Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color

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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 Štenclová, Lukáš Ondi#, Martin Ledinsky, Ji#í Pangrác, Ondrej Vanek, Jan Lipov, Alexander Kromka, and Bohuslav Rezek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14436 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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ACS Applied Materials & Interfaces

Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color Centers via Seeding by 2 nm Detonation Nanodiamonds Stepan Stehlik1*, Marian Varga1, Pavla Stenclova1, Lukas Ondic1, Martin Ledinsky1, Jiri Pangrac1, Ondrej Vanek2, Jan Lipov3, Alexander Kromka1, Bohuslav Rezek1,4 1

Institute of Physics ASCR, Cukrovarnická 10, Prague, 16200, Czech Republic

2

Department of Biochemistry, Faculty of Science, Charles University, Hlavova 2030/8, Prague,

12840, Czech Republic 3

Department of Biochemistry and Microbiology, University of Chemistry and Technology,

Technická 3, Prague, 16628, Czech Republic 4

Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague,

16627, Czech Republic KEYWORDS: detonation nanodiamond, surface chemistry, hydrogenation, zeta potential, nucleation density, nanocrystalline diamond, SiV center

ACS Paragon Plus Environment

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ABSTRACT

Color centers in diamond have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report chemical vapor deposition (CVD) growth of as thin as 5-6 nm nanocrystalline diamond (NCD) films 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 Si/SiOx substrate which enabled the CVD growth of such ultra-thin NCD films in two different and complementary microwave (MW) CVD systems: i) focused MW plasma with ellipsoidal cavity resonator and ii) pulsed MW plasma with 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, the nucleation layer as well as the ultra-thin 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.

Introduction In recent years, color centers in diamond 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)


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centers have demonstrated their potential1–4. In comparison to the NV center with broad emission spectrum at room temperature, the SiV center offer great advantage in its narrow roomtemperature zero-phonon-line (ZPL) at around 738 nm in which 70% of its photoluminescence (PL) is concentrated5. 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 surface chemistry of diamond or diamond nanoparticles in which they are incorporated6 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 to the diamond surface is crucial to have a good response of a sensor to a change of surrounding environment. Nanocrystalline diamond (NCD) film is a suitable material for hosting 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 a Si substrate or from a piece of Si placed to the reaction plasma during the NCD growth7,8. 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 NCD layer is essential. If a thicker diamond layer is for some reason needed (i.e. for efficient heat transfer), then growing firstly a thick layer under conditions which minimize the presence of SiV centers and secondly 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 non-diamond substrates a high nucleation density of diamond is essential; i.e. the higher the density of the nucleation centers, the lower thickness of continuous diamond film can be achieved. Here detonation nanodiamonds9 (DNDs) with their typical size of 4-66 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


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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 potential (ZP) in water10. The former is usually achieved by oxidation in acids or in ambient air atmosphere11 while the latter is typically reached by hydrogenation either in hydrogen plasma12 or hydrogen gas at elevated temperatures13,14. This variability of the DND surface charge offers broad possibilities to form high density seeding layer on many materials by an electrostatic interaction between oppositely charged DNDs and a substrate15–17. The density of the nucleation routinely achieved by using DNDs is in the order 1011 DND particles per cm2. There are several other seeding techniques like bias enhanced nucleation18, ultrasonic treatment of various particles19 or use of chemical precursors20, but none of these techniques currently provide 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 negatively charged SiO2 substrate and positively charged H-DNDs lead to a nucleation density of 8×1011 cm-2 and root mean square (RMS) surface roughness of ~2 nm at optimized pH of the HDND colloid17. Such nucleation density still corresponds to only 10% coverage assuming 4 nm particles and monolayer-like assembly. Recently, Yoshikawa et al.24 used an electrolyte (KCl) addition to H-DND colloid in order to intentionally weaken its ZP. This resulted in higher screening effect between individual H-DNDs and subsequent formation of very dense and flat nucleation monolayers composed of single 4-66 nm DND particles with RMS of 1.7 nm on Si substrate, however the nucleation density was not reported. In the two above mentioned


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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 ultra-thin NCD films but also for exploration of quantum phenomena in diamond, biomedical detection and drug delivery3. However, such nanodiamonds have not been available so far in a reasonable amount despite of several top-down25,26 and bottom-up attempts27. In this work we utilized our recently developed high-yield technique which provided DNDs with the mean size around 2 nm (volumetric distribution) by means of controllable size reduction of conventional 4-6 nm DNDs via oxidative etching in air28,29. We performed hydrogenation of these 2 nm DNDs by annealing in hydrogen. We characterized DNDs size distributions in colloidal dispersions depending on various stages of processing. Surface chemistry and structure of the 2 nm DNDs before and after hydrogenation were studied by 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 as thin as 5-6 nm continuous NCD films with embedded optically active SiV centers whose PL is sensitive to the NCD film surface chemistry.

Experimental


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As a reference DND material in terms of size and material quality we used 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 minutes providing oxidized DND 520/50. The details of this process can be found elsewhere29. The hydrogenation of the size reduced oxidized DND 520/50 took place in a quartz chamber at 600°C for 6 hours 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 one hour. Such prepared colloidal dispersions were centrifuged at 14000 × g for 3 hours (Eppendorf Mini plus) to eliminate as much of 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 14000 × g for one hour and resulting supernatants were further ultracentrifuged at 250000 × g for 1 hour (Optima Ultra, Beckman Coulter). Again, 1 mL of the supernatant after ultracentrifugation was carefully separated by a micropipette, the estimated concentration is 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 10-3M 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


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plasma (Diener Electronic - FEMTO, frequency 13.56 MHz, r.f. power 45 W, process pressure 50 Pa, time 1 min) to ensure their surface 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 minutes. After the nucleation treatment, the samples were rinsed by DI water or by 10-3M KCl solution and blown by nitrogen. Size distribution of the reference DNDs as well as 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 sedimentation velocity experiment. Samples of DNDs in DI water were spun at 10000 – 30000 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 DNDs partial specific volume was estimated as inverse of 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 instrument Zetasizer Nano ZS equipped with a helium-neon laser (633 nm); the scattering angle was 173°. The refractive index of bulk diamond (2.4), 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 5 subsequent runs. DND surface chemistry and efficiency of hydrogenation treatment was explored by FTIR spectroscopy in grazing-angle reflectance (GAR) arrangement. IR absorbance spectra were measured using a N2-purged Thermo Nicolet 870 spectrometer equipped with the KBr beam


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splitter and MCT detector cooled by liquid nitrogen. A 100–200 µL of the aqueous suspension was applied on the Au mirror by a drop casting just before the GAR-FTIR measurement. HDNDs on Au mirrors were heated at 100 °C for 2 min to evaporate the bulk water31. 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 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. 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 nominal tip radius 1-3 nm. The AFM images were acquired in a non-contact regime (free vibrational amplitude = 1-2 nm, setpoint 80%) to avoid possible distracting tip-particles interactions. Resolution of the AFM images was 512 × 512 points, scan ranges 200 × 200 nm2 up to 1 × 1 µm2. The nucleation density from the AFM images was estimated as detectable number of particles in the given area. In order to reliably determine thickness of the nucleation layers, the nucleated substrates were gently scratched by a toothpick, thus revealing the Si substrate. This further enabled to determine also 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 magnification of 300000 × (MAIA 3, Tescan) in the regime of secondary electrons. The diamond growth was performed in two different MW CVD systems: i) focused MW plasma with ellipsoidal cavity resonator (Aixtron P6) and ii) pulsed MW plasma with linear


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antenna arrangement. The deposition conditions used for focused MW CVD were as follows: gas flow of hydrogen equal to 300 sccm, gas flow of methane equal to 15 sccm, working pressure p = 60 mbar, microwave power P = 3 kW, deposition time t = 2, 4, 6 and 8 minutes, and the substrate temperature approx. = 700 °C. The main source of 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 hours, 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 spot of 1 µm on the sample in a direction perpendicular to the sample plane using a 100x 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 ultra-thin NCD films was extended to 60 min for 12 nm and 16 nm samples and to 5 hours in case of 8 nm NCD film. PL signal from the ultra-thin NCD films was accumulated for 1 min.

Results and discussion


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Size distribution of the size-reduced DNDs before and after hydrogenation In order to obtain accurate size distribution data from the AUC we first made AFM size analysis of the reference DNDs (AFM images not shown) in the same way as established earlier28,29,32. 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 contribution from larger particles or residual core agglutinates. Both distributions were fitted by a lognormal function. The accurate and statistically relevant AFM size analysis of the reference DNDs provided 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 distribution33. This is supported by a good agreement in the line shapes of the AFM volume distribution (Fig. 1b, grey histogram) and AUC size distribution of the reference DNDs (Fig. 1b, violet), but 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 non-ideal sedimentation behavior thus complicating the AUC data analysis34. However, in our case when comparing nanodiamonds of relatively similar size, it could be treated only as a systematic error occurring due to recalculation of the sedimentation


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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 factor 2.6 in order to normalize it to the AFM volumetric distribution. After this step we obtained AFM-corrected AUC particle size distribution shown in the upper X axis in the 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 volumetric mean size of the DND 520/50 sample obtained by AFM was 2.3 nm29. According to 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 which 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 nanodiamond35. Although some carbon atoms obviously desorb from the DND surface during the hydrogenation26 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 atoms desorption26. Therefore other effects like strength of hydrogen bonding network in


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the hydration shell, viscosity of the colloids etc. are most probably involved. The detailed analysis of these effects is however beyond the scope of this paper. Very important and demonstrative is 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 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 size dependent properties of diamond on nanometer scale, including quantum phenomena36, phonon confinement effect37 etc.

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


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of the size reduced DND 520/50 (red) and subsequently hydrogenated DND 520/50-H (blue) after centrifugation (c).

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 sizereduced 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 distribution can be made. Still, obvious correlation of the DLS data with the AUC data can be found. In case of the DND 520/50-H the DLS volumetric mean sizes fluctuates in 0.7-1.9 nm range which would give slightly lower average mean size than 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 obtained by AUC (2.6 nm). This considerable difference between the size of oxidized and hydrogenated DNDs in DLS data is again given by different surface chemistry and 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 results29, and for the sake of clarity


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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 ODNDs (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 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 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 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 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 bending and stretching region. The OH bonds mostly originate from surface bound water31,39. In the OH stretching region there is rise of two sharp peaks at 3690 and at 3620 cm-1. These peaks originate from free OH bond stretching, 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-DNDs31 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


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hydration layer around conventional 5 nm H-DNDs exhibits a long range disruption of the water hydrogen bond network which is accompanied by an electron transfer from a H-DND particle to surrounding water molecules which is evidenced by a shift to lower frequency of the OH bending mode in the FTIR spectra38. 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 supporting information, 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. broadened and shifted diamond peak at 1326 cm-1 and a sp2 carbon-related 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 peak29. 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 character down to 1 nm29,32. From the


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herein presented results it is obvious that this stability limit may be applied also to H-DNDs. Note, that both oxidized and hydrogenated 2 nm DNDs contain significantly lower amount of non-diamond 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 non-diamond carbon content29.


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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).

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 conventional size of 4-6 nm (left column), by 2 nm H-DNDs-DI water based colloid obtained by centrifugation (middle column), and by 2 nm H-DNDs dispersed in 10-3 M KCl solution (right column), also obtained by centrifugation. Attachment of H-DNDs on Si/SiOx substrate is driven by electrostatic attraction between negatively charged Si/SiOx surface and positively charged H-DNDs17. The negative ZP of the Si/SiOx surface at neutral pH can be explained by a deprotonation of acidic Si-OH surface groups21. 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 diamond42 thus leaving the positively charged DND38. Although the reference DNDs host both oxygen containing (such as C-O) and hydrogen containing (C-H) surface groups38, they exhibit a positive ZP (+48 mV) similarly to hydrogenated DNDs and therefore both are suitable for nucleation on negatively charged Si/SiOx substrates. SEM image (Fig. 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 Fig. 3d). The average nucleation density obtained from AFM data is 4×1011 cm-2 which is only twice lower than highest reported value obtained after pH optimization of the H-DND colloid in order to maximize the electrostatic attraction between H-DNDs and SiO2 substrate17.


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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 4-6 nm H-DND colloid also leads to an increase of the nucleation density, unfortunately no value was provided24. 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 ZP24. 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 sizereduced) 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 instability region (< ± 30 mV), which is only advantageous for formation of ultra-dense nucleation layers. Tuning of the DND zeta potential to increase the nucleation density may be also driven by the level of hydrogenation. For example, when the 2 nm O-DNDs were hydrogenated at 700°C for 6 hours we obtained the ZP value of +42 mV, i.e. higher than +36 mV for the 2 nm H-DNDs hydrogenated at 600°C for 6 hours. This correlates with a significantly lower intensity of the residual carbonyl peak for DNDs hydrogenated at 700°C (see Supporting Information, Figure S2). For further study we used only the 2 nm H-DNDs hydrogenated at 600 °C and having a lower ZP value, i.e. closer to ZP stability limit for colloids. Figure 3b shows SEM image of the nucleation layer formed by 2 nm H-DNDs dispersed in DI water. It is obvious, that the Si/SiOx


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substrate is much more densely and homogeneously covered by the DNDs compared to Fig. 3a. From the detailed 200 × 200 nm2 AFM scan (Fig. 3e) we estimate the nucleation density to be around 2×1012 cm-2 which is 5 times more compared to nucleation density obtained by conventional 4-6 nm reference DNDs.

Figure 3. SEM and AFM comparison of the formed nucleation layers on 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)).


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Figures 3c and 3f show SEM and AFM images of the nucleation layer formed by 2 nm H-DNDs dispersed in 10-3 M KCl solution. Although the morphology visualized by SEM is similar to nucleation layer of 2 nm H-DNDs dispersed in DI water (Fig. 3b), the AFM image indicates that the coverage of the Si/SiOx substrate is even somewhat higher than in case of DI water based colloid. The increase of the nucleation density estimated from the 200 × 200 nm2 AFM scan (4×1012 cm-2) can be related with an additional decrease of DNDs ZP value down to +27 mV after the KCl addition. However, it is challenging to get a reliable statistical data at such high nucleation density due to an unavoidable convolution of the AFM tip having radius similar or larger than the particles. In such a case the individual particles are hardly resolvable, the substrate coverage is overestimated, and nucleation density underestimated. Figure 4 shows a comparison of 200 × 200 nm AFM and SEM images and derived surface coverage images of sample deposited from 2 nm H-DND dispersed in DI H2O. The AFM image (Fig. 4 a) appears as if the 2 nm H-DNDs already cover majority of the surface. An image processing using a thresholding and masking gives about 90% coverage (Fig. 4 b). In contrast, SEM image of the same sample (Fig. 4 c) still reveals some empty space between the 2 nm H-DND deposits. In fact, the SEM image suggests only ~ 50% coverage using analogous processing to the AFM image (Fig. 4 d). Theoretical nucleation density reachable by 2 nm particles approximated as circles and forming a monolayer is ~ 3.2 × 1013 cm-2. This is nearly 10 times more than 4×1012/cm-2 derived from the AFM images and it simply does not correlate to the 90% coverage suggested by AFM image which would correspond to ~ 2.9 × 1013 cm-2. Therefore it is obvious that the convolution of the tip with the 2 nm particles cannot be neglected and the resolution of the AFM, despite using supersharp tips, is limited on this size scale. On the other hand, the SEM image provides more realistic data. The 50% coverage corresponds to 1.6 × 1013 cm-2 and if we


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take into account certain blur of the SEM image (estimated as 20 %), still the SEM data suggest 40% coverage, which in turn yields ~ 1.3 × 1013 cm-2 nucleation density which is up to two orders of magnitude higher than commonly reported. Although significant simplifications (2 nm particles only, monolayer geometry) are involved in this calculation, we assume that the nucleation density derived from the SEM image is realistic.

Figure 4. 200 × 200 nm AFM scan (a) and corresponding image after thresholding and masking showing 90% coverage (b). 200 × 200 nm SEM image (c) and corresponding image after


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thresholding and masking showing 50% coverage (d). The data comes from 2 nm H-DNDs deposited on Si/SiOx substrate from DI H2O based centrifuged colloid.

The RMS values of the nucleation layers obtained from the AFM data varied between 0.5 – 0.8 nm for both DI water and 10-3 M KCl based 2 nm H-DND colloids, highlighting dense and homogeneous coverage reached by 2 nm H-DNDs. Note, that the 2 nm H-DNDs deposited from the 10-3 M KCl colloids were often prone to aggregation on the Si/SiOx substrate when rinsed by DI water after the ultrasonic treatment. This effect did not appear when the samples nucleated from 10-3 M KCl-H-DNDs colloids were rinsed by the 10-3 M KCl solution. This is clear evidence that the stability of the samples nucleated from solution containing an electrolyte (KCl) depends on the presence of K+ and Cl- ions in the hydration shell of the H-DNDs. Once the ions are washed away by DI water such as during the rinsing, local ZP of individual H-DNDs increases due to the suppressed shielding effect and thus their aggregation occurs24. To reliably determine thickness of the nucleation layers we performed 1 × 1 µm2 AFM scans at a nucleation layer/Si/SiOx interface created by a mechanical removal of the DNDs by gentle scratch using a toothpick. Figure 5 shows a representative histogram of Z values obtained from AFM scans at the interface (inset in Fig. 5) for the 2 nm H-DNDs-H2O colloid. Thickness of the nucleation layer was determined from the peaks of the mean Z values corresponding to the Si/SiOx substrate defined as zero and the average height of the nucleation layer. The thickness in this particular case is around 2 nm and varied between 1.8 and 2.5 nm as measured on several samples for both DI water and 10-3 M KCl based 2 nm H-DND colloids. The nucleation layer thickness corresponds well to DNDs volume distributions obtained by AUC and to some extent


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to DLS measurements (Figures 1b, c). It must be emphasized here that the use of ultracentrifuged H-DNDs colloids with mean volumetric size of 1.6 nm for nucleation surprisingly did not lead to noticeably different morphology or thinner nucleation layer as one would expect when using sub-2 nm DNDs. This suggests that the sub-2 nm H-DNDs form a quasi-3D nucleation structure rather than a monolayer during the ultrasonic treatment on the Si/SiOx surface.

Figure 5. Histograms of AFM Z values (black) fitted by two Gaussians (blue) showing height of the DND layer ~2 nm. The AFM Z values were obtained from 1 × 1 µm2 AFM scan of 2 nm HDND nucleation layer-Si/SiOx substrate interface shown in the inset. The Z scale of the AFM image is 5 nm.

Si-doped ultra-thin NCD films from a focused MW plasma system


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For the growth of Si-doped ultra-thin NCD films we used the Si/SiOx substrates nucleated by both the DI water- and 10-3 M KCl-based 2 nm H-DNDs colloids obtained by centrifugation. Although AFM mapping suggests higher nucleation density reached by the 10-3 M KCl-H-DNDs colloids, we did not observe any considerable difference in the resulting NCD films in terms of morphology (grain size, surface roughness) and density of pinholes. Due to unresolvable difference in the NCD films (morphology and properties) grown from DI water and 10-3 M KCl 2 nm H-DND colloids we do not distinguish between them in the following sections. Figures 6a-c show representative SEM images of NCD films obtained by CVD growth for 2, 4, 6, and 8 min, respectively. It is important to mention that all the NCD films were microscopically uniform over nearly whole sample area. On nanoscale, the NCD film grown only for 2 min is possibly not entirely continuous, since certain number of pinholes is observable by SEM (Figure 6a). As the deposition time increases the pinhole density decreases together with increasing diamond crystals size. According to SEM images the NCD film looks practically pinhole-free after 6 min of the CVD growth (Figure 6c). In order to determine the thickness of the NCD films we used the same way as for the nucleation layers, i.e. the AFM scan of the NCDSi/SiOx substrate interface was performed. The areas of Si/SiOx substrate from which the DNDs were removed prior the CVD growth remained smooth and uncovered by the NCD film and therefore served as a good height reference for the NCD thickness analysis by AFM. Example of a typical 1 x 0.5 µm2 AFM scan of the thinnest 2 min NCD film- Si/SiOx interface is shown in the inset of the Figure 6e. The Z values histograms for all three NCD films- Si/SiOx interfaces are shown in the graph of Figure 6e. Mean thickness of the NCD films determined by AFM was 5.5 ± 1.5 nm, 8 ± 2 nm, 12 ± 2.7 nm, and 16 ± 3.7 nm for the NCD films grown for 2, 4, 6, and 8 min, respectively. Fabrication of 5.5 nm thin NCD film clearly demonstrate advantage of using 2


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nm DNDs for seeding since such thickness is not achievable with conventional 4-6 nm DNDs. It is remarkable that the obtained NCD films are also very smooth thanks to the high nucleation density, deposition conditions used, and low thickness of the films. The RMS determined by AFM from 1 × 1 µm2 scans (not shown here) were 1.3 nm, 1.7 nm, 2.1 nm, and 2.9 nm for the NCD films grown for 2, 4, 6, and 8 min, respectively. SEM images comparison of the NCD films grown from the 4-6 nm DNDs with 4×1011 cm-2 nucleation density (Figure 3, left column) and 2 nm DNDs with 1.3 × 1013 cm-2 nucleation density (Figure 3, middle or right column) both after 8 min of growth is shown in supporting information, Figure S3. The comparison clearly demonstrates the positive effect of more than one order higher nucleation density on the morphology of the resulting NCD films.


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Figure 6. SEM images of the Si-doped NCD films grown for 2 min (a) 4 min (b), 6 min (c) and 8 min (d). Z value histograms (e) of the NCD-Si/SiOx substrate interfaces for particular samples showing thickness of the NCD films. Representative 1 x 0.5 µm2 AFM scan of the 5.5 nm NCD film-Si/SiOx interface is shown in the inset of the graph. Z scale of the AFM image is 9 nm.

In order to verify diamond character of the NCD films we performed Raman spectroscopy, the spectra are shown in the Figure 7a. Raman spectrum of the thinnest 8 nm NCD film (red) does not show any hint of the diamond peak at 1333 cm-1. In fact it nearly perfectly matches to the spectrum of Si background acquired at the same conditions (black), i.e. there is no detectable Raman signal coming from such thin NCD film containing very small (mostly sub-10 nm) diamond crystals. Therefore, Raman spectrum of the thinnest 5.5 nm NCD film is not plotted in the Figure 7a due to the detection limit of the used setup. The peak rising below 1200 cm-1 and the peak at 1450 cm-1 are obviously from the Si/SiOx substrate (first and second overtone of Si, respectively), the sharp peak at 1551 cm-1 comes from air oxygen appearing only at long accumulation times. Raman spectra of 12 nm (wine) and 16 nm (blue) NCD films already exhibit distinct diamond peak at 1333 cm-1 which is a clear evidence of their diamond character. When the diamond film thickness increases, the individual non-diamond phases (Raman peaks/bands) in the region 1450-1650 cm-1 combine into a broad G-band superimposed to the 1450 cm-1 peak of Si/SiOx substrate. The Raman spectrum of the thickest 16 nm NCD film also reveals a peak at 1150 cm-1 which is regularly assigned to trans-polyacetylene (t-PA) chains arising at the grain boundaries43.


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Truly diamond character of all the NCD films is evidenced by the PL spectra shown in Figure 7b. The NCD films exhibit PL peak at around 739 nm superimposed on a broad PL band of the nanodiamond. This peak is the ZPL from the SiV centers that were introduced into the layers during the growth and it is a direct proof of the layers diamond character. It has been shown that SiV centers may stably exist in as small as 1.6 nm nanodiamonds of meteoritic origin3 and our results only confirms this since we identified them even in the thinnest 5.5 nm NCD film. After subtraction of the undoped nucleation DND layer (~2 nm) it becomes evident that the SiV PL signal comes in average from 3.5 nm SiV doped diamond nanocrystals. The absolute PL intensity of the SiV centers peak increases with the diamond film thickness. Also the ZPL spectral position shifts from 740 to 739 nm with increasing layer thickness. The measured linewidth of the ZPL decreases from approx. 12 to 10 nm with increased layer thickness. This means that PL changes are not only due to increased amount of SiV centers in the PL detection volume but also due changing surroundings of the SiV centers. Both the spectral shift and the broad linewidth of the ZPL can be explained by the presence of the residual mechanical stress between crystallites44,45. Due to the stress, each SiV center from the ensemble may have slightly different ZPL position which causes the observed inhomogeneous broadening of the linewidth compared to minimal linewidth 0.7 nm of a single SiV center45. Further, by increasing the thickness of the layer the stress changes which in turn affects the position of the PL peak. The absolute PL intensity of the SiV center is also sensitive to surface termination. The scales in the Figure 7b are the same for direct comparison. It is obvious that all the as-grown films (dashed grey lines) exhibit lower PL intensity than after a surface oxidation treatment (full lines). In addition, the 5.5 nm and 8 nm samples do not clearly show the SiV PL peak when left


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in the as grown state. Positive effect of the oxygen surface termination on the SiV PL intensity in NCD films has been demonstrated recently and ascribed to presence of surface C=O bonds rather than to a purification effect (removal of sp2 carbon)6. Since we used relatively mild oxidation treatment (450°C, 30 min) and we did not observe any change in the NCD film thickness, morphology, and Raman spectra after the oxidation step we also suppose that the increase of the SiV PL intensity after the oxidation treatment is due to change from hydrogen to oxygen surface termination. This clearly demonstrates applicability of the ultra-thin SiV films for sensing or PL switching purposes. Thickness of the diamond film/volume in which the SiV PL is switched on/off by oxygen/hydrogen surface terminations can be deduced as follows: first, the 2 nm nucleation layer has to be subtracted for all samples. Then, the thickness lower than 10 nm is identified as the critical one due to barely detectable and not detectable SiV PL in 12 nm and 8 nm as grown films, respectively. In this meaning the sub-10 nm diamond films are highly advantageous for exploitation of full SiV PL switching under the used conditions and experimental setup.


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Figure 7. Raman spectra of the ultra-thin NCD films (8 nm; red, 12 nm; wine, and 16 nm; blue) and the Si/SiOx substrate (black) background (a). Photoluminescence spectra showing the PL from SiV center at 739 nm of the ultra-thin Si-doped NCD films. The dashed grey lines correspond to PL of the NCD films in as-grown state, while the full lines (5.5 nm; cyan, 8 nm; red, 12 nm; wine, and 16 nm; blue) show an increase of the SiV PL after surface oxidation (b).

Ultra-thin NCD films from a linear antenna MW plasma system The MW plasma CVD process leading to ultra-thin NCD films with SiV PL presented above took place at relatively high temperatures (cca 700°C) and high plasma densities (i.e. also high concentration of atomic hydrogen). Such conditions, among others, result in relatively fast


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all directional growth and accurate control of the NCD layer thickness with nanometer precision is somewhat challenging. High concentration of atomic hydrogen might also lead to a certain reduction of the nucleation density due to possible etching of nanodiamond seeds. To suppress these effects we performed the NCD growth at lower pressure and lower temperature (460°C). We chose process conditions (see Experimental part) at which the NCD growth is very slow compared to the focused plasma system. It is important to note here that such grown NCD films do not exhibit SiV PL due to its quenching by CO2, which if necessary, can be also used in focused MW plasma CVD8. The slow growth enables highly accurate control of the film morphology and thickness due to long incubation period which is characteristic by dominant lateral growth, i.e. closing of the NCD film in the initial stage of the growth20, 46. Once the film is closed, the vertical growth prevails. Figure 8 shows SEM images of the nucleation layer (a), and NCD films obtained after 2h (b), 3h (c) and 5h (d) of growth. Figure 8e shows the dependence of the NCD film thickness on the growth time. The film thickness was determined by AFM in the same manner as for the Si-doped NCD films. The error bars represent a full-width at halfmaximum of the AFM Z values of the film. It is obvious that the thickness increases only slightly (0.8 nm) during first three hours of the deposition. This observation well correlates with the SEM images showing a progressive closing of the film, i.e. lateral growth is dominant. After five hours, the film thickness increases to 5 nm ± 2.5 nm and the vertical growth starts to dominate the film growth. Indeed, the SEM image (Figure 8d) reveals a highly coalesced and continuous film. This growth regime change is also accompanied by increase of the films surface roughness. Still, the RMS of 2.1 nm for the 5 nm NCD film is very low and in fact comparable to values reported for the nucleation layer formed by conventional 4-6 nm DNDs17, 24. A comparison of NCD films grown 5 hours in the linear antenna MW plasma system for samples nucleated either


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by conventional 4-6 nm DNDs (4×1011 cm-2) and or by 2 nm DNDs (1.3×1013 cm-2) is shown the Figure S3.


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Figure 8. SEM image of 2 nm DND nucleation layer with 1.3×1013 cm-2 nucleation density (a), NCD films obtained in low density plasma regime after 2h (b), 3h (c), and 5h (d) of the growth. The RMS values are provided for each sample in the corresponding SEM image. The graph shows dependence of the NCD layer thickness on the time of the growth (e). The line is to guide the eye.

Conclusions In this work we demonstrated the feasibility of obtaining hydrogenated 2 nm detonation nanodiamonds by thermal hydrogenation of oxidized DNDs whose size was at first reduced by controlled annealing in air. The analytical ultracentrifuge served here as accurate and useful tool for analyses and adjustment of size distribution of DND colloidal dispersions on nanometer scale. FTIR and Raman spectroscopy revealed an effective surface hydrogenation of the 2 nm DND via their annealing at 600°C in hydrogen without detectable affecting the 2 nm diamond crystalline core. This provides new insight for understanding and applications of 2 nm nanodiamonds and documents that characteristically large variability of DNDs surface chemistry (e.g. hydrogenated to oxidized) and corresponding zeta potential variations (positive to negative) is available down to 2 nm or below. For instance, a desired electrostatic interaction between 2 nm DND and the substrate can thus be achieved. In our case, positive zeta potential reaching colloidal stability region and nearly molecular size of the 2 nm H-DNDs enabled formation of homogeneous, extremely dense (1.3 × 1013 cm-2), thin (2 nm), and smooth (RMS < 0.8 nm) nucleation layers on Si/SiOx substrates from aqueous (deionized or 10-3 M KCl based) colloids. Such unprecedented quality of the nucleation layer was crucial in the subsequent CVD growth


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(linear antenna MW plasma and focused MW plasma) of highly coalesced sub-6 nm NCD films which could not be achieved by conventional 4-6 nm DND seeds. We employed the focused MW plasma CVD system under conditions optimized for incorporation of SiV centers to the NCD films and we observed distinct PL from SiV centers in NCD films as thin as 5.5 nm. Moreover, the SiV PL intensity was tunable by variation of the NCD films surface chemistry. The critical thickness (or size) of the diamond film within which the SiV PL can be fully switched between on and off states by oxygen and hydrogen surface terminations was estimated to 10 nm. Our results bring new opportunities for extremely thin diamond films, and improve understanding to diamond at nanoscale.

ASSOCIATED CONTENT Supporting Information FTIR spectra of 5 nm H-DNDs and 2 nm H-DNDs hydrogenated at 600°C, FTIR spectra of 2 nm H-DNDs hydrogenated at 600°C and 700°C, and SEM images of the NCD films grown at different growth regimes on differently nucleated substrates. AUTHOR INFORMATION Corresponding Author * Stepan Stehlik * E-mail: [email protected] Author Contributions


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S.S., B. R., and A. K. conceived and supervised the work. S.S. and M. V. designed the experiments. S.S. was primarily responsible for the nanodiamond part of the work (DND size reduction, preparation of the colloids, nucleation, and size AFM/DLS experiments). M. V. was responsible for the NCD part of the work (growth and characterization). P. S. measured the FTIR spectra, L. O. measured the PL spectra, M. L. measured the Raman spectra, J. P. performed the hydrogenation of the DNDs, O. V. made the AUC experiments, and J. L. performed ultracentrifugation of DND colloids. S.S. wrote the manuscript with help and contributions of other coauthors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by the Czech Science Foundation project P108/12/G108, by Charles University project UNCE 204025/2012, by the Ministry of Education, Youth and Sports of the Czech Republic projects LD15003 and LTC17065 within the frame of the COST Actions MP1403 “Nanoscale Quantum Optics” and CA15126 "Molecular Biophysics in Europe", respectively, and by the European Regional Development Fund project CZ.02.1.01/0.0/0.0/15 003/0000464. We also gratefully acknowledge P. Bauerova for SEM work. REFERENCES (1)

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TOC graphics


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Figure 1 175x47mm (300 x 300 DPI)


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Figure 2 82x151mm (300 x 300 DPI)



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Figure 3 170x112mm (300 x 300 DPI)


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Figure 4 85x94mm (300 x 300 DPI)



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Figure 5 85x76mm (300 x 300 DPI)


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Figure 6 85x171mm (300 x 300 DPI)



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Figure 8 85x238mm (300 x 300 DPI)



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TOC 76x35mm (300 x 300 DPI)


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Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color

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