Nanometer-scale ordered arrangement of diamond nanoparticles on substrates via electrostatic deposition

Diamond nanoparticles (DNPs) are exciting candidates for a diverse array of applications ranging from lubrication, ¹⁾ qubits in quantum computing, ^2,3) semiconductor quantum dots for biomedical imaging, ^4,5) nanoscale magnetic sensors, ^6–8) drug delivery, ^9–13) through globular protein mimics ¹⁴⁾ and reflectors for low-energy neutrons, ¹⁵⁾ to nucleation sites for chemical vapor deposition of diamond films on non-diamond substrates. ^16,17) Any of these applications can be categorized into two groups; the applications using DNP suspensions and the applications based on solid-state substrate-supported DNP systems. The suspension-based applications have significantly progressed toward their practical realization over the last few decades. This must be owing to the thorough technology development on material preparation including disintegration of stubborn aggregates and/or mono-dispersion via surface graphitization and oxidation, ¹⁸⁾ insertion of surfactants into suspending mediums, ¹⁹⁾ stirred-media milling with microbeads of ceramic ²⁰⁾ or zirconia, ^21,22) high-temperature annealing in hydrogen gas, ²³⁾ and surface functionalization with specific molecules. ²⁴⁾ Meanwhile, exploring of the substrate-based applications has been hampered by the inherent technical difficulty of placing DNPs on any substrate surfaces with desired densities and locations at a fine length scale. The only approach currently available to address this issue appears to be the inkjet printing of DNP suspensions, enabling deposition of DNPs on substrates with a designed geometry at a length scale of, in the finest case, some tens of microns. ^25,26)

In this letter, we demonstrate a nanometer-scale ordered arrangement of DNPs on SiO₂ surfaces via the technique of electrostatic deposition, which is, in some cases, called as electrostatic self-assembly or seeding. ^16,17,27) SiO₂ was chosen for surfaces to place DNPs as it is the most commonly used transparent substrate material for DNP single-photon emitters and also the omnipresent natural oxide of all Si wafers. The signs of surface charges, i.e. zeta potentials, of SiO₂ surfaces were locally inversed through a combination of electron beam lithography (EBL) and surface functionalization with 3-aminopropyltriethoxysilane (APTES). As a result, selective electrostatic deposition of DNPs onto the substrates was successfully performed with line-and-space and dot array patterns at a length scale of ≥100 nm.

In order to locally invert the zeta potential of SiO₂ surfaces, formation of patterned resist layers followed by vapor-phase deposition of APTES was carried out as described in Fig. 1. First, a positive resist (ZEP-520A7, Zeon, Japan) diluted to 50% of the initial concentration by adding an anisole solvent was spin-coated onto RCA-SC1-cleaned SiO₂ surfaces to form resist layers with a thickness of 100 nm. Second, EBL was done with 2000–100 nm pitch line-and-space and dot array patterns using a point-electron-beam system (JBX-9300FS, JEOL, Japan). Next, the samples were developed in a solution of pentyl acetate for 5 min and dried with a nitrogen gas flow. Here, a SiO₂ surface untreated with the above-described patterning process was also prepared for reference. All the SiO₂ surfaces (with and without the patterned resist layers) were exposed in the APTES vapor atmosphere for 20 h in an airtight chamber and then baked in air at 110 °C for 15 min, allowing APTES molecules covalently attached with silanol groups on the SiO₂ surfaces [see Fig. 1(b)]. ²⁸⁾ Afterward, the samples were immersed in a toluene solution for 5 min to remove the patterned resist layers. Finally, the samples were rinsed with acetone and pure water and spin-dried. The above-described surface functionalization with APTES was performed also on silica nanospheres (SiO₂MS-2.0 0.166 μm–1 g, Cospheric, USA) to be dispersed in water and investigated in terms of zeta potential using a dynamic light scattering apparatus (Zetasizer Nano ZS, Malvern Instruments, UK). For the electrostatic deposition of DNPs onto the obtained SiO₂ surfaces, two types of 1 wt% aqueous colloidal suspensions of DNPs synthesized via the detonation in-house ²⁹⁾ (DINNOVARE™: ζ+NDs in water and ζ−NDs in water, Daicel, Japan), having positive and negative zeta potentials as a result of hydrogen and air annealing, respectively, were used (the pH dependence of their zeta potentials is shown afterwards in Fig. 2). We label these materials as "ζ+NDs" and "ζ−NDs" hereafter. To perform the electrostatic deposition, all the prepared substrates (SiO₂ surfaces) were immersed in ζ+NDs or ζ−NDs for 5 min, rendering the DNPs, when the DNPs and the SiO₂ surfaces have opposite signs for their zeta potentials, attached onto the SiO₂ surfaces due to the Derjaguin–Landau–Verwey–Overbeek interaction. ³⁰⁾ The substrates were subsequently rinsed in pure water to wash unwanted large aggregates of DNPs away and then blown dry under nitrogen gas. To observe the DNPs placed on the substrates, the SiO₂ surfaces were characterized by an atomic force microscope (AFM) (Dimension Icon, Bruker, USA) in noncontact (Peak Force QNM) mode with a silicon nitride AFM tip (SCANASYST-AIR, Bruker, USA) of <12 nm tip apex, 70 kHz tapping frequency, and 0.4 N m⁻¹ spring constant.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The zeta potentials of ζ+NDs, ζ−NDs, as-received silica nanospheres, and APTES-attached silica nanospheres dispersed in water are shown in Fig. 2 as a function of pH. As mentioned in the previous paragraph, ζ+NDs and ζ−NDs, respectively, show positive and negative zeta potentials in a wide pH range. The positive zeta potential for ζ+NDs, obtained by high-temperature (600 °C) annealing of DNPs in hydrogen atmosphere, which makes the DNPs hydrogen-terminated, is ascribed to the interaction of electrons in the DNPs with hydronium ions in the water. ²³⁾ The negative zeta potential for ζ−NDs and its downward trend with increasing pH are explained by the de-protonation of the carboxy and hydroxy groups present at the air-annealed DNP surfaces. ²³⁾ Meanwhile, the as-received silica nanospheres exhibit a negative zeta potential over the whole examined pH range as previously reported in the literature. ³¹⁾ For the APTES-attached silica nanospheres, such an originally negative zeta potential is inverted to positive in an acidic pH range presumably due to the amino group with a positive charge induced via the vapor-phase deposition of APTES. Most importantly, it can be visually judged from Fig. 2 that, for each combination between DNPs and SiO₂ surfaces, the electrostatic repulsive/attractive interaction can be maximized when the pH is around 6.0 (indicated with a dashed black line in the figure). It should be stated here that the electrostatic interaction force between a sphere and a planar surface in an aqueous suspension F is obtained by: ³²⁾

$$\begin{eqnarray}&&F=64\pi {\varepsilon }_{0}{\varepsilon }_{{\rm{r}}}{\left(\frac{{k}_{{\rm{B}}}T}{ze}\right)}^{2}\,\tanh \left(\frac{ze{\psi }_{{\rm{s}}}}{{k}_{{\rm{B}}}T}\right)\tanh \left(\frac{ze{\psi }_{{\rm{p}}}}{{k}_{{\rm{B}}}T}\right)r\kappa {e}^{-\kappa h},\end{eqnarray} \tag{ 1 }$$

where ${\varepsilon }_{0}$ is the electric constant, ${\varepsilon }_{{\rm{r}}}$ is the relative permittivity of the suspending medium, ${k}_{{\rm{B}}}$ is the Boltzmann constant, T is the temperature, z is the valence of the ionic species, e is the elementary charge, ${\psi }_{{\rm{s}}}$ and ${\psi }_{{\rm{p}}}$ are the surface potentials (or zeta potentials) of the sphere and the planar surface, respectively, $\kappa$ is the Debye–Hückel parameter, and h is the separation distance. These electrostatic interactions were examined from the perspective of electrostatic deposition of DNPs onto non-patterned SiO₂ surfaces at pH ≅ 6.0 and the AFM results are shown in Fig. 3. In accordance with the zeta potentials plotted in Fig. 2, the DNPs are densely deposited onto the SiO₂ surfaces only when the electrostatic attractive interaction works between them: completely no particle deposition is observed when the DNPs and the SiO₂ surfaces have the same sign of zeta potentials. This phenomenon was directly applied to the selective electrostatic deposition of DNPs onto the patterned SiO₂ surfaces prepared through the aforementioned procedure and the representative AFM height images are now shown in Fig. 4. For both line-and-space and dot array patterns, the DNPs are selectively deposited only on the SiO₂ surface areas where the electrostatic attractive interaction works even at a pitch of 100 nm, which is the finest length scale among the examined pattern designs. Although the deposited DNPs appear to be slightly aggregated as judged from the AFM height images, such aggregates would be disintegrated by precisely adjusting the ionic strength of suspensions (ζ+NDs and ζ−NDs) as previously reported by the author. ³⁰⁾ Considering these results, it can now be stated that the concept of local zeta potential inversion for SiO₂ surfaces by the combination of EBL and surface functionalization with APTES works for the nanometer-scale ordered arrangement of DNPs on Si-related substrates via electrostatic deposition.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

In summary, the present study has demonstrated the nanometer-scale ordered arrangement of DNPs on SiO₂ surfaces via electrostatic deposition. Using the lift-off process combining EBL and surface functionalization with APTES, the originally negative zeta potential of SiO₂ surfaces was locally inversed to positive. Consequently, the DNPs were deposited only on the limited SiO₂ surface areas with ≥100 nm pitch line-and-space and dot array patterns where the electrostatic attractive interaction works. This achievement would surely contribute to manufacturing smaller diamond-based micro/nano electromechanical system (MEMS/NEMS) devices with higher operating frequencies. ²⁶⁾ As an ultimate goal of this technique, in addition, single DNPs separately placed on a substrate and arranged at a <20–30 nm separation distance, required for diamond-based quantum computing due to the cut off distance of quantum entanglement between single NV center-containing DNPs, ³³⁾ might become accessible depending on the future development of lithography technology. Furthermore, since this approach does not rely on any specific material property other than zeta potential, it would be applicable to any material combination of charged nanoparticles and substrates. It can therefore be expected that the present work contributes to, for example, further miniaturization of silver nanoparticle-based flexible circuits or electrodes. ³⁴⁾

Acknowledgments