Laser Chemical Vapor Synthesis (LCVS) of the BN-Nano-Structured Materials using Borazine Decomposition by Radiation of Fundamental and Second Harmonic Mixture of the YAG-Laser.

Arturo Hidalgo,1 Vladimir I. Makarov,2 Dachun Huang,2 Gerardo Morel,1 Brad R. Weiner.2

1Department of Physics, University of Puerto Rico, San Juan, PR, USA1Department of Chemistry, University of Puerto Rico, San Juan, PR, USA

Experimental Procedure

Conclusions

Results

Acknowledgments

Introduction

 In the past decade, high interest has generated in the synthesis of nitride nano-structured materials [1-3]. This interest is based on unique physical and chemical properties of such materials. The materials of interest can be used as very stable coatings, in microelectronics, as well as sensors to detect different chemical compounds and photo-sensors for different spectral ranges. Special interest has been created in the boron-nitride and carbon-nitride nano-structured material, because they have very high thermal and mechanical stability. Development of new methods for synthesis of such material with high yield of the final product is the goal of many researches [4-6]. In the present study, we concentrate our attention to synthesis of the BN-nano-structured materials.

The BN nano-structured materials can be represented by the cubic-nano-structured material, hexagonal-nano-structured material, nano-fibers and nano-tubes. Formation of the respective modifications of the BN-nano-structured material is dependent of the synthesis methods applied to material obtaining, synthesis conditions. It is well known that the cubic nano-structured BN material is close to diamond, and it has higher sensitivity to volume doping by the electro active admixtures of both positive and negative signs. The hexagonal nano-structured BN material is close to graphite, and it is electro conductive in direction of the hexagonal structured nano-net. Electric properties both of cubic and hexagonal BN nano-structured materials have been investigated [7,8]. Since the considered BN nano-structured materials can be interesting for practical application in electronics as well as the stable anticorrosive coatings, very high interest has been created in deposition of both cubic and hexagonal nano-structured BN thin films on different surfaces. For deposition of the BN-nano-structured thin films, the Laser Ablation (LA) method has been applied [9-12]. We will not discuss this method in detail here, as the goal of the present study is related to synthesis and characterization of the BN-nano-structured materials using borazine compound and Laser Chemical Vapor Synthesis (LCVS) method.

Abstract

We report a new method of BN-nanotube synthesis using Laser Chemical Vapor Decomposition (LCVD) of borazine by simultaneous radiation of both the fundamental and second YAG laser harmonics. It was found that yield of the nano-structured BN material is about 30%. The synthesis has been explained using simplest phenomenological chemical kinetics model, which includes degenerated chain reaction mechanism. Average concentration of active chemical species per single laser short was estimated as well as the average effective rate constant of the interaction of these species with borazine molecule was also estimated.

This research project is being carried out under the auspices of:

• Institute for Functional Nanomaterials (NSF Grant No. 0701525).

• NASA Training Grant NNG05GG78H (PR Space Grant), NASA Cooperative Agreement NCC5-595 (PR NASA EPSCoR)

•NASA Grant NCC3-1034 (NASA CNM URC).

The used reactor schematically represented in Figure 1. The reactor body comprises of a quartz tube with an external diameter of 1.5” and wall thickness of 0.04”. This tube was installed on stainless steel frames (O-ring seal) and connected to vacuum system. In input side of reactor, the quartz lens with focus length of 10 cm was installed (O-ring seal). Laser radiation was focused by this lens in the reactor center. Total reactor volume is about 211.4 cm3. Reactor was evacuated by mechanical pump to rest pressure of about 10-2 Torr, and then it was evacuated by turbo-pump (Varian-Turbo V-550) to the rest pressure of about 10-6 Torr. Reactor was filled with high purity nitrogen to atmospheric pressure, and evacuated to the rest pressure 10-6 Torr. The last process was repeated two or three times. Then reactor was filled with borazine vapor (Boro Science, Canada, Inc.) to a pressure of about 68.4 Torr. The borazine pressure was controlled by a pressure gauge (Vacuum Instruments, MKS-Instruments, MPS Products, A-900) during the synthesis.

Time (s)

790 800 810 820 830 840 850 860 870

Rel

ativ

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tens

ity

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0.0

0.5

1.0

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3.5Borazine photoionization induced by 193 nm radiation; Ar + Borazine (1 %) mixture

Figure 1. Schematic representation of the used reactor.

Figure 2. Time dependence of pressure evolution during borazine LCV decomposition at initial borazine pressure 68.4 Torr, fundamental YAG laser harmonic radiation energy density of 8.9 J/cm2 and second harmonic radiation energy density 0.051 J/cm2, laser repetition rate of 10Hz.

Figure 3. TOFMS of borazine measured for mixture of Ar + Borazine (1 %) at input mixture pressure of 760 Torr. The studied mixture was photoionized by radiation of the ArF laser (193 nm; 3 mJ/pulse; beam profile - 22 mm2; beam was focused in the center of the TOFMS ionization area by quartz lens; profile of the focused laser beam is 0.30.3 mm2).

Figure 4. Raman spectrum of the BN-nano-structured material obtained during the LCVS from borazine vapor at 68.4 Torr initial pressure of sample.

Figure 5. FTIR spectrum of the BN-nano-structured material obtained during the LCVS from borazine vapor at 68.4 Torr initial pressure of sample.

Figure 6. X-Ray spectra of the BN-nano-structured material obtained during the LCVS from borazine vapor at 68.4 Torr initial pressure of sample and BN-commercial powder measured at the same experimental conditions.

Figure 7. (a) SEM-image (resolution 10 m/grid) and (b) TEM-image (100 nm/grid) of the BN-nano-structured material obtained during the LCVS from borazine vapor at 68.4 Torr initial pressure of sample.

A new method of LCVS of BN-nano-tubes with high yield of final product was developed. We found hBN and cBN structures show in FTIR and Raman spectroscopy. Synthesis was carried out using two radiations (fundamental and second harmonic YAG laser radiation) excitation of borazine vapor. It was proposed that the laser radiation induces the primary processes of formation of electronically excited borazine molecules that subsequently undergo photo-induced dissociation to B3N3H5 and H fragments. The degenerated chain “dark” reactions of these

radicals determine the BN-nano-structured material formation.

200 400 600 800 1000 1200 1400 1600

cBN LO

803

1356

Powder Po=68.4 Torr

Inte

nsity

(a. u

.)

Raman shift (cm-1)

hBN

hBN

cBN TO

500 1000 1500 2000 2500 3000 3500 40000.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1068

1365

802.23

cBN

Tran

smis

sion

(a. u

.)

Energy (cm-1)

hBN

hBN

Po = 68.4 Torr

20 30 40 50 60 70 80

hBN

110

hBN

104

hBN

103

hBN

004

hBN

102

hBN

101

hBN

100

BN Nano Structured Material

Powder commercial 99.5 % hexagonal

Inte

nsity

(a. u

.)

2 (Degree)

hBN

002

Po = 68.4 Torr

0 10 20 30 40 50 60

100

120

140

160

mass BN / mass borazina

Pre

ssur

e m

Bar

Time (min)