Photoluminescence of nanodiamonds influenced by charge transfer from silicon and metal substrates
Graphical abstract
Introduction
Nanodiamonds (NDs) belong to a family of carbon-based nanomaterials that are promising for numerous applications [1]. The most topical nowadays are applications of NDs in biology [2] such as imaging and drug delivery as well as in spintronics, photonics and quantum information technology [3], [4], [5]. The development of these applications gives rise to important progresses in surface modifications of nanodiamonds [6]. Photoluminescence (PL) is the key property in most of these applications. PL of nanodiamonds in visible spectral range can originate from dislocations and point defects [7], various color centers such as nitrogen or silicon vacancy centers (NV, SiV), and surface-related transitions in sp2 carbon phase (often forming ND shell), surface defects, and surface chemical moieties [8], [9], [10], [11].
Photoluminescence of NDs is indeed to a great extent determined by the structure and chemistry of the surface due to considerable surface to volume ratio. For instance, ND surface termination was shown to control photoluminescence of NV centers. In particular, hydrogenation of NDs leads to quenching of luminescence related to negatively charged (NV−) centers and as a result produced color shifts from NV− (638 nm) to neutral NV0 (575 nm) photoluminescence [12]. Similarly, high quality surface oxidation [13] or fluorination (on both bulk diamond [14] and nanodiamonds on Pt substrate [15]) lead to enhancement of NV− over NV0 color center population compared to hydrogen-terminated diamonds.
Here we report another, novel effect that arises from the substrate the NDs reside on. Effect of substrate on electronic and PL properties of nanostructures such as graphene oxide [16] or MoS2 [17] has been recently demonstrated. Similarly, we have demonstrated that NDs inherently accommodate their electrical potential to the substrate, resulting in up to 0.4 V different potential of the same nanoparticles on Au and Si substrates [18]. This effect was attributed to charge transfer and trapping on the nanoparticles which was dependent on the substrate material as well as on the surface termination of NDs. The effect was also independently confirmed by observation of different emission of secondary electrons in a scanning electron microscope (SEM). Here we show that the inherent charge transfer between NDs and substrate influences also photoluminescence of NDs in terms of PL spectral shifts and lifetimes. We also show that the PL changes depend on whether NDs are hydrogen or oxygen terminated and we discuss the data based on naturally present NV centers and their phonon sideband.
Section snippets
Experimental
As diamond nanoparticles we used detonation nanodiamonds (DNDs) provided by the NanoCarbon Research Institute Co., Ltd. (Japan) with nominal size of 5 nm. We compare hydrogenated (H-DNDs) and oxidized (O-DNDs) prepared by micro-wave enhanced plasma hydrogenation [19] or oxidation by air annealing [20]. The DNDs were dispersed in water by ultrasonication. The DNDs were de-agglomerated when dispersed in solution. The DLS, TEM and other basic characteristics of the employed DNDs are provided in our
Results
Fig. 1 shows optical and fluorescence images of H-DNDs measured at Au/Si and Ni/Pt boundary. Optical images in Fig. 1a and c show that the surface is coated mostly by a thin DND layer (thickness 20–50 nm estimated by atomic force microscopy). Dark dots correspond to large DND clusters arising from the deposition process. Thus the fluorescence originates mostly from DND clusters and not from single particles. Yet note that as the DNDs were de-agglomerated in the solution the DND clusters on the
Discussion of PL spectra
Broad band PL of nanodiamonds can generally span from 500 nm to 800 nm in dependence on particle size and excitation wavelength [22]. Such broadband PL typically originates from volume dislocations and point defects or from surface defects, sp2 carbon phase, and surface chemical moieties. So-called A-band of volume defects is typically at around 430 nm, i.e. in blue spectral region [7]. Recent study of surface chemistry-dependent PL study on nanodiamonds observed a blue PL component (around 430 nm)
Conclusions
The presented PL spectra and PL decay data evidenced novel effect of substrate on PL of DNDs. Broadband PL at 600–800 nm was attributed the phonon sideband of naturally present NV color centers. The fluorescence microscope showed clear differences in PL intensity of the same DNDs on different substrates. However, this was mainly due to different reflectivity of the substrates. On the other hand, the PL spectra shifts (by about 20 nm) and changes of PL decay times (by as much as 50%) show clear
Prime novelty statement
Here we show that the inherent charge transfer between detonation nanodiamonds (DNDs) and substrates with different workfunction influences photoluminescence of DNDs in terms of PL spectral shifts and lifetimes. We also show that the PL changes depend on whether DNDs are hydrogen or oxygen terminated and we discuss the data based on naturally present NV centers and their phonon sideband.
Acknowledgments
This work was supported by the GACR research project 15-01809S (BR, SS). It occurred in the frame of the LNSM infrastructure. I.A. is the recipient of an Australian Research Council Discovery Early Career Research Award (Project No. DE130100592).
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