Deposition of detonation nanodiamonds by Langmuir–Blodgett technique

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Abstract

Langmuir–Blodgett technique, which classically allows preparing monomolecular films, was here used to deposit a film of detonation nanodiamond particles.

We proceeded to the functionalization of the nanodiamond (nD) particles so as to obtain hydrophobic nanodiamonds. Cetyltrimethylammonium chloride (C19H42ClN) (CTAC) was used for this purpose in order to form an ionic complex ND–COO(NH3)+–R with the functional groups of the nanodiamonds.

Compressions with various strengths (10 to 30 mN/m) were performed on the Langmuir–Blodgett device in order to prepare different types of deposits on mica substrates. Atomic Force Microscopy was used to characterize these deposits. It was shown that compressions with low intensity result in discontinuous distributions of the particles on the surface. Conversely, very dense and continuous deposits were observed for higher compression strengths. By optimizing the nD/CTAC ratio in the suspension, a very regular deposit with a monoparticle height was obtained.

Introduction

Detonation nanodiamonds have become a material of great interest these last years. Numerous applications are aimed for these nanoparticles, the most cited are biological applications [1], reinforcement of materials [2], seeding for Chemical Vapour Deposition (CVD) growth [3].

Some of them need the preparation of well defined nanodiamond deposits. For instance, CVD growth requires a well defined surface roughness and a controlled thickness of nanocrystalline diamond films. The surface density of nanodiamonds on substrates has a direct influence on these two characteristics.

Different methods are already used in order to obtain these kinds of deposits [4], [5]. The most widespread technique, which allows obtaining the highest nucleation sites density (up to 1010 particles/cm2) is the seeding by ultrasonic treatment in a suspension of nanodiamond in an appropriate liquid depending on the nature of the substrate. However, the main problem with this technique is that the nanodiamond colloids tend to aggregate into clusters that are subsequently found in the deposits. Williams et al. [6] worked with very well dispersed nanodiamonds by using the wet milling with zirconia micro-beads under fast rotation which was developed by Osawa [7]. They obtained a regular deposit with a nucleation sites density higher than 1011 sites/cm2. However, there is still more than one order of magnitude between this result and the highest theoretical possible density of 4 × 1012 particles/cm2 for particle with a diameter of 5 nm. In addition, the use of zirconia microbeads to obtain well dispersed nanodiamonds may introduce some zirconia impurities in the deposit.

Langmuir–Blodgett technique, which classically allows preparing monomolecular films [8], [9], can also be used to deposit a monolayer film of particles [10], [11].

Kim et al. used this technique in order to obtain well oriented BaCrO4 nanorods monolayer films [10]. Paul et al. obtained a very dense and well organized monolayer of 10 nm gold nanoparticles by Langmuir–Blodgett [11].

This work reports the first attempt to apply the Langmuir–Blodgett method for the deposition of detonation nanodiamonds in order to obtain very well ordered films of single nanodiamond particles with a very high density. The study of the influence of the surface pressure on the morphology and the density of the deposit was carried out.

Section snippets

Experimental

Nanodiamonds with a size between 2–8 nm were synthesized at the ISL by the detonation process of hexogen/trinitrotoluene charges 30/70. The detonation soot was treated by using a 25/75 wt.% mixture of hydrofluoric (HF, 40 wt.%) and nitric (fuming HNO3) acids to remove metallic particles coming from the walls of the tank in which the explosive charge is fired. To eliminate sp2 carbon species such as graphite and amorphous carbon, an oxidation treatment was carried out under air at 420 °C in a muffle

Results and discussion

The isotherm curve obtained from the compression of the nanodiamonds deposited at the surface of the water on the Langmuir–Blodgett trough is shown in Fig. 1.

Depending on the isotherm shape, the constant surface pressure to apply for the deposition of the particles layers have to be carefully chosen to ensure sufficient cohesion in the monolayer, i.e. the attraction between the particles in the monolayer must be high enough so that the monolayer does not break during the transfer on the solid

Conclusions

Nanodiamond particles functionalized with CTAC were used to obtain Langmuir–Blodgett films, the nanodiamond particles were grafted by ionic bond with CTA+ ions on the nanodiamond carboxylic groups. For this first attempt to use this technique for detonation nanodiamonds, compressions with various strengths (10 to 30 mN/m) were performed on the Langmuir–Blodgett device in order to prepare different types of deposits on mica substrates. It was shown that compressions with low intensity result in

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