Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color

Oct 13, 2017 - Color centers in diamonds have shown excellent potential for applications ..... These peaks originate from free OH bond stretching and ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38842-38853

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Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color Centers via Seeding by 2 nm Detonation Nanodiamonds Stepan Stehlik,*,† Marian Varga,† Pavla Stenclova,† Lukas Ondic,† Martin Ledinsky,† Jiri Pangrac,† Ondrej Vanek,‡ Jan Lipov,§ Alexander Kromka,† and Bohuslav Rezek†,∥ †

Institute of Physics ASCR, Cukrovarnická 10, Prague 16200, Czech Republic Department of Biochemistry, Faculty of Science, Charles University, Hlavova 2030/8, Prague 12840, Czech Republic § Department of Biochemistry and Microbiology, University of Chemistry and Technology, Technická 3, Prague 16628, Czech Republic ∥ Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague 16627, Czech Republic

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‡

S Supporting Information *

ABSTRACT: Color centers in diamonds have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report chemical vapor deposition (CVD) growth of nanocrystalline diamond (NCD) films as thin as 5−6 nm with photoluminescence (PL) from silicon-vacancy (SiV) centers at 739 nm. Instead of conventional 4−6 nm detonation nanodiamonds (DNDs), we prepared and employed hydrogenated 2 nm DNDs (zeta potential = +36 mV) to form extremely dense (∼1.3 × 1013 cm−2), thin (2 ± 1 nm), and smooth (RMS roughness < 0.8 nm) nucleation layers on an Si/SiOx substrate, which enabled the CVD growth of such ultrathin NCD films in two different and complementary microwave (MW) CVD systems: (i) focused MW plasma with an ellipsoidal cavity resonator and (ii) pulsed MW plasma with a linear antenna arrangement. Analytical ultracentrifuge, infrared and Raman spectroscopies, atomic force microscopy, and scanning electron microscopy are used for detailed characterization of the 2 nm H-DNDs and the nucleation layer as well as the ultrathin NCD films. We also demonstrate on/off switching of the SiV center PL in the NCD films thinner than 10 nm, which is achieved by changing their surface chemistry. KEYWORDS: detonation nanodiamond, surface chemistry, hydrogenation, zeta potential, nucleation density, nanocrystalline diamond, SiV center

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Nanocrystalline diamond (NCD) film is a suitable material for hosting a high concentration of SiV centers, because they can be incorporated into the diamond lattice directly during the microwave plasma-assisted chemical vapor deposition (MWPCVD) process either directly from an Si substrate or from a piece of Si placed to the reaction plasma during the NCD growth.7,8 In order to bring the SiV centers close to the surface for full utilization of their sensing potential, fabrication of, hypothetically, as thin as possible of an NCD layer is essential. If a thicker diamond layer is for some reason needed (i.e., for efficient heat transfer), then growing first a thick layer under conditions that minimize the presence of SiV centers and, second, growing an ultrathin “sensing” layer with SiV centers on the top seems reasonable. However, for the growth of ultrathin continuous NCD films with SiV on nondiamond substrates, a high nucleation density of diamond is essential; the

INTRODUCTION In recent years, color centers in diamonds have shown excellent potential for applications in quantum information processing, photonics, and biology. The most extensively studied are, without any doubt, nitrogen-vacancy (NV) centers, but recently, also silicon-vacancy (SiV) centers have demonstrated their potential.1−4 In comparison to the NV center with a broad emission spectrum at room temperature, the SiV center offers a great advantage in its narrow room-temperature zero-phononline (ZPL) at around 738 nm, in which 70% of its photoluminescence (PL) is concentrated.5 The narrow emission of the SiV center at room temperature can be interesting also for optical sensing applications. It was shown that the SiV centers are sensitive to the surface chemistry of the diamonds or diamond nanoparticles in which they are incorporated,6 but it is not yet elucidated to which depth the SiV PL can be influenced by surface chemical groups. Here, the high density of the SiV centers located in the vicinity of the diamond surface is crucial to have a good response of a sensor to a change of the surrounding environment. © 2017 American Chemical Society

Received: September 22, 2017 Accepted: October 13, 2017 Published: October 13, 2017 38842

DOI: 10.1021/acsami.7b14436 ACS Appl. Mater. Interfaces 2017, 9, 38842−38853

Research Article

ACS Applied Materials & Interfaces

Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. We show that 2 nm hydrogenated DNDs can be employed to form extremely thin and smooth nucleation layers on Si/SiOx substrates, with the highest recorded nucleation density (∼1.3 × 1013 cm−2) estimated from SEM images. Such seeded substrates enable to grow continuous NCD films as thin as 5−6 nm with embedded optically active SiV centers whose PL is sensitive to the NCD film surface chemistry.

higher the density of the nucleation centers, the lower the thickness of a continuous diamond film can be achieved. Here, detonation nanodiamonds9 (DNDs) with their typical size of 4−6 nm and narrow size distribution are often used to form a seeding layer on a substrate where individual DNDs serve as nucleation centers for subsequent CVD growth of diamond films. DNDs consist of a rigid diamond core and a chemically active surface that can host various functional groups. Rich DND surface chemistry enables formation of DNDs with both negative and positive zeta potentials (ZP) in water.10 The former is usually achieved by oxidation in acids or in ambient air atmosphere,11 while the latter is typically reached by hydrogenation either in hydrogen plasma12 or hydrogen gas at elevated temperatures.13,14 This variability of the DND surface charge offers broad possibilities to form a high density seeding layer on many materials by an electrostatic interaction between oppositely charged DNDs and a substrate.15−17 The density of the nucleation routinely achieved by using DNDs is in the order of 1011 DND particles per cm2. There are several other seeding techniques like bias enhanced nucleation,18 ultrasonic treatment of various particles,19 or use of chemical precursors,20 but none of these techniques currently provide a sufficiently high nucleation density to enable growth of continuous sub-30 nm diamond films. Silicon covered by a native SiOx or other forms of SiO221 including SiO2 spheres22 and fibers23 used as substrates are frequently seeded by hydrogen-terminated DNDs (H-DNDs) with positive ZP in water-based colloidal dispersions. In this case, the electrostatic interaction between the negatively charged SiO2 substrate and the positively charged H-DNDs leads to a nucleation density of 8 × 1011 cm−2 and a root-meansquare (RMS) surface roughness of ∼2 nm at the optimized pH of the H-DND colloid.17 Such nucleation density still corresponds to only 10% coverage assuming 4 nm particles and a monolayer-like assembly. Recently, Yoshikawa et al.24 used an electrolyte (KCl) addition to an H-DND colloid in order to intentionally weaken its ZP. This resulted in a higher screening effect between individual H-DNDs and subsequent formation of very dense and flat nucleation monolayers composed of single 4−6 nm DND particles with an RMS of 1.7 nm on Si substrate; however, the nucleation density was not reported. In the two above-mentioned approaches, the seeding process was carefully optimized, and it may appear that the nucleation density cannot be further increased using conventional 4−6 nm DNDs. In principle, the nucleation density could be further increased by using nanodiamond particles that are smaller than conventional 4−6 nm DNDs, e.g. with mean size around 2 nm or lower. Such small nanodiamonds currently attract increasing attention since they are promising not only for growth of ultrathin NCD films but also for exploration of quantum phenomena in diamonds, biomedical detection, and drug delivery.3 However, such nanodiamonds have not been available so far in a reasonable amount despite several topdown25,26 and bottom-up attempts.27 In this work, we utilized our recently developed high-yield technique, which provided DNDs with a mean size around 2 nm (volumetric distribution) by means of controllable size reduction of conventional 4−6 nm DNDs via oxidative etching in air.28,29 We performed hydrogenation of these 2 nm DNDs by annealing in hydrogen. We characterized the DNDs’ size distributions in colloidal dispersions depending on various stages of processing. The surface chemistry and structure of the 2 nm DNDs before and after hydrogenation were studied by

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EXPERIMENTAL SECTION

As a reference, in terms of size and material quality for the DND material, we used a NanoAmando aqueous dispersion (concentration of the stock solution 5.0 wt %, median diamond grain size of 4.8 ± 0.6 nm (98.8 wt %), particle density of 288 × 1015 particles per 1 mL). The stock colloid was diluted by deionized water in the ratio of 1:40. For preparation of the 2 nm DNDs, we used DND powder from New Metals and Chemicals, which consists of aggregated 4−6 nm DND particles. The size reduction via air oxidation was carried out at 520 °C for 50 min, providing oxidized DND 520/50. The details of this process can be found elsewhere.29 The hydrogenation of the sizereduced oxidized DND 520/50 took place in a quartz chamber at 600 °C for 6 h at atmospheric pressure of pure hydrogen gas, providing hydrogenated DND 520/50-H. Colloidal dispersions of both DND 520/50 and DND 520/50-H were prepared from 10 mg of corresponding DND powder and 2 mL of laboratory grade demi (DI) water using an ultrasonic probe (Hielscher UP200S, f = 24 kHz) at the power of 200 W, lasting 1 h. Such prepared colloidal dispersions were centrifuged at 14 000g for 3 h (Eppendorf Mini plus) to eliminate as much of the DND aggregates as possible. After centrifugation, 1 mL of supernatant was carefully separated by a micropipette. The estimated concentration is 1 mg/mL. Alternatively, the colloidal dispersions were centrifuged at 14 000g for 1 h, and resulting supernatants were further ultracentrifuged at 250 000g for 1 h (Optima Ultra, Beckman Coulter). Again, 1 mL of the supernatant after ultracentrifugation was carefully separated by a micropipette, with the estimated concentration being 0.5 mg/mL or lower. These treatments provided final colloids that were used for size distribution analysis, FTIR, and Raman characterizations, and for the nucleation of Si substrates either as DI water or 10−3 M KCl-based colloids. As substrates, we used p-type Si covered by a native oxide, further labeled as Si/SiOx. Prior to the nucleation process, the substrates were treated in oxygen plasma (Diener ElectronicFEMTO, f = 13.56 MHz, r.f. power = 45 W, process pressure = 50 Pa, time = 1 min) to ensure their surface was clean, wettable, and free of possible hydrocarbon contaminants. The nucleation of the Si substrates in the H-DNDs colloids took place in an ultrasonic bath (Elma Transsonic T 490 DH, f = 40 kHz) for 10 min. After the nucleation treatment, the samples were rinsed by DI water or by 10−3 M KCl solution and blown by nitrogen. The size distribution of the reference DNDs as well as the oxidized DNDs 520/50 and hydrogenated DNDs 520/50-H was explored in an analytical ultracentrifuge (AUC) ProteomeLab XL-I equipped with An-50 Ti rotor (Beckman Coulter) using a sedimentation velocity experiment. Samples of DNDs in DI water were spun at 10 000− 30 000 rpm at 20 °C, and 100−200 scans with 0.003 cm spatial resolution were recorded in 2−6 min steps using absorbance and interference optics. Solvent density and viscosity values for water at 20 °C (ρ = 0.99823 g/mL; η = 1.0020 mPa·s) were used, whereas the DNDs’ partial specific volume was estimated as the inverse of the diamond density as 0.2841 mL/g. Data were analyzed with Sedfit30 using a c(s) continuous size distribution model and recalculated to Stokes hydrodynamic radii. DLS and ZP measurements were performed on a Malvern Zetasizer Nano ZS equipped with a helium−neon laser (633 nm); the scattering angle was 173°. The refractive index of bulk diamond (2.4) and the viscosity of pure water (1.0020 mPa·s) were used to convert the measured intensity/size distributions to volume/size distributions. Each sample was analyzed by five subsequent runs. 38843

DOI: 10.1021/acsami.7b14436 ACS Appl. Mater. Interfaces 2017, 9, 38842−38853

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Figure 1. AFM number (black) and volume (violet) size distribution acquired by particle analysis of AFM images for reference DNDs. (a) Size distribution data from analytical ultracentrifuge for the reference DNDs (violet), size-reduced DND 520/50 (red), and subsequently hydrogenated DND 520/50-H (solid blue) after centrifugation and DND 520/50-H after ultracentrifugation (dashed blue). The lower x axis shows raw AUC particle size data, while the upper x axis shows the AFM-corrected AUC particle size data. (b) DLS volume distributions of the size-reduced DND 520/50 (red) and subsequently hydrogenated DND 520/50-H (blue) after centrifugation. (c) The DND surface chemistry and the efficiency of hydrogenation treatment was explored by FTIR spectroscopy in a grazing-angle reflectance (GAR) arrangement. IR absorbance spectra were measured using a N2-purged Thermo Nicolet 870 spectrometer equipped with the KBr beam splitter and MCT detector cooled by liquid nitrogen. A 100−200 μL aliquot of the aqueous suspension was applied on the Au mirror by drop casting just before the GAR-FTIR measurement. HDNDs on Au mirrors were heated at 100 °C for 2 min to evaporate the bulk water.31 Optical absorbance was calculated in the standard absorbance units as A = −log(R/R0), where R is the spectrum measured with DNDs, and R0 is the reference (background) spectrum recorded using a clean Au mirror prior to the DNDs application. In all cases, the spectra represent an average of 128 scans recorded with a resolution of 4 cm−1. The morphology of nucleation layers was inspected by AFM (Ntegra Prima, NTMDT) using high aspect ratio super sharp Si tips covered by diamond-like carbon whiskers (NSG01_DLC, NTMDT) working at 150 kHz with a nominal tip radius of 1−3 nm. The AFM images were acquired in a noncontact regime (free vibrational amplitude = 1−2 nm, set point 80%) to avoid possible distracting tip−particle interactions. Resolution of the AFM images was 512 × 512 points, and the scan ranges were 200 × 200 nm2 up to 1 × 1 μm2. The nucleation density from the AFM images was estimated as the detectable number of particles in the given area. In order to reliably determine the thickness of the nucleation layers, the nucleated substrates were gently scratched by a toothpick, thus revealing the Si substrate. This also further enabled the determination of the thickness of the grown NCD layers. All the scanning electron microscopy (SEM) images of DND nucleation layers as well as NCD films were acquired at 15 kV and a magnification of 300 000× (MAIA 3, Tescan) in the regime of secondary electrons. The diamond growth was performed in two different MW CVD systems: (i) a focused MW plasma with an ellipsoidal cavity resonator (Aixtron P6) and (ii) a pulsed MW plasma with a linear antenna arrangement. The deposition conditions used for focused MW CVD were as follows: gas flow of hydrogen = 300 sccm, gas flow of methane = 15 sccm, working pressure p = 60 mbar, microwave power P = 3 kW, deposition time t = 2, 4, 6, and 8 min, and the substrate temperature ≈ 700 °C. The main source of the Si atoms were additional pieces of bare crystalline Si wafer and the nucleated substrates themselves. The deposition conditions used for linear antenna MW CVD were as follows: gas compositions of a CH4/CO2/H2 atmosphere (5:20:300 sccm); working pressure p = 0.15 mbar; microwave power of 2 × 1.7 kW; deposition time t = 2, 3, and 5 h; and the substrate temperature of 460 °C. The Raman and PL spectra were measured employing an InVia Reflex Renishaw setup. The sample was excited by a HeCd continuous wave 442 nm laser (intensity of 20 mW) focused to a spot of 1 μm on

the sample in a direction perpendicular to the sample plane using a 100× objective with NA = 0.9. The Raman and PL signals were collected via the same objective and imaged through a spectrograph on a silicon CCD camera. The Raman signal from each DND sample was accumulated for 1 min, and spectra from three different spots were added. Due to a certain PL background, the spectra were baseline corrected and normalized to the diamond peak. The acquisition time for the ultrathin NCD films was extended to 60 min for 12 and 16 nm samples and to 5 h in the case of 8 nm NCD film. PL signal from the ultrathin NCD films was accumulated for 1 min.

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RESULTS AND DISCUSSION Size Distribution of the Size-Reduced DNDs before and after Hydrogenation. In order to obtain accurate size distribution data from the AUC, we first performed AFM size analysis of the reference DNDs (AFM images not shown) in the same way as established earlier.28,29,32 Figure 1a shows the size distribution of the reference DNDs, revealing the mean size of 4 nm (black). Volume distribution (violet) calculated from the number distribution (particles are approximated by spheres) shifts the mean size to 4.7 nm and highlights the contribution from larger particles or residual core agglutinates. Both distributions were fitted by a log-normal function. The accurate and statistically relevant AFM size analysis of the reference DNDs provided a reasonable calibration for the size distribution data obtained by AUC. Both interference and absorbance detections in AUC are optical techniques based on a material contrast (absorbance or refractive index) between the analyte (DNDs) and the solvent (water); therefore, the obtained AUC data represent the mass/volume distribution rather than the number distribution.33 This is supported by a good agreement in the line shapes of the AFM volume distribution (Figure 1b, gray histogram) and the AUC size distribution of the reference DNDs (Figure 1b, violet), but the mean size provided by AUC is obviously overestimated. The conversion of sedimentation coefficient distributions to a particle size distribution highly relies on the knowledge of the density of the sedimenting particle. For very small nanoparticles, this issue could cause the conversion to become difficult, as the effective density of the particles is heavily influenced by the presence of a stabilizing shell or surface attached solvent due to DND charge stabilization, which may lead to a nonideal sedimentation behavior, thus, complicating the AUC data analysis.34 However, in our case, when comparing nanodiamonds of relatively similar size, it 38844

DOI: 10.1021/acsami.7b14436 ACS Appl. Mater. Interfaces 2017, 9, 38842−38853

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ACS Applied Materials & Interfaces

distribution can be made. Still, an obvious correlation of the DLS data with the AUC data can be found. In the case of DND 520/50-H, the DLS volumetric mean sizes fluctuates in the 0.7−1.9 nm range, which would give a slightly lower average mean size than that obtained from AUC (2 nm). On the other hand, the DLS volumetric mean sizes for the DND 520/50 are slightly higher (2.6−3.9 nm) than that obtained by AUC (2.6 nm). This considerable difference between the size of oxidized and hydrogenated DNDs in DLS data is again given by a different surface chemistry, corresponding thickness, and hydrogen bonding environment of their hydration shells38 rather than their significantly different size. Finally, it must be mentioned here that in contrast to AUC, DLS did not reveal any noticeable difference between centrifuged and ultracentrifuged DND 520/50-H samples (data not shown), and both samples provided similar DLS data. This signalizes a limited performance of the DLS to provide reliable data of ND size distribution in the sub-2 nm range. Due to the obtained size characteristics, our previous results,29 and for the sake of clarity, we further use the following labeling of the samples: 2 nm O−DNDs (DND 520/50 cen.) and 2 nm H-DNDs (DND 520/50-H cen.). Surface Chemistry and Structure of 2 nm H-DNDs. Figure 2a shows FTIR spectra obtained from the size-reduced, i.e. oxidized 2 nm O−DNDs (before hydrogenation; red) and hydrogenated 2 nm H-DNDs (blue). It is obvious that the hydrogenation leads to formation of C−H bonds (C−H

could be treated only as a systematic error occurring due to recalculation of the sedimentation coefficient to the particle size, thus, allowing for calibration of AUC data using particle size direct observation by AFM. After comparison of AFM volume distribution and AUC data of reference DNDs, we divided the original AUC particle size data by a factor of 2.6 in order to normalize it to the AFM volumetric distribution. After this step, we obtained the AFMcorrected AUC particle size distribution shown in the upper x axis in Figure 1b. The obtained volumetric mean DND sizes are then 2.6 nm for DND 520/50 after centrifugation (solid red; DND 520/50 cen.), 2.1 nm for the DND 520/50-H after centrifugation (solid blue; DND 520/50-H cen.), and 1.6 nm for the DND 520/50-H after ultracentrifugation (dashed blue; DND 520/50-H ucen.). The value for centrifuged DND 520/ 50 samples is in good agreement with our AFM analysis published previously, where the volumetric mean size of the DND 520/50 sample obtained by AFM was 2.3 nm.29 According to the AUC data, it seems that the hydrogenation of the size-reduced DND 520/50 further noticeably reduces the DND size. It was shown recently that the hydrogenation of a DND powder is a radical reaction that involves C3 radical desorption, which incites a free radical reaction through the reduction of molecular hydrogen to atomic hydrogen. Consequently, released atomic hydrogen facilitates C−H adsorption on the surface of the nanodiamond.35 Although some carbon atoms obviously desorb from the DND surface during the hydrogenation,26 it can hardly explain the mean size reduction from 2.6 to 2.1 nm, which would correspond to about 47% weight loss. The hydrogenation of the size-reduced DND 520/50 powder at the used conditions is in fact accompanied by only 7% weight loss, which might be simply explained by replacement of heavier oxygen atoms by lighter hydrogen atoms accompanied by a minor size reduction due to carbon atom desorption.26 Therefore, other effects like the strength of the hydrogen bonding network in the hydration shell, viscosity of the colloids, and etc. are most probably involved. The detailed analysis of these effects is, however, beyond the scope of this paper. Very important and demonstrative is the comparison of centrifuged and ultracentrifuged samples of DND 520/50-H by AUC. The data from AUC clearly distinguishes both samples in terms of mean size and size distribution. It is evident that the ultracentrifugation results in a further shift of the volumetric mean size down to 1.6 nm accompanied by significant narrowing of the size distribution in comparison to the centrifuged sample. The data clearly shows that one can isolate a fraction of truly sub-2 nm DNDs by ultracentrifugation, which is extremely important for further investigation of sizedependent properties of diamond on nanometer scale, including quantum phenomena,36 phonon confinement effect,37 and etc. To demonstrate the excellent relative accuracy of the AUC method in comparison to DLS as a traditional tool for size distribution analysis of colloidal dispersions, we performed a DLS analysis of the DND 520/50 and DND 520/50-H centrifuged samples as well as of the DND 520/50-H ultracentrifuged sample. Figure 1c shows five characteristic DLS runs of the size-reduced DNDs before hydrogenation (red; DND 520/50 cen.) and after hydrogenation (blue; DND 520/50-H cen.). The presented DLS data were acquired on centrifuged samples. It is clear that from such scattered DLS data only rough estimations of the mean size and the size

Figure 2. FTIR spectra of oxidized 2 nm DNDs before hydrogenation (red) and after hydrogenation treatment (blue). (a) Raman spectra of 2 nm O−DND (red) and 2 nm H-DND (blue). The black spectrum corresponds to 5 nm reference DNDs. (b) 38845

DOI: 10.1021/acsami.7b14436 ACS Appl. Mater. Interfaces 2017, 9, 38842−38853

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Figure 3. SEM and AFM comparison of the formed nucleation layers on the Si/SiOx substrate using 4−6 nm reference DNDs (left column, (a,d)); 2 nm H-DNDs dispersed in DI water (middle column, (b,e)); and 10−3 M KCl (right column (c,f)).

stretching region at 2800−3000 cm−1, C−H2 bending at 1465 cm−1, and C−H3 bending at 1380 cm−1) and at the same time significantly reduces the intensity of the features coming from oxygen-containing functional groups such as C−O bonds in alcohols, ethers, or carboxyl groups (1000−1500 cm−1) or the carbonyl CO bond (∼1800 cm−1). The carbonyl peak in particular has been nevertheless indicated also in the spectrum of hydrogenated DNDs, although with a significantly lower intensity and wavenumber (1720 cm−1), which indicates reduction of the lactones or anhydrides to ketones or carboxyl acids. It means that the hydrogenation is not yet fully completed at the used conditions. The hydrogenation has also an effect on the structure of the OH bonds, which is obvious in both the bending and stretching region. The OH bonds mostly originate from surface bound water.31,39 In the OH stretching region, there is a rise of two sharp peaks at 3690 and at 3620 cm−1. These peaks originate from free OH bond stretching and facing a hydrophobic surface, i.e. their frequency is not damped by hydrogen bonding network. The peak at 3690 cm−1 has been identified in FTIR spectra of H-DNDs,31 but the peak at 3620 cm−1 was not yet reported. We may speculate here that the two −OH-related sharp peaks correspond to hydrophobic interfaces between most frequent and facets and surrounding water molecules in the hydration layer. Recently, it has been shown that the hydration layer around conventional 5 nm H-DNDs exhibits a long-range disruption of the water hydrogen bond network and is accompanied by an electron transfer from a H-DND particle to surrounding water molecules, which is evidenced by a shift to a lower frequency of the OH bending mode in the FTIR

spectra.38 H-DNDs of a conventional size (4−6 nm) thus exhibit an unusual combination of hydrophobicity on one hand and ability to form stable colloids via electron transfer to surrounding water molecules on the other hand. We show that the H-DNDs keep these characteristics down to 2 nm or below as evidenced by rise of multiple bands in the OH bending region (1550−1670 cm−1), similarly to 5 nm H-DNDs (see Figure S1). In other words, the 2 nm H-DNDs keep the characteristic features of 5 nm H-DNDs, namely, the formation of stable colloids via electron transfer to surrounding water molecules. The two sharp peaks at 1330 and 1197 cm−1 in the spectrum of 2 nm H-DND most probably come from incorporated nitrogen or nitrogen-related defects13,40,41 inside the DNDs and appears only after hydrogenation due to suppression of C−O bonds in this region. Figure 2b shows Raman spectra of 2 nm O−DND (red), 2 nm H-DND (blue), and 5 nm reference DNDs (black). The Raman spectra of both 2 nm DNDs still resemble a typical Raman spectrum of conventional 5 nm DNDs, i.e. a broadened and shifted diamond peak at 1326 cm−1 and a sp2 carbonrelated band at around 1620 cm−1. In accordance to our previous results, the size reduction of DNDs down to or below 2 nm does not result in any significant shift or broadening of the diamond-related peak.29 It is obvious that Raman spectrum of 2 nm H-DNDs is very similar to 2 nm O−DNDs. Based on the FTIR and Raman results, we conclude that the hydrogenation under the used conditions changes only the DND surface chemistry and does not ruin the structure/crystallinity of the 2 nm nanodiamonds. We recently showed that oxidized nanodiamonds keep their crystallinity and nanodiamond 38846

DOI: 10.1021/acsami.7b14436 ACS Appl. Mater. Interfaces 2017, 9, 38842−38853

Research Article

ACS Applied Materials & Interfaces character down to 1 nm.29,32 From the herein presented results, it is obvious that this stability limit may be applied also to HDNDs. Note that both oxidized and hydrogenated 2 nm DNDs contain a significantly lower amount of nondiamond carbon than the 5 nm DNDs. This is given by certain selectivity of the oxidative etching to sp2 carbon during the size reduction at 520 °C, which in turn results in 2 nm nanodiamonds with significantly reduced nondiamond carbon content.29 Nucleation of 2 nm H-DNDs on Si/SiOx Substrates. Figure 3 shows SEM and AFM images of the nucleation layers formed by reference DNDs having a conventional size of 4−6 nm (left column), by 2 nm H-DNDs-DI water-based colloids obtained by centrifugation (middle column), and by 2 nm HDNDs dispersed in 10−3 M KCl solution (right column), also obtained by centrifugation. Attachment of H-DNDs on the Si/ SiOx substrate is driven by electrostatic attraction between the negatively charged Si/SiOx surface and the positively charged H-DNDs.17 The negative ZP of the Si/SiOx surface at neutral pH can be explained by a deprotonation of the acidic Si−OH surface groups.21 The positive ZP of H-DNDs is attributed to the presence of C−H surface groups, which enable an electron transfer to surrounding water molecules, similarly to the hydrogenated bulk diamond,42 thus, leaving the positively charged DND.38 Although the reference DNDs host both oxygen-containing (such as C−O) and hydrogen-containing (C−H) surface groups,38 they exhibit a positive ZP (+48 mV) similarly to hydrogenated DNDs, and therefore, both are suitable for nucleation on negatively charged Si/SiO x substrates. The SEM image (Figure 3a) shows that individual DND particles are clearly resolvable with a relatively large empty space between them (better visible in 200 × 200 nm2 detailed AFM scan shown in Figure 3d). The average nucleation density obtained from AFM data is 4 × 1011 cm−2, which is only twice lower than the highest reported value obtained after pH optimization of the H-DND colloid in order to maximize the electrostatic attraction between H-DNDs and SiO2 substrate.17 Still, the highest reported value of nucleation density (8 × 1011 cm−2) corresponds to only 10% substrate coverage when 4 nm particles are used for the calculation. Recently, it has been shown that addition of an inert electrolyte such as KCl to a 4−6 nm H-DND colloid also leads to an increase of the nucleation density; unfortunately, no value was provided.24 Inert electrolyte addition leads to a reduction of Debye screening length, which finally results in reduced electrostatic repulsive interaction between charged colloidal particles. In other words, the nucleation density may be tuned by ZP adjustment. In this context, it is fair to mention that the nucleation density reached by the reference DNDs could be also possibly increased by pH adjustment17 or intentional reduction of its ZP.24 In the case of 2 nm DNDs, the hydrogenation is accompanied by ZP change from −30 mV for the starting 2 nm O−DNDs to +36 mV for the 2 nm H-DNDs. It is important to mention that the ZP of 4−6 nm H-DNDs counterparts (i.e., not size-reduced) hydrogenated at the same conditions was +45 mV, which indicates possible dependence of ZP on H-DND size. Although the origin of this size effect is at the moment unknown, we assume that reduction of H-DND size shifts the ZP values closer to the instability region (