorption from the surfaces of Au NPs is balanced with the absorption of B2O3 molecules on the surfaces of Au NPs at the growth temperature of

689


870 °C (because it is approximately equal to the critical evaporation temperature calculated by Eq. (5)). As a result, the surfaces of Au NPs are covered by these B2O3 molecules. Under the electric field formed near the silicon substrate by plasma, the NH+3 ions fast reach the surfaces of Au NPs, thus the reaction (4) can fast occur on the surfaces of gold NPs because the reaction (4) can occur at
600 °C [32]. As a consequence, the BN molecules formed in reactions (4) assemble into h-BN nanosheets on the surfaces of gold NPs [17]. During the formation of BN molecules on the surfaces of Au NPs, the deposition of B2O3 molecules simultaneously occurs on the surfaces of silicon substrate among the Au NPs. However, the rate of reaction (4) (i.e., formation of BN molecules) is slow due to low chemical potential. According to the studies on the chemical potential of adatoms on liquid nanoparticles [41], the chemical potential l of B2O3 molecules on the Au NPs is expressed by:

l ¼ l1 þ

2cLV V m ; r

ð7Þ

where l1 is the chemical potential of B2O3 molecules on plain surface, cLV is the interface energy between gas and the Au nanoparticle, Vm is the volume per molecule. For the Au NPs, they locate on the silicon surface in the sphere-like shapes, thus r is larger than zero. According to Eq. (7), the chemical potential of B2O3 molecules on the Au NPs is larger than that on the silicon surface, i.e., the B2O3 molecules on the Au NPs are unstable. As a result, the reaction (4) is easy to occur on the surfaces of Au NPs and the h-BN nanosheets mainly form on the surfaces of gold NPs. After the h-BN nanosheets are formed, an excellent thermal conductivity of h-BN causes the fast cooling of gold NPs [42], which leads to the precipitation of carbon and boron from the surfaces of Au NPs and incorporation of carbon and boron atoms in h-BN nanosheetes as the heteroatoms. Furthermore, the reaction (4) is not completed, which leads to the residue of some B2O3 molecules in the h-BN nanosheets. Simultaneously, the carbon atoms deposited on the h-BN nanosheets are incorporated into the h-BN nanosheets. These atoms as the heteroatoms incorporate into the h-BN nanosheets to produce a stress in the h-BN nanosheets. During the formation of h-BN nanosheets, the deformation of h-BN nanosheets and gold NPs is inevitable due to local heating by ion bombardment [43–47], which further improves the stress. On the sites with high stress, the h-BN nanosheets break into the cracks and the cracks bend upward due to the stress [48,49], i.e., the vertical h-BN nanosheets are formed. Due to the continuous deposition, some carbon atoms, NH+3 ions, BN and B2O3 molecules can deposit on the edges of vertical h-BN nanosheets. In addition, some BN and B2O3 molecules formed on the substrate surface move toward the edges of vertical h-BN nanosheets via the side surfaces of vertical h-BN nanosheets under the electric field formed near the substrate by the plasma [50]. As a result, the reaction (4) occurs on the edges of h-BN nanosheets to form the BN molecules and promote the growth of h-BN nanosheets. As the above analyses show, the vertical h-BN nanosheets contain some heteroatoms, i.e., some defects exist in the vertical h-BN nanosheets. When the BN and B2O3 molecules moving on the side surfaces meet these defects, their motion is blocked. Hence, the reaction (4) will occur on these sites with the defects to start the growth of new h-BN sheets and result in the side growth of vertical h-BN nanosheets. These are reasons that the thick vertical C and O co-doped h-BN nanosheets form, which are confirmed by the Raman results. Since the reaction (4) can occur on the edges and surfaces of vertical h-BN nanosheets, it results in the growth of h-BN nanosheests along different directions. From Fig. 6(a) and 7(a) one can obviously see that the edges of h-BN nanosheets are rough, i.e., the edges contain a defined number of islands. According to Eq. (7), reaction (4) on the edges of h-BN nanosheets is easy compared to the surfaces of BN nanosheets, thus the edges of h-BN

690

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

nanosheets have a higher growth rate than the side faces of h-BN nanosheets. This is a possible reason that the h-BN nanosheets form. 3.3. PL properties of B and N co-doped tube-like carbon nanorods and C and O co-doped h-BN nanosheets Fig. 11 is the PL spectra obtained from different areas of samples A-D. From Fig. 11 one can clearly see that the samples generate multi-PL bands including the ultraviolet (UV), blue, green and red PL bands. Importantly, one can further find that there are significant differences in the PL properties of the samples, i.e., samples C and D generate more and stronger PL bands than samples A and B. These differences in the PL properties of these nanomaterials may be related to their structures and composition. In the next sections, the PL properties of these nanomaterials are discussed. 3.3.1. Generation of PL bands from B and N co-doped tube-like carbon nanorods and C and O co-doped h-BN nanosheets For the PL emission of C and O co-doped h-BN nanosheets, the UV PL emission is attributed to the transition between the VN3 defect level and oxygen level in the C and O co-doped h-BN nanosheets, where the VN3 defect level is a nitrogen vacancy level called three boron center (VN3) [10,17]. This is due to the VN3 defect and oxygen impurity levels below the conduction band about 1.0 and 4.5 eV in the BNCO structure, respectively [10]. In the BNCO structure, the carbon impurity level is located below the conduction band about 4.1 eV [10], thus the blue PL emission originates from the transition between the VN3 defect level and carbon impurity level in the C and O co-doped h-BN nanosheets. Due to the

doping of carbon in h-BN, the carbon clusters can form in h-BN and the energy gap of p⁄ and p bands is about 2.0–3.0 eV for some carbon clusters [9,51]. Thus, the transition between p⁄ and p bands in the carbon clusters can generate the blue PL emission and green PL emission [17,51,52]. It is the possible reason why Fig. 11(c) and (d) show the multi-PL bands related to blue light. The red PL emission may be related to the oxygen level in the carbon clusters. The XPS results indicate the formation of CAO bonds, thus the oxygen level can form between p⁄ and p bands. As a result, the transition occurring between the oxygen level and p band can generate the red PL emission, which is further evidenced by the PL emission from graphene treated by oxygen plasma [53]. In Ref. [53], the few layer graphene treated by oxygen plasma emits the PL band at about 700 nm and it is considered to be related to the CO group. For our samples, the carbon clusters in the C and O co-doped h-BN nanosheets may be large, thus the reduced energy gap of p⁄ and p bands results in the generation of red PL bands with the wave length larger than 700 nm [51]. Furthermore, the XPS results shown in Fig. 10(o) and (p) exhibit the formation of different CAO groups, which implies that there are different oxygen levels to be formed, thus the PL spectra in Fig. 11(c) and (d) show the multi-PL bands related to red light. The tube-like nanorods are composed of carbon materials doped with B and N, thus the PL emission from them is naturally determined by carbon. The Raman and XPS results indicate that there are BN bonds to form in the B and N co-doped tube-like carbon nanorods, thus the UV emission is related to BN in the B and N co-doped tube-like carbon nanorods according to the above analyses. Furthermore, Fig. 10(e) and (f) indicate that there are C-N bonds formed in the B and N co-doped tube-like carbon nanorods.

Fig. 11. PL spectra obtained from different areas of samples A-D, which show the UV bands at about 380–383 nm, blue bands at about 409–480 nm, green PL bands at about 505–513 nm, and red PL bands at about 730–822 nm, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

Besides, Figs. 4 and 5 indicate that the B and N co-doped tube-like carbon nanorods contain amorphous phases. In other words, the amorphous carbon phases contain some carbon nitride phases. According to the PL emission of amorphous carbon nitride materials [54], the blue PL emission can be attributed to the transition between r⁄ band and lone-pair valence band of nitride and the green PL emission is related to the transition between p⁄ band and p band. For the red emission, it is associated to the transition between the oxygen level and p band according to the above analyses. From Fig. 11(a) and (b) one can see that the blue and green PL bands of sample B have a red shift compared to the PL bands of sample A, which may result from the increasing size of carbon clusters in sample B due to long growth time [51]. Since the change in the size of carbon clusters can result in the change of blue and green PL bands, one naturally could ask why the red PL bands have no change in Fig. 11(a) and (b)? It may be explained by the oxygen position. The B and N co-doped tube-like carbon nanorods are grown via the vapor-liquid–solid model, thus it is difficult for oxygen to locate in interior of nanorods. In other words, oxygen mainly locate on surface of nanorods to form the surface states, thus the red PL bands have no change. 3.3.2. Analysis for differences in PL properties of B and N co-doped tube-like carbon nanorods and C and O co-doped h-BN nanosheets As shown in Fig. 11, there are significant differences in PL properties of samples, i.e., samples C and D emit more and stronger PL bands than samples A and B. What are the reasons for the observed differences in PL properties of samples? In this section, the origination of differences is analyzed. Although the synthesized nanomaterials are composed of boron, nitrogen, carbon and oxygen elements, there are significant differences in their structure and composition. As a result, there are the significant differences in their PL properties. The data listed in Table 2 indicates that the ratios of B to N atoms in samples A and B are
0.17 and 0.28, respectively, which imply that there are small quantity of BN in samples A and B. In other words, there are small number of UV PL emitting units, thus samples A and B emit the weaker UV PL bands than samples C and D. According to Tables 2 and 3, the main component of samples A and B is carbon including sp2 and sp3 carbon. Furthermore, Fig. 10 (i) and (j) indicate that the amount of sp2 carbon exceeds that of the sp3 carbon, thus samples A and B have a narrow bandgap [55]. Due to the exponential increase of PL efficiency of amorphous carbon materials with their bandgap [55], the blue and green PL efficiency of samples A and B is low so that their blue and green PL intensity is weak. For samples C and D, carbon is doped into h-BN nanosheets. Due to the aggregation of some carbon atoms, some carbon cluster can be formed in samples C and D [9]. However, Table 2 indicates that there is a small quantity of carbon in samples C and D, i.e., the carbon clusters in samples C and D exist in the form of quantum dots. As a result, these carbon clusters have a wide bandgap and a high PL efficiency [51,55], thus the PL intensity of samples C and D related to carbon is strong. Due to the different size of carbon cluster, the samples C and D generates the multi-PL bands related to carbon [51]. The above analyses indicate that the significant differences in the PL properties originate from the significant changes of their composition and structure; however these changes are caused by the growth temperature and the quantity of B4C. The studies in Sections 3.1 and 3.2 indicate that the changes of growth temperature and quantity of B4C result in the formation of nanorods and nanosheets. As a result, carbon in these different nanomaterials exists in different forms, which leads to the significant differences in the PL properties. In other words, the changes of growth temperature and quantity of B4C result in the formation of different car-

691

bon structure in the synthesized nanomaterials, which makes the PL properties of samples have significant differences. 4. Conclusion In summary, the 1D B and N co-doped tube-like carbon nanorods and 2D vertical C and O co-doped h-BN nanosheets were synthesized on silicon substrates coated with Au films by PEHFCVD, where boron carbide was used as the carbon and boron sources and N2 and H2 were used as the reaction gases. The characterization results of SEM, TEM, micro-Raman and XPS spectroscopes indicates that the changes in the quantity of boron carbide and the growth temperature lead to the formation of different nanomaterials, i.e., 1D B and N co-doped tube-like carbon nanorods are formed at a high growth temperature using the low quantity of boron carbide, while 2D vertical C and O co-doped h-BN nanosheets are grown at a low growth temperature using the large quantity of boron carbon. The studies indicate that the formation of different nanomaterials is related to the behaviors of intermediate product B2O3 deposited on the Au NPs, i.e., whether the desorption of B2O3 molecules deposited on the Au NPs occurs mainly determinates the formation of different nanomaterials. The formation of 2D vertical carbon and oxygen co-doped boron nitride nanosheets results from the high growth rate of edges of nanosheets compared to their side surfaces. Furthermore, the PL properties of 1D B and N co-doped tubelike carbon nanorods and 2D vertical C and O co-doped h-BN nanosheets were studied at room temperature. The PL results indicate that 1D B and N co-doped tube-like carbon nanorods and 2D vertical C and O co-doped h-BN nanosheets can generate the UV, blue, green and red PL emission, but there are significant differences in the number and intensity of PL bands. The UV, blue, green and red PL emission is related to the transition between the VN3 defect level and oxygen level in h-BN, the transition between the VN3 defect level and carbon level in h-BN, the transition between p⁄ band and p band of carbon clusters, and the transition between the oxygen level and p band of carbon clusters, respectively. The significant differences in the number and intensity of PL bands originate from different carbon structure in these nanomaterials. These results can be used to synthesize and control the structures of different nanomaterials and contribute to the development of next generation optoelectronic nanodevices related to 1D and 2D nanomaterials [56,57]. Acknowledgements This work was partially supported by CSIRO’s OCE Science Leadership Scheme and the Australian Research Council. I.L. acknowledges the support from the School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology. References [1] Y. Liu, Y. Shen, L. Sun, J. Li, C. Liu, W. Ren, F. Li, L. Gao, J. Chen, F. Liu, Y. Sun, N. Tang, H.-Mi. Cheng, Y. Du, Elemental superdoping of graphene and carbon nanotubes, Nat. Commun. 7 (2016) 10921-9. [2] A. Ariharan, B. Viswanathan, V. Nandhakumar, Nitrogen doped graphene as potential material for hydrogen storage, Graphene 6 (2017) 41–60. [3] Y. Zhang, X. Zhang, X. Ma, W. Guo, C. Wang, T. Asefa, X. He, A facile synthesis of nitrogen- doped highly porous carbon nanoplatelets: efficient catalysts for oxygen electroreduction, Scient. Rep. 7 (2017) 43366–43410. [4] R. Vishwakarma, S.M. Shinde, M.S. Rosmi, C. Takahashi, R. Papon, R.D. Mahyavanshi, Y. Ishii, S. Kawasaki, G. Kalita, M. Tanemura, Influence of oxygen on nitrogen-doped carbon nanofiber growth directly on nichrome foil, Nanotechnology 27 (2016) 365602–365611. [5] K. Guo, H. Qi, F. Wang, Y. Zhu, Fabrication of boron- and nitrogen-doped carbon nanoparticles by stress from pyrolysis of borazine-containing arylacetylene, RSC Adv. 4 (2014) 6330–6336.

692

B.B. Wang et al. / Applied Surface Science 442 (2018) 682–692

[6] H.K. Sadhanala, K.K. Nanda, Boron and nitrogen co-doped carbon nanoparticles as photoluminescent probes for selective and sensitive detection of picric acid, J. Phys. Chem. C 119 (2015) 13138–13143. [7] A.H.M.A. Wasey, S. Chakrabarty, G.P. Das, C. Majumder, h-BN monolayer on the Ni(111) surface: a potential catalyst for oxidation, ACS Appl. Mater. Interfaces 5 (2013) 10404–10408. [8] S. Saha, M. Jana, P. Khanra, P. Samanta, H. Koo, N.C. Murmu, T. Kuila, Band gap engineering of boron nitride by graphene and its application as positive electrode material in asymmetric supercapacitor device, ACS Appl. Mater. Interfaces 7 (2015) 14211–14222. [9] Q. Weng, X. Wang, X. Wang, Y. Bando, D. Golberg, Functionalized hexagonal boron nitride nanomaterials: emerging properties and applications, Chem. Soc. Rev. 45 (2016) 3989–4012. [10] X. Zhang, L. Li, Z. Lu, J. Lin, X. Xu, Y. Ma, X. Yang, F. Meng, J. Zhao, C. Tang, Effects of carbon and oxygen impurities on luminescence properties of BCNO phosphor, J. Am. Ceram. Soc. 97 (2014) 246–250. [11] W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Porous boron nitride nanosheets for effective water cleaning, Nat. Commun. 4 (2013) 1777–1787. [12] K. Ostrikova, E.C. Neyts, M. Meyyappan, Plasma nanoscience: from nano-solids in plasmas to nano-plasmas in solids, Adv. Phys. 62 (2013) 113–224. [13] B.B. Wang, Q.J. Cheng, X. Chen, K. Ostrikov, Enhancement of electron field emission of vertically aligned carbon nanotubes by nitrogen plasma treatment, J. Alloys Compd. 509 (2011) 9329–9334. [14] Z.L. Tsakadze, I. Levchenko, K. Ostrikov, S. Xu, Plasma-assisted self-organized growth of uniform carbon nanocone arrays, Carbon 45 (2007) 2022–2030. [15] S. Mao, Z. Wen, S. Ci, X. Guo, K. Ostrikov, J. Chen, Perpendicularly oriented MoSe2/graphene nanosheets as advanced electrocatalysts for hydrogen evolution, Small 11 (2015) 414–419. [16] I. Levchenko, K.K. Ostrikov, J. Zheng, X. Li, M. Keidar, K.B.K. Teo, Scalable graphene production: perspectives and challenges of plasma applications, Nanoscale 8 (2016) 10511–10527. [17] B.B. Wang, K. Zheng, D. Gao, I. Levchenko, K. Ostrikov, M. Keidar, S.S. Zou, Plasma-chemical synthesis, structure and photoluminescence properties of hybrid graphene nanoflake–BNCO nanowall systems, J. Mater. Chem. C 4 (2016) 9788–9797. [18] B.B. Wang, D. Gao, I. Levchenko, K. Ostrikov, M. Keidar, M.K. Zhu, K. Zheng, B. Gao, Self-organized graphene-like boron nitride containing nanoflakes on copper by low temperature N2 + H2 plasma, RSC Adv. 6 (2016) 87607–87615. [19] A. Pakdel, Y. Bando, D. Golberg, Morphology-driven nonwettability of nanostructured BN surfaces, Langmuir 29 (2013) 7529–7533. [20] A. Pakdel, X. Wang, Y. Bando, D. Golberg, Nonwetting ‘‘white graphene” films, Acta Mater. 61 (2013) 1266–1273. [21] A. Pakdel, C. Zhi, Y. Bando, T. Nakayama, D. Golberg, Boron nitride nanosheet coatings with controllable water repellency, ACS Nano 5 (2011) 6507–6515. [22] A. Pakdel, Y. Bando, D. Shtansky, D. Golberg, Nonwetting and optical properties of BN nanosheet films, Surf. Innovat. 1 (2013) 32–39. [23] R. Arenal, A.C. Ferrari, S. Reich, L. Wirtz, J.-Y. Mevellec, S. Lefrant, A. Rubio, A. Loiseau, Raman spectroscopy of single-wall boron nitride nanotubes, Nano Lett. 6 (2006) 1812–1816. [24] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Physical Electronics Division, USA, 1979. [25] B.B. Wang, K. Zheng, Q.J. Cheng, L. Wang, M.P. Zheng, K. Ostrikov, Formation and electron field emission of graphene films grown by hot filament chemical vapor deposition, Mater. Chem. Phys. 144 (2014) 66–74. [26] A. Rath, J.K. Dash, R.R. Juluri, A. Rosenauer, P.V. Satyam, Temperaturedependent electron microscopy study of Au thin films on Si (1 0 0) with and without a native oxide layer as barrier at the interface, J. Phys. D: Appl. Phys. 44 (2011) 115301–115309. [27] H. Nagai, S. Takashima, M. Hiramatsu, M. Hori, T. Goto, Behavior of atomic radicals and their effects on organic low dielectric constant film etching in high density N2 /H2 and N2/NH3 plasmas, J. Appl. Phys. 91 (2002) 2615–2621. [28] B.B. Wang, B. Zhang, Effects of carbon film roughness on growth of carbon nanotip arrays by plasma-enhanced hot filament chemical vapor deposition, Carbon 44 (2006) 1949–1953. [29] H. Okamoto, T.B. Massalski, The Au-C (gold-carbon) system, Bull. Alloy Phase Diagr. 5 (1984) 378–379. [30] J. Yang, Investigation of low energy nitrogen ion implantation into gold, Adv. Mater. Res. 461 (2012) 840–843. [31] Z.-Z. Li, J. Baca, S.H. Yun, J. Wu, Gold/boron core–shell nanocables synthesized from gold–boron eutectic droplets, Nanotechnol. 19 (2008) 055606–55616.

[32] X.M. Gu, Y.S. Gong, X.W. Zhang, K.L. Tang, Y.Y. Lu, W.Z. Zeng, Inorganic Chemistry Series, Science Press, Beijing, Vol. 2, 1990, pp. 195–286 (in Chinese). [33] J.P. Bromberg, Physical Chemistry, Allyn and Bacon Inc, Boston, 1980. [34] J.M. Xiao, Alloy Phases and Phase Transformation, Metallurgy Industry Press, Beijing, 1987 (in Chinese). [35] H. Okamoto, T.B. Massalski, The Au-N (Gold-Nitrogen) System, Bull. Alloy Phase Diagr. 5 (1984) 381. [36] D. Takagi, Y. Kobayashi, H. Hibino, S. Suzuki, Y. Homma, Mechanism of goldcatalyzed carbon material growth, Nano Lett. 8 (2008) 832–835. [37] B.B. Wang, S.l. Lee, X.Z. Xu, S. Choi, H. Yan, B. Zhang, W. Hao, Effects of the pressure on growth of carbon nanotubes by plasma-enhanced hot filament CVD at low substrate temperature, Appl. Surf. Sci. 236 (2004) 6–12. [38] Y. Hu, Physical Chemistry, Higher Education Press, Beijing, 2017 (in English). [39] Au-B Phase diagram, Binary Systems. Part 5: Binary Systems Supplement 1 pp 1-3, Part of the Landolt-Börnstein-Group IV Physical Chemistry book series (vol. 19B5), . [40] M. Aizenshtein, N. Froumin, N. Frage, Experimental study and thermodynamic analysis of high temperature interactions between boron carbide and liquid metals, Eng. 6 (2014) 849–868. [41] V.G. Dubrovskii, N.V. Sibirev, J.C. Harmand, F. Glas, Growth kinetics and crystal structure of semiconductor nanowires, Phys. Rev. B 78 (2008) 235301– 235310. [42] N. Jain, T. Bansal, C.A. Durcan, Y. Xu, B. Yu, Monolayer graphene/hexagonal boron nitride heterostructure, Carbon 54 (2013) 396–402. [43] K. Ostrikov, I. Levchenko, U. Cvelbar, M. Sunkara, M. Mozetic, From nucleation to nanowires: a single-step process in reactive plasmas, Nanoscale 2 (2010) 2012–2027. [44] P. Liu, S.C. Mao, L.H. Wang, X.D. Han, Z. Zhang, Direct dynamic atomic mechanisms of strain-induced grain rotation in nanocrystalline, textured, columnar-structured thin gold films, Scr. Mater. 64 (2011) 343–346. [45] L. Wang, J. Teng, P. Liu, A. Hirata, E. Ma, Z. Zhang, M. Chen, X. Han, Grain rotation mediated by grain boundary dislocations in nanocrystalline platinum, Nat. Commun. 5 (2014) 4402–4407. [46] K. Zheng, C. Wang, Y.-Q. Cheng, Y. Yue, X. Han, Z. Zhang, Z. Shan, S.X. Mao, M. Ye, Y. Yin, E. Ma, Electron-beam-assisted superplastic shaping of nanoscale amorphous silica, Nat. Commun. 1 (2010) 24–28. [47] I. Levchenko, M. Keidar, S. Xu, H. Kersten, K. Ostrikov, Low-temperature plasmas in carbon nanostructure synthesis, J. Vac. Sci. Technol. B 31 (2013) 050801–50816. [48] A. Malesevic, R. Vitchev, K. Schouteden, A. Volodin, L. Zhang, G. V. Tendeloo, A. Vanhulsel, 1 C. V. Haesendonck, Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition, Nanotechnology 19 (2008) 305604-6. [49] Z. Bo, S. Mao, Z.J. Han, K. Cen, J. Chen, K. Ostrikov, Emerging energy and environmental applications of vertically-oriented graphenes, Chem. Soc. Rev. 44 (2015) 2108–2121. [50] J. Zhao, M. Shaygan, J. Eckert, M. Meyyappan, Mark H. Rümmeli, A growth mechanism for free-standing vertical graphene, Nano Lett. 14 (2014) 3064– 3071. [51] G. Eda, Y.-Y. Lin, C. Mattevi, H. Yamaguchi, H.-A. Chen, I.-S. Chen, C.-W. Chen, M. Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv. Mater. 22 (2010) 505–509. [52] B.B. Wang, Q.J. Cheng, L.H. Wang, K. Zheng, K. Ostrikov, The effect of temperature on the mechanism of photoluminescence from plasmanucleated, nitrogenated carbon nanotips, carbon 50 (2012) 3561–3571. [53] T. Gokus, R.R. Nair, A. Bonetti, M. Böhmler, A. Lombardo, K.S. Novoselov, A.K. Geim, A.C. Ferrari, A. Hartschuh, Making graphene luminescent by oxygen plasma treatment, ACS Nano 3 (2009) 3963–3968. [54] Y. Iwano, T. Kittaka, H. Tabuchi, M. Soukawa, S. Kunitsugu, K. Takarabe, K. Itoh, Study of amorphous carbon nitride films aiming at white light emitting devices, Jap. J. Appl. Phys. 47 (2008) 7842–7844. [55] J. Robertson, Recombination and photoluminescence mechanism in hydrogenated amorphous carbon, Phys. Rev. B 53 (1996) 16302–16305. [56] I. Levchenko, K. Bazaka, M. Keidar, S. Xu, J. Fang, Hierarchical multicomponent inorganic metamaterials: Intrinsically driven self-assembly at the nanoscale, Adv. Mater. 30 (2018) 1702226. [57] I. Levchenko, K. Bazaka, Y. Ding, Y. Raitses, S. Mazouffre, T. Henning, P.J. Klar, S. Shinohara, J. Schein, L. Garrigues, M. Kim, D. Lev, F. Taccogna, R.W. Boswell, C. Charles, H. Koizumi, S. Yan, C. Scharlemann, M. Keidar, S. Xu, Space micropropulsion systems for Cubesats and small satellites: from proximate targets to furthermost frontiers, Appl. Phys. Rev. 5 (2018) 011104.