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Carbon 109 (2016) 352e362

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Carbon

journal homepage: www.elsevier .com/locate /carbon

Conversion of vertically-aligned boron nitride nanowalls tophotoluminescent CN compound nanorods: Efficient composition andmorphology control via plasma technique

B.B. Wang a, M.K. Zhu b, K. Ostrikov c, d, e, I. Levchenko f, *, M. Keidar g, R.W. Shao h,K. Zheng h, D. Gao h

a College of Chemistry and Chemical Engineering, Chongqing University of Technology, 69 Hongguang Rd, Lijiatuo, Banan District, Chongqing 400054, PRChinab College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR Chinac Institute for Future Environments and School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000,Australiad CSIRO-QUT Joint Sustainable Materials and Devices Laboratory, Commonwealth Scientific and Industrial Research Organization, P.O. Box 218, Lindfield,NSW 2070, Australiae Plasma Nanoscience, School of Physics, The University of Sydney, Sydney, NSW 2006, Australiaf School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000, Australiag George Washington University, 20052 Washington DC, USAh Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, PR China

a r t i c l e i n f o

Article history:Received 8 May 2016Received in revised form9 August 2016Accepted 11 August 2016Available online 12 August 2016

* Corresponding author.E-mail address: Igor.Levchenko@qut.edu.au (I. Lev

http://dx.doi.org/10.1016/j.carbon.2016.08.0290008-6223/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Two-dimensional BN/CN nanomaterials of various composition and morphology were synthesized inN2eH2 plasma by plasma-enhanced hot filament chemical vapor deposition, with B4C used as precursor.The results of field emission scanning electron microscopy, X-ray diffractometer, transmission electronmicroscopy, micro-Raman and X-ray photoelectron spectroscopy evidence that the hexagonal boronnitride nanowalls with carbon and oxide phases were synthesized under about 1:1 ratio of H2 to N2,while the CN nanorods with boron and oxide phases are formed under other ratios of H2 to N2, and thenanowires made of oxide phases are grown without the plasma. Efficient control over the nanostructurecomposition and morphology was demonstrated, thus proposing a cheap and convenient fabricationtechnique for the advanced composite boron nitride/graphene materials. The photoluminescenceproperties of the synthesized BN and CN nanomaterials were also studied using the 325 nm line ofHeeCd laser as the excitation source. The fabricated nanomaterials can generate the ultraviolet, blue andgreen multi-PL bands, which are associated to the defects formed in these nanomaterials and the carbonclusters, respectively. These outcomes can make a great contribution to the design of functional BN- andCN-nanomaterials of various morphologies and compositions for the applications in various electronicand carbon-based optoelectronic nanodevices.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon-based nanomaterials are very fascinating structureswhich have recently attracted an enormous attention due to theirpotential application in various areas including biomedical sci-ences, chemical and catalyst industry, environment engineering,energy storage and conversion, and many others [1e3]. Since

chenko).

discovery of two-dimensional (2D) graphene, the 2Dmaterials havebecome a subject of extensive research efforts due to their uniqueproperties and potential applications in nanoelectronics, opto-electronics, sensing, energy conversion and many other areas[4e7]. The vertically oriented 2D materials grown on solid sub-strates offer unique features, capable of boosting the characteristicsof many emerging applications. For example, three dimensionalinter-networked arrays of vertically-aligned graphene nanowallswith exposed edges are of paramount importance for diverse ap-plications such as supercapacitors and biosensors [8,9]. Among

Table 1Gas flow rate, bias current (Ib) and growth time (t).

Sample N2 (sccm) H2 (sccm) Ib (mA) t (min)

A 50 50 e 20B 50 50 160 15C 50 50 160 20D 50 60 160 20E 80 20 160 20

B.B. Wang et al. / Carbon 109 (2016) 352e362 353

others 2D materials, hexagonal structures such as graphene andhexagonal boron nitride (h-BN) attract the major attention due totheir exceptional electronic, mechanical, and other properties. Asan isomorph of graphene, h-BN has the same structure except for asmall difference in the lattice constant, and many properties of h-BN are similar to those of graphene [10,11]. Moreover, h-BN/gra-phene hybrid structures exhibit excellent electronic properties,high charge mobility, small charge fluctuation level, outstandingchemical stability and very low chemical reactivity [12,13].Furthermore, the atomic level hybrid h-BN/graphene materials aresemiconductors with tunable composition and bandgap [14e17].Thus, very encouraging results are expected from the complex h-BN/graphene 2D materials.

Another perspective application niche for the complex h-BN/graphene 2D materials is the electric propulsion techniques whereh-BN/graphene structures can be used as base for the sophisticatedmetamaterials [18] and potentially self-healing material for theaccelerating channels of Hall-type thrusters [19]. Low secondaryelectron emission and very high chemical stability of the boronnitride, combined with unique electronic and mechanic propertiesof graphene, can pave the way to the advanced self-adapting pro-pulsion devices [20]. For this specific application, a sophisticatedcontrol of the morphology and composition is required to producevirtually any required BN/CB structure.

Despite extensive studying of the 2D h-BN/graphene materialsand hybrid metamaterials, several challenges still undergo activeexploration: (i) it is very difficult to arrange the vertical alignmentof two-dimensional nanosheets, and (ii) it is also very difficult tocontrol the elemental and phase composition (e.g., BN and CN),reduce the oxygen content, and optimize the photoluminescenceproperties of these advanced materials. In this work, we haveemployed the plasma-enhanced hot filament chemical vapordeposition (PEHFCVD) system to synthesize the vertical 2D BN/CNnanowalls, as well as CN nanorods and nanowires made of oxidephases in the N2eH2 plasma by changing the process conditions(specifically, ratio of nitrogen to hydrogen flow rates). Efficientcontrol over the nanostructure composition and morphology wasdemonstrated, thus proposing a cheap and convenient solution forthis problem of immense complexity. Besides, the photo-luminescence (PL) properties of these nanomaterials were studied,and the generation of multi-PL bands different with the single PLband of the hybridized materials of BN and graphite synthesized bysintering were also demonstrated [15,17].

2. Experimental

2.1. Samples preparation

Prior to growth of nanomaterials, the surfaces of silicon waferswere cleaned using chemical method described elsewhere [21],then a gold filmwith a thickness ofz15 nmwas deposited onto thewafers.

Nanomaterials were synthesized in a PEHFCVD systemdescribed in detail in our previous work [21]. Prepared wafer wasinstalled in a CVD chamber equipped with heating and biasingsystems. The heating system included three coiled tungsten fila-ments of 1 mm dia., heated toz1800 �C. The bias systemwas fittedwith a direct current (DC) power supply unit, which anode andcathode were connected to the tungsten filaments and substrateholder, respectively. The B4C sheets pressed of the B4C powderwereplaced near the siliconwafer on the substrate holder. Due to a shortdistance between the filaments and substrate (z10 mm), thesubstrate was heated to a high (z870 �C) temperature by the hotfilaments.

Before nanomaterial synthesis, CVD chamber was evacuated to a

basic pressure not exceeding 2 Pa. Then, reactive gases (nitrogenand hydrogen) were supplied into the chamber, gas pressure wasstabilized at about 2� 103 Pa using vacuumvalve, and the tungstenfilaments were heated by AC current. Once the substrate washeated to z870 �C, the DC power supply was turned on to igniteplasma. In these experiments, five samples (A e E) were preparedunder the synthesis conditions listed in Table 1. The sample A wasprepared without igniting the plasma in the chamber (i.e., in aneutral process environment).

2.2. Characterization

Morphology, structure, and composition of the synthesizednanomaterials were extensively and systemically studied usingcharacterization instruments including Hitachi S-4800 field emis-sion scanning electron microscopy (FESEM) operated at 15 KV, FEITECNAT G2 transmission electron microscopy (TEM) operated at200 kV, Rigaku Ultima X-ray diffractometer (XRD), HR 800 micro-Raman spectroscope using a 532 nm line of semiconductor laseras the excitation source, and ESCALAB 250 X-ray photoelectronspectroscope (XPS) using an Al Ka X-ray source, respectively. The PLproperties of the synthesized nanomaterials were studied at roomtemperature in LabRAMHR Evolution system using the 325 nm lineHeeCd laser as the excitation source. During the PL measurements,laser power was 1mW for samples B and C, and 10mW for samplesA, D and E, respectively.

3. Results

3.1. FESEM, Raman and XRD results

Fig. 1 shows the FESEM images of all five samples fabricated bythe above described technique. From this figure one can see thatthe process conditions drastically affect the morphology of theresultant nanostructures. Specifically, the sample A is composed ofentangled nanowires, the samples B and C feature vertically-aligned nanosheets, and the samples D and E are made of a mixof vertically-aligned nanorods and nanotips, respectively. The in-sets in Fig. 1(b) and (c) are the high-magnification FESEM images ofsamples B and C, which clearly show the edges of the vertically-aligned nanosheets. Moreover, Fig. 1(e) also shows that the tips ofsome nanotips are broken (apparently, due to ion bombardmentduring the growth in plasma); the red dot rings highlight some ofthe disrupted nanotips.

Fig. 2(aec) is the Raman spectra of all five samples, grouped bythe process conditions and different morphologies described in theabove section. As can be seen in these figures, all spectra feature thepeaks at about 1363e1369 and 1592e1615 cm�1, which areattributed to the vibration of BeN bonds in the h-BN materials[22e24], and to the vibration of CeB or C]C bonds, respectively[25e27]. The further analysis of the results reveals the peaks atabout 1164 and 1166 cm�1 in the spectra of samples A and D relatedto the BeOeH groups [28], and the peak at about 1905 cm�1 in thespectrum of sample D which results from the CeO bonds [29]. Weshould note that the origin of peaks at about 2125, 2127, 2134 and

Fig. 1. FESEM images of samples. Entangled nanowires on sample A (a), vertically-aligned nanosheets on samples B and C (b, c), and a mix of vertically-aligned nanorods andnanotips on samples D and E (d, e). The insets in (b) and (c) are the high-magnification FESEM images of samples B and C with clearly visible edges of the vertically-alignednanosheets. (A color version of this figure can be viewed online.)

B.B. Wang et al. / Carbon 109 (2016) 352e362354

1709 cm�1 in the Raman spectra of samples A, D and E is notcompletely clear. The peaks at about 2125, 2127 and 2134 cm�1 maybe related to the CeN groups since the close peaks at 2120 and2210 cm�1 originate from the C]N and C^N bonds, respectively[30]. Since the peak at 1709 cm�1 related to C]O group wasobserved by FTIR [31], it may be associated to the C]O group. FromFig. 2 one can also see that no peaks related to C]N, C^N, CeO, andC]O groups appear in the Raman spectra of samples B and C; we

explain this by the fact that these groups are too small to bedetected by the Raman spectroscopy.

Fig. 2(d) is the XRD patterns of the fabricated samples. In thesepatterns, the peaks at about 2q z 38.5� and 61.9� result from thediffraction of B2O (003) (JCPDS card: No. 41-0624) and b-C3N4 [32].However, the XRD pattern of carbon has a diffraction peak at2q z 61.9� (JCPDS card: No. 79-1468), thus the peak at about 61.9�

in our spectra relates to carbon. The broad diffraction peaks at

Fig. 2. (aec) Raman spectra of the samples. All spectra feature the peaks at about 1363e1369 and 1592e1615 cm�1. The peaks at about 1164 and 1166 cm�1 in the spectra of samplesA and D relate to BeOeH groups, and the peak at about 1905 cm�1 in the spectrum of sample D which results from the CeO bonds. (d) XRD patterns of the samples. (A colourversion of this figure can be viewed online.)

B.B. Wang et al. / Carbon 109 (2016) 352e362 355

about 2qz 25.3� can be attributed to themixture of BN and carbon,or they relate to the amorphous phases [33].

3.2. XRD, XPS and TEM analysis results

Fig. 3 is the XPS spectra of all samples, which show the B 1s, C 1s,N 1s and O 1s XPS peaks at about 191, 285, 399 and 533 eV,respectively. The two peaks at about 102 and 154 eV are the Si 2p

Fig. 3. XPS spectra of samples. The B 1s, C 1s, N 1s and O 1s peaks are present at about191, 285, 399 and 533 eV. The two peaks at about 102 and 154 eV are the Si 2p and Si 2sXPS peaks. The peaks related to B 1s are absent in the spectra of samples A, D and E,thus indicating small amount of boron in these samples. (A color version of this figurecan be viewed online.)

and Si 2s XPS peaks [34]. Importantly, the peaks related to B 1s areabsent in the XPS spectra of samples A, D and E, and this could beexplained by small amount of boron in these specimens. Indeed,the atomic concentrations of B element in samples A, D and E listedin Table 2 (the data taken from the XPS results) are significantlylower than that of samples B and C, and thus the XPS spectra ofsamples A, D and E do not show the obvious B 1s XPS peaks.

Furthermore, the ratios of boron to nitrogen atoms are about0.64, 1.16, 1.08, 0.24 and 0.24 for the samples A-E, respectively; thismeans that the ratios of boron to nitrogen atoms in the samples Band C approach to the stoichiometric ratios intrinsic to the h-BNstructure. In addition, Table 2 also indicate that the samples B and Cfeature low atomic concentrations of carbon and oxygen, while thesamples A and E have high atomic concentration of oxygen.

Since the B, C and N elements represent the main interest in ourwork, we have shown the B 1s, C 1s, and N 1s XPS spectra inseparate Fig. 4. To analyze the binding states of the B, C and N el-ements, the B 1s, C 1s and N 1s XPS spectra were fitted using thestandard XPS fitting software after Shirley background subtraction,and the fitted peaks (positions of fitted XPS peaks) are summarizedin Table 3. The C1, C2, C3 and C4 peaks are attributed to the CeB, C]B, C]C and C]N or CeO bonds [14,35e37], the N1, N2 and N3peaks are related to the SieC]N, BeN, and C]N bonds [38,39], and

Table 2Atomic concentration of B, C, N and O elements.

Sample B (atm.%) C (atm.%) N (atm.%) O (atm.%)

A 1.93 8.14 3 51.53B 43.66 10.17 37.54 7.93C 34.65 16.5 31.58 17.27D 4.46 17.68 18.58 27.22E 5.12 25.07 21.33 48.48

Fig. 4. B1s, C1s, and N1s XPS spectra of samples. The spectra were fitted using the standard XPS fitting software after Shirley background subtraction. The integral intensity ratiosN2/N3 peaks are 1.23 and 1.72 for samples B and C, and the N1/N2 ratios peaks are 1.44 and 2.77 for samples D and E; this evidences that samples B and C contain more boronnitride, while samples D and E contain more CN compound. (A color version of this figure can be viewed online.)

B.B. Wang et al. / Carbon 109 (2016) 352e362356

the B1, B2 and B3 peaks are associated with the BeN, NeBeO andBeO bonds, respectively [39,40]. According to Fig. 4, the integralintensity ratios of N2 to N3 peaks are 1.23 and 1.72 for samples Band C, and the ratios of N1 to N2 peaks are 1.44 and 2.77 for thesamples D and E, respectively. These data imply that the samples B

and C contain more BN component while the samples D and Econtain more CN components, i.e., the transformation from BN toCN structures can be realized through altering the properties ofplasma.

From the above XPS results we can conclude that the samples B

Table 3Positions of fitted XPS peaks in Fig. 4.

Sample C1 (eV) C2 (eV) C3 (eV) C4 (eV) N1 (eV) N2 (eV) N3 (eV) B1 (eV) B2 (eV) B3 (eV)

A 282.6 283.2 284.8 286.4 e 397.8 398.7 190.4 e 192.9B e 283.6 284.8 286.5 e 398.0 398.8 190.4 191.0 e

C e 283.6 284.8 286.3 e 397.8 398.5 190.3 191.0 e

D e 283.4 284.8 286.3 397.4 397.9 e 190.6 191.4 e

E e 283.7 284.8 286.3 397.3 397.8 e 190.4 191.7 192.4

B.B. Wang et al. / Carbon 109 (2016) 352e362 357

and C are the h-BN structure with carbon and oxide phases,whereas the samples D is the CN structure with boron and oxidephases, and the samples A and E contain oxide phases. Since thesamples B and C are the h-BN structure with carbon and oxidephases, one can ask why the XRD patterns do not show thediffraction peaks of h-BN? Most likely, this is related to h-BNstructure and impurities. Our samples B and C are composed of h-BN fragments (i.e., nanosheets) mixed with carbon and oxygen,which results in broadened XRD peaks at about 25.3� due to smallsizes of h-BN fragments and carbon particles. Furthermore, thestructural difference between h-BN and carbon nanoparticles andthe edges of h-BN nanosheets can introduce some amorphousphases in the h-BN nanosheets. These amorphous phases furtherbroaden the XRD peaks. Since strong XRD peaks of h-BN and h-graphite are located about 26.7� and 26.4� (JCPDS cards: No. 45-0896 and No. 41-1487), it is difficult to distinguish h-BN and carbonfrom the broadened XRD peaks. As a result, the XRD patterns do notshow the diffraction peak of h-BN. According to the JCPDS cards, thebroadened XRD peaks of samples B and C should be located at ~26�,while they are located at about 25.3�; this may be due to thestructural difference of film with powder and the measurementerror. For example, the h-BN powders generate a strong and sharppeak at about 26.5� [41], while the XRD patterns of the thick h-BNfilm show a weak and broad peak at about 2q z 25.9� [22]. Thisevidences the changes of XRD peaks caused by the structuralchange, thus broadened XRD peaks at about 25.3� are attributed tothe h-BN and carbon.

The oxygen content in the fabricated samples is one moreimportant question. Indeed, the XPS results show a large amount ofoxygen for sample A, and thus, an obvious question requires clar-ification e is this due to the use of silicon substrate, and is the maincomponent of nanowires silicon oxide or carbon oxide? The largeamount of oxygen for sample A is related to the silicon substrateand the nanowires, but the nanowires are not related to the siliconsubstrate. From Fig. 1(a) one can see that the nanowires form mi-cropores, i.e., the silicon substrate cannot be fully covered by thenanowires. As a result, there is enough Si to be detected by XPS,which is confirmed by the strong Si signal of sample A in Fig. 3.Although hydrogenwas employed during the growth of nanowires,it is possible that hydrogen does not completely react with thenative oxide layer of silicon substrate. Thus, some of oxygen in thenative oxide layer of silicon substrate is detected by XPS. However,silicon substrate is covered with gold film, and hence the Si sub-strate weakly evaporated during heating, i.e. there is only few Siatoms in the gas environment. Furthermore, carbon can easilyseparate from AueC nanoparticles to form carbon nanotubes andthe carbon nanowires can be formed from AueSi nanoparticles[42,43], while the Si separation from AueSi nanoparticles was notobserved [43]. In addition, Takagi et al. grew the carbon nanowiresfrom SiO2 substrates coated with gold film [43], and hence, carbonnanowires can be catalytically formed by gold. Due to the interac-tion of carbon with the residual oxygen, carbon oxide can beformed on the surfaces of carbon nanowires, thus the nanowires inthe sample A are mostly composed of carbon oxide rather thansilicon oxide. The above analyses indicate that the large amount of

oxygen for sample A is related to the silicon substrate and nano-wires, while the nanowires are not related to SiOx. Due to the sta-bility of h-BN [12,13], it is difficult to react with oxygen, hence thesamples B and C contain a small amount of oxygen compared to thesamples D and E, which are evidenced by the XPS results.

To further confirm the structure of the synthesized nano-materials, samples A, B, D and E were studied by TEM technique,and the results are shown in Fig. 5. As one can see from these im-ages, the samples A, B, D and E are indeed composed of nanowires,nanosheets and nanorods, respectively, and this is consistent withthe above described SEM results. The selected area electrondiffraction (SAED) patterns in the insets evidence the crystallinestructure of the synthesized nanomaterials. However, the disper-sion rings in the SAED patterns indicate that the samples containsome amorphous phases, and this is consistent with the XRD re-sults. Furthermore, the mixture of diffraction spots and dispersionrings in the SAED pattern of sample B further evidences thatsamples B and C are the multilayer structure (the thickness ofmonolayer H-BN is ~0.7 nm [44], but the thickness of nano sheets inFig. 1(b) is over 4.7 nm) due to the structural similarity between h-BN and graphene, and the SAED patterns of graphene structurechanging from the spots to rings with the increase of graphenelayers [44,45].

The blue-bordered inset in Fig. 5(a) shows rough surfaces whichmay be an indicator of a possibility of pore presence in the struc-ture. Nitrogen sorption (BET) analysis showed negative sorption(not shown here) which is usually attributed to the insufficientamount of the analysed material (see supporting information (SI)).Our data thus indicate that porosity is indeed possible, but theseindications are inconclusive at this stage and further research isrequired to scale up the BNCO material production to synthesis thesufficient amount for BET analysis.

3.3. PL results

Fig. 6 shows the PL spectra of all five samples. As seen in thefigure, the PL spectra reveal the ultraviolet (UV) bands at about348e355 nm, blue bands at about 409e415 nm, and green bands atabout 505e526 nm, respectively. In addition, Fig. 6 also shows thestronger PL intensity of sample B over sample C, weak PL intensityof samples A and D, and a veryweak PL for sample E. From Fig. 6 onecan see that there is a great difference in the PL properties ofdifferent samples, which are related to the differences in theirstructure and chemical composition.

According to the above analyses, the results can be summarizedas below. When the plasma is off and the nanostructures are grownin a neutral process environment, only non-oriented entanglednanowires made of oxide phases are formed, which are capable ofproducing a weak photoluminescence. When the plasma is on, thevertically oriented nanomaterials are synthesized. The samplesgrown in the 50/50 hydrogen-nitrogen mixture are the h-BNnanostructures with carbon and oxide phases. The sample B grownfor 15 min exhibits the lowest carbon and oxygen content andgenerates the strongest photoluminescence. The sample C grownfor 20 min shows a bit lower concentrations of boron and nitrogen

Fig. 5. TEM images of samples A, B, D and E. The insets are the corresponding SAED patterns of samples A, B, D and E. Samples A, B, D and E are composed of nanowires, nanosheetsand nanorods. SAED patterns in the insets show the presence of crystalline structure. (A color version of this figure can be viewed online.)

B.B. Wang et al. / Carbon 109 (2016) 352e362358

and demonstrates weaker photoluminescence. We recall here thatall above samples were grown in the 50/50 hydrogen-nitrogenmixture, whereas the increase of hydrogen flow by even 10 sccmleads to drastic changes in the composition and morphology. Thenanostructures grown in the 60/50 hydrogen-nitrogen mixture(sample D) contain more CN compound than boron nitrogen andfeature the one-dimensional nanorod morphology with weaker PLphotoluminescence. In contrast, the nanostructures grown in the20/80 hydrogen-nitrogen mixture (sample E) also consist mostly ofCN compound with even larger oxygen content, they exhibit veryhigh crystallinity and very weak photoluminescence. The mainresults are summarized in Table 4.

4. Discussion

4.1. Formation and growth of BN and CN nanomaterials

The results indicate that the nanostructures of variousmorphology (nanowires, nanosheets, nanorods and nanotips) andvarious composition (BN, CN and oxides) were formed underdifferent conditions. Apparently, this could be related to the re-actions among precursors, as well as unique properties of theN2eH2 plasma. Here, starting from the properties of N2eH2 plasma,the formation and growth of different nanomaterials are analysed.

In the early work, Nagai et al. [46] has studied the properties ofH2eN2 plasma and found that hydrogen and nitrogen are ionized toform various ions such as Hþ, H3

þ, Nþ, N2þ, NH2

þ, NH3þ, NH4

þ, N2Hþ,

etc.With the increase of hydrogen content in the reactive gases (i.e.,H2/(H2 þ N2)), the number of Hþ and H3

þ ions gradually increases,but the density of Nþ and N2

þ ions gradually reduce; however, theNH2

þ, NH3þ, NH4

þ and N2Hþ ions reach their maximum nearby thecentre for the content of hydrogen in the reactive gases.

Due to the high temperature of filaments (z1800 �C), some ofboron carbide can be evaporated to form boron atoms by thefollowing reaction [47],

1/4 B4C (solid) / B (gas) þ 1/4 C (solid). (1)

After ignition of the N2eH2 plasma, the B4C sheets are sputteredby the ions produced by the plasma. As a result, the neutral boronand carbon atoms, as well as B4C molecules are produced. Due tothe presence of residual oxygen in the CVD chamber, boron isoxidized in the gas environment:

4B þ 3O2 / 2B2O3. (2)

However, heavy B4C molecules have low speed and hence, theyquickly deposit onto the substrate surface. On the other hand, thegold film form the molten gold nanoparticles on the hot siliconsurface [48], thus the reaction of B4C with oxygen mainly occurs onthe surfaces of gold nanoparticles:

B4C þ 4O2 / 2B2O3 þ CO2. (3)

The reaction (3) can be evidenced by the reaction of B4C with

Fig. 6. Photoluminescence spectra of the samples. Ultraviolet (UV) bands at about348e355 nm, blue bands at about 409e415 nm, and green bands at about505e526 nm are present in the spectra. Sample B features strongest PL intensity,whereas samples A and D demonstrate weak intensity, and sample E has the weakestintensity. (A color version of this figure can be viewed online.)

B.B. Wang et al. / Carbon 109 (2016) 352e362 359

H2O, i.e., B4C þ 7H2O / 2B2O3 þ 7H2 þ CO [49].

4.1.1. Formation and growth of h-BN with C and O admixturesDue to the presence of plasma, a strong electric field is formed

near the substrate [50]. Under the action of the electric field, the

Table 4Summary of the results.

Plasma on/off Sample Structure

off A Nanowireson B Vertical nanosheetson C Vertical nanosheetson D Vertical nanorodson E Vertical nanotips

NH3þ ions move toward the substrate, and the B2O3 molecules react

with the NH3þ ions to form the BN molecules [41],

B2O3 þ 2NH3þ þ 2e / 2BN þ 3H2O. (4)

The BN molecules serve as the nucleation centres of the BNnanosheets on the surfaces of gold nanoparticles. The BNmoleculesformed in the gas environment deposit on the gold nanoparticlesand diffuse along their surfaces without any reaction with inertgold. Then the BN molecules coalesce on the surfaces of goldnanoparticles and nucleate the BN nanosheets. Along with this, thecarbon and boron atoms, as well as B2O3 molecules, NH3

þ ions andother atoms, molecules and ions deposit on the surfaces of goldnanoparticles and diffuse toward the edges of growing BN nano-sheets [51]. Once the B2O3 molecules collide with the NH3

þ ions inthe process of diffusion, they form the BN molecules according toEq. (4). When these BN molecules reach the edges of nanosheets,they bond to the activated edges [52], thus sustaining continuousgrowth.

However, the B2O3 molecules cannot react with NH3þ ions

completely, thus there are a small number of B2O3 molecules tobond to the edges of BN nanosheets. Simultaneously, some carbonatoms can also bond to the edges of BN nanosheets, and these re-actions are the possible reasons why the samples B and C containcarbon and oxide phases.

The incorporation of the carbon and oxygen atoms as thehetero-atoms in the BN nanosheets during their growth leads to astress in the BN nanosheets. As a result, the growth of BN nano-sheets on the surfaces of gold nanoparticles switches to the upwardgrowth, which releases the stress accumulated within the nano-sheets and results in the formation of the vertical BN nanosheets[8].

The further growth proceeds as follows. After formation ofvertical nanosheets, the distribution of plasma-generated electricfield near the substrate changes, i.e., the electric field focuses at theupper edges of vertical nanosheets [53]. Under the action of theelectric field, most of NH3

þ ions move to the upper edges of verticalnanosheets, thus the reaction (4) is accelerated on the upper edges.As a result, the vertical nanosheets grow with a high rate along thedirection of electric field. Simultaneously, some electric field linesterminate on the side faces of the vertical nanosheets [53], thussome NH3

þ ions move toward the side faces and form the BN mol-ecules on them. Due to the stress in the BN nanosheets, some de-fects caused by the stress serve as nucleation sites for the newnanosheets [8], and hence, the new BN layers are formed on sidesurfaces of the nanosheets. The NBmolecules formed on the surfaceof silicon substrate diffuse toward the edges of new BN layers underthe action of electric field, and contribute to the growth of the newBN layers [52]. The electric field on the side surfaces of verticalnanosheets is weak compared to that of the upper edges, so thereaction rate of NH3

þ ions with B2O3 molecules on the side surfacesis lower than that on the edges. Eventually, the vertically-alignedBN nanosheets are formed on the substrate, as shown in Fig. 1.

From the reaction (4) one can see that the NH3þ ions play an

important role in the formation of BN nanosheets. The results ofNagai et al. indicate that the highest density of NH3

þ ions in the

Materials produced PL

Oxide phases Weakh-BN with C and oxide phases Strongesth-BN with C and oxide phases StrongCeN with B and oxide phases WeakOxide phases Very weak

B.B. Wang et al. / Carbon 109 (2016) 352e362360

N2eH2 radiofrequency discharge plasma is reached when thehydrogen content in the reaction gases is about 70% [46]. Duringfabrication of the samples B and C, the gas flows of 50 sccm for H2and 50 sccm for N2 were used (see Table 1). Under this condition,the NH3

þ ions may have the maximum density since we used theplasma produced by DC glow discharge in our experiment. It is theprobable reason why the samples B and C are mostly composed ofthe NB nanosheets.

According to the reactions (2)e(4), oxygen is spent in the for-mation of BN molecules, thus the oxygen content in the BN nano-sheet samples is small, as confirmed by the XPS results.

Apparently, some carbon, boron and nitrogen atoms formed bynitrogen ions can be dissolved in the gold nanoparticles at theinitial stages of the BN nanosheet growth [48]. However, the for-mation of BN nanosheets inhibits the further dissolution of thecarbon, nitrogen and boron atoms in the gold nanoparticles, i.e., thesaturation is not reached. As a consequence, the carbon, nitrogenand boron atoms cannot precipitate from the gold nanoparticles toform other nanomaterials such as nanorods and nanowires.

4.1.2. Formation and growth of CN nanorods and nanotips with Band O admixtures

When the hydrogen content in the reaction gases deviates from50%, it is possible that the NH3

þ ions are reduced as shown by Nagaiet al. [46]. As a result, the formation rate of the BN molecules de-creases, so the BN nanosheets cannot be formed on the surfaces ofgold nanoparticles, and the dissolution of carbon, nitrogen andboron atoms becomes the dominant process. Here we should stressthat the nitrogen atoms are formed through the reaction of thehydrogen ions with the nitrogenous ions absorbed on the surfacesof gold nanopaticles. Due to low mass of boron atoms, the boronatoms absorbed on the surfaces of gold nanopaticles are easilydesorbed, thus only carbon and nitrogen atoms are dissolved in thegold nanoparticles. The solubility of carbon in the molten gold isvery low [54], and hence the dissolution is rapidly saturated. As aresult, the carbon and nitrogen atoms precipitate from the goldnanoparticles and eventually form the CN nanorods [48]. Duringthe growth of CN nanrods, the carbon and boron atoms, B2O3 andBN molecules and nitrogenous ions formed in the plasma depositonto the surfaces of CN nanorods. However, the boron atoms easilydesorb from the surface, and the nitrogenous ions easily react to thehydrogen ions to form the nitrogen atoms. As a result, the carbonand nitrogen atoms, as well as the B2O3 molecules deposit onto thesurfaces of CN nanorods and contribute to the radial growth of CNnanorods. Due to the incorporation of B2O3 molecules, the CNnanorods contain a small number of boron. Furthermore, the re-sidual oxygen in the CVD chamber easily bonds to carbon on thesurfaces of CN nanorods, and the CN nanorods (sample D) contain ahigh content of oxygen as evidenced by the XPS results.

The sample E is mainly composed of the CN nanotips with boronand oxygen admixtures, which may originate from the strongsputtering. We recall here that 80% of N2 was used during thegrowth of sample E, thus a large number of nitrogen ions wasproduced. As a result, the CN nanorods were strongly sputtered by anumber of nitrogen ions to form the CN nanotips [55]. In contrast,the low flow rate of nitrogen and high flow rate of hydrogenresulted in a great number of hydrogen ions during the growth ofsample D [46]. Importantly, the lightest mass of hydrogen ionsgreatly weakens the sputtering yield which significantly decreaseswith the ion mass [55], thus the sample D is composed of thenanorods. Since the gold nanoparticles were removed from the topsof CN nanorods (see Fig. 1(e)), there were more carbon atoms of theCN nanorods and hence, more oxygen atoms bond to the carbonatoms; as a result, sample E has a higher concentration of oxygenthan the sample D, and sample E could be termed as the mixed

oxide phases.

4.1.3. Formation of nanowires made of mixed oxide phasesFigs. 1(a) and 5(a) show that the nanowires made of oxide

phases (sample A) were also formed in the HFCVD system. Wesuppose that these nanowires were formed similarly to the for-mation of the CN nanorods with boron and oxide phases, i.e., thecarbon and nitrogen atoms dissolved in the molten gold nano-particles and then precipitated from gold to form the nanowires.However, the formation of carbon and boron atoms depends on theevaporation of B4C by the hot filaments, hence there was only asmall number of carbon and boron atoms in the gas environment.Furthermore, the number of nitrogen atoms is very small due to thestability of N2 molecules. On the other hand, the gold nanoparticlesformed in the process of heating are non-uniform [48], and thusonly small nanoparticles can be saturated by the dissolution of thecarbon and nitrogen atoms. As a result, the sample A is composed ofthe CN nanowires of small diameters, compared to the CN nano-rods. Due to the high surface energy of CN nanowires, the residualoxygen easily bonds to the carbon atoms on the surfaces of CNnanowires and forms the carbon oxide. Simultaneously, the B2O3

molecules deposit onto the surfaces of the CeNeB nanowires andincorporate in the nanowires, thus the nanowires contain a smallnumber of boron atoms and a great number of oxygen atoms, asevidenced by the XPS results. Thus, the sample A mainly consists ofoxide phases.

4.2. PL properties of BN and CN nanomaterials

Fig. 6 shows the multi-PL bands, and specifically, the strongestphotoluminescence intensity for sample B, somewhat weaker PLfor sample C, weak PL for samples A and D, and very weak PL forsample E. These great differences in the PL properties relate tostructure and composition of the fabricated nanostructures, andthe reasons are analysed in the following sections.

To well understand the PL generation of samples, the opticalbandgap should be obtained [56]. However, silicon is non-transparent; here the solid diffuse reflectance (SDR) spectra ofsamples were recorded from UV-3600 Plus UV-VIS-NIR spectro-photometer (see Fig. S2 in SI section). Themultipeaks present in theSDR spectra may be related to multi-components of the samplessuch as h-BN, B2O and carbon [57]. Furthermore, the appearance ofmultipeaks in the SDR spectra indicate that there are multiple ab-sorption to occur in the samples, thus the generation of multi-PLbands are related to the multi-components of the samples. Ac-cording to Kubelka-Munk equation, we have obtained the relationcurves of (Fhn)2 with photonic energy (see Fig. S3 in the SI avail-able). However, it is difficult to accurately obtain the bandgaps (thedetail discussion can be found in the SI), thus here we analyze thePL generation of samples according to the electronic structure ofBNCO nanomaterials [17].

4.2.1. PL properties of samples B and CIn Section 3 we have analysed the structure and composition of

the BN and CN nanomaterials and suggested that the samples B andC have the h-BN nanostructure with carbon and oxide phases.Zhang et al. studied electronic structure of similar nanostructuresand concluded that the nitrogen vacancy levels termed as the threeboron centre (VN3) and the one boron centre (VN1) are locatedbelow the conduction band at about 1.0 and 0.7 eV, and the levelsrelated to the carbon and oxygen dopants blow the conductionband about 4.1 and 4.5 eV, respectively [17]. According to this bandstructure, the UV PL bands at about 356 and 357 nm (~3.5 eV) canbe attributed to the transition between the VN3 level and the oxy-gen impurity level, whereas the blue PL bands at about 409 and

Table 5Summary of the PL results.

Sample UV PL band (nm) Blue PL band (nm) Green PL band (nm) Possible cause

A 355 412 526 Transition from VN3 to O levels for UV PL bands; transitionfrom VN3 to C levels for PL bands at about 409 and 410 nm;transition from p* band to LP band for PL bands at about411, 412 and 415 nm; transition between p* and p bandsfor green PL bands.

B 357 410 505C 356 409 506D 348 411 521E e 415 512

B.B. Wang et al. / Carbon 109 (2016) 352e362 361

410 nm (~3.03 eV) result from the transition between the VN3 leveland the carbon impurity level.

Concerning the green PL bands at about 479, 505 and 506 nm,Liu and Zhang et al. suggest that they result from the red shift of theUV PL band at about 356 nm caused by the increase of carbondoping [15,17]. However, in our case the PL bands at about 479, 505and 506 nm appear simultaneously in the same PL spectra togetherwith the PL band at about 356 nm; thus, the PL bands at about 479,505 and 506 nm have the mechanism differing from that of the PLband at about 356 nm. Examination of data in Table 2 evidencesthat the atomic concentrations of carbon element in sample B and Care about 10.17% and 16.5%, i.e., some carbon clusters are formed inthese samples, as confirmed by the XRD and SDR results. Due to thenon-uniformity of the clusters, they have different bandgaps [58].According to the PL mechanism of CeN nanomaterials, the green PLbands at about 479, 505 and 506 nm may be related to the tran-sition between p* and p bands of these carbon clusters [58].

Due to the formation of carbon clusters, the dangling bonds anddistorted states are formed in the h-BN nanosheets, and this causethe quenching of photoluminescence [59,60]. The data shown inTable 2 indicate that sample C has more carbon than sample B,which implies that sample C has more carbon clusters (quenchingcentres of photoluminescence) than sample B. Thus, Fig. 6 showsthat the PL intensity of the sample B is stronger than that ofsample C.

4.2.2. PL properties of samples A, D and EAccording to Table 2, the samples A, D, and E have high con-

centration of carbon relative to boron and nitrogen, thus their PLproperties should be determined by the properties of carbon ma-terials. Table 3 indicates that there is the h-BN phase in thesessamples, thus the UV PL bands at about 348 and 355 nm in samplesA and D are related to the h-BN phase, i.e., they originate from thetransition between the VN3 level and the oxygen impurity level ofthe h-BN phase. The green PL bands at about 512, 521 and 526 nmin samples A, D, E can be attributed to the transition between p*and p bands of the carbon clusters [58]. In addition, Table 3 alsoshows that there are the CeN bonds in theses samples, thus theblue PL bands at about 411, 412 and 415 nm can be attributed to thetransition between p* band and the lone pair (LP) band formed bythe nitrogen atoms [58].

Fig. 6 shows a very weak PL intensity for sample E. The reasonfor this finding is not completely clear, and possibly can be theresult of a small number of PL units. On the other hand, one can seefrom Table 2 that carbon content dominates in sample E, and thismeans that there are some large carbon clusters. The crystallinestructure of sample E indicates that the large carbon clusters haveno bandgap or very small bandgap (e.g., they are similar to gra-phene), thus only small carbon clusters can generate the obvious PLbands. Due to the formation of large carbon clusters, the smallcarbon clusters relatively reduce, i.e., the number of PL units re-duces. This is the possible reasonwhy the PL intensity of sample E isvery weak.

According to the above analyses, the generation of different PLbands can be summarized as follows: the UV PL bands originatefrom the transition from VN3 to O levels; the blue PL bands generate

the transition from VN3 to C levels or the transition from p* band toLP band; and the green PL bands are attributed to the transitionfrom p* band to p band. For clarity, the PL results are summarizedin Table 5.

5. Conclusion

In summary, the BN and CN nanomaterials of various structureand morphology were synthesized by PEHFCVD technique underdifferent conditions. The results of FESEM, XRD, TEM, micro-Ramanand XPS characterization show that the 2D h-BN nanosheets withcarbon and oxide phases are synthesized under about 1:1 ratio ofH2 to N2, while the CN nanorods and nanotips with boron and oxidephases are formed under other ratios of H2 to N2. The nanowires ofoxide phases are grown in the neutral environment withoutplasma. According to the characterization results, the formationand growth of different BN and CN nanomaterials can be explainedby the specific reactions of precursors and unique properties of thereactive plasmas [61,62]. The photoluminescence properties of thesynthesized BN and CN nanomaterials were also studied. The BNand CN nanomaterials can generate the ultraviolet, blue and greenmulti-photoluminescence bands, which originate from the transi-tion between the VN3 level and the oxygen impurity level, transitionbetween the VN3 level and the carbon impurity level, transitionbetween p* band and the LP band of carbon clusters, and transitionbetween p* and p bands, respectively. These outcomes are prom-ising to effectively control the structure of BN or CN nanomaterialsfor the applications in the development of BN- and carbon-basedoptoelectronic, energy conversion, and other nanodevices [63].

Acknowledgments

This work was partially supported by CSIRO's OCE ScienceLeadership Scheme and the Australian Research Council. I.L. ac-knowledges the support from the School of Chemistry, Physics andMechanical Engineering, Science and Engineering Faculty,Queensland University of Technology.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.carbon.2016.08.029.

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