Photoluminescence of nanodiamonds influenced by charge transfer from silicon and metal substrates

Significance Statement

Nanodiamonds, being miniature diamond particles with sizes down to 1 nm (Stehlik et al. 2015), have shown a great promise in numerous modern high-tech applications including biomedical imaging, drug delivery, as well as photonics and solar energy conversion.

Photoluminescence represents a key property in those nanodiamonds applications. Although diamond is a wide band gap semiconductor, nanodiamonds exhibit photoluminescence in a visible spectra range due to surface disorder, dislocations, point defects as well as specific color centers arising from natural or intentional impurities such as nitrogen or silicon to name just few.

Moreover, nanodiamonds photoluminescence is greatly influenced by surface chemical groups. For example, hydroxyl groups give rise to specific photoluminescence bands. Hydrogenation of nanodiamonds leads to quenching of the color centers photoluminescence while oxidation or fluorination of nanodiamonds leads to its enhancement.

In a recent article published in Diamond & Related Materials, Stehlik et al. (2016) showed that inherent charge transfer between nanodiamonds and various substrates influences photoluminescence of nanodiamonds in terms of photoluminescence spectral shifts and lifetimes. Furthermore, the researchers showed that the observed effects depended on whether nanodiamonds were hydrogenated or oxidized.

Stehlik et al. (2016) used detonation nanodiamonds (DNDs) with nominal size of 5nm for the experiments. Hydrogenated (H-DNDs) and oxidized (O-DNDs) nanodiamonds were prepared by using microwave enhanced plasma hydrogenation or oxidation by air annealing. Detonation nanodiamonds were dispersed in water by ultrasonication and colloidal dispersions were appropriately diluted in order to deposit only a thin layer of DNDs on substrate by a drop casting.

Substrate were n-type Silicon (Si) wafers with metals {Gold (Au), Platinum (Pt) and Nickel (Ni)} deposited with thickness of 50 nm on top providing wide range of surface work functions. Laboratory-built confocal fluorescence microscope setup was used to measure photoluminescence spectra and time-resolved photoluminescence. Olympus IX71 inverted microscope was used for photoluminescence imaging.

The photoluminescence measurements revealed several noticeable features. Comparison of photoluminescence intensity of the same DNDs on Au and Si substrate showed an order of magnitude difference. On the other hand, no noticeable difference in the PL intensity was seen between Ni and Pt substrates. The photoluminescence intensity is thus dominated by different substrate reflectance rather than the substrate work function.

Yet interestingly, photoluminescence spectra of O-DNDs had maximum at 680 nm for Si and Au while the maximum shifted to about 700 nm on Ni. For H-DNDs the photoluminescence maximum was about the same at 660 nm for Si and Ni while it shifted to 640 nm on Au substrate. The photoluminescence spectra thus shift in dependence on substrate work function.

Time-resolved photoluminescence profiles exhibited fast decay for both H-DNDs and O-DNDs, however, there is a pronounced difference in lifetime depending on substrate work function. Lifetime of O-DNDs remains almost the same between Si and Au work functions and then rises towards higher work functions of Pt and Ni. Lifetime of H-DNDs exhibits a minimum on Au and increases towards lower (Si) and higher (Ni) work functions. Photoluminescence lifetimes of nanodiamonds thus depend on the substrate material in similar fashion as the spectral shifts, as shown in the Figure.

By using Kelvin probe force microscopy and scanning electron microscopy Stehlik et al. (2013) confirmed different charging of nanodiamonds. This suggested that variations in Contact Potential Difference (CPD) can explain the work function trends in H-DNDs and O-DNDs photoluminescence spectra and lifetimes. In case of H-DNDs, there is a minimal CPD on Au due to the lowest work function differences between Au and H-terminated diamond. As work function increases or decreases, charging of detonation nanodiamonds increases and thereby also spectral shifts and lifetimes. In case of O-DNDs, surface states on oxidized diamond are reportedly about 5 eV below vacuum level which means that holes can be trapped on Pt or Ni surface with higher substrate work functions. This is correlated with increasing spectral shifts and lifetimes on higher work function substrates.

Results obtained in this study thus have broad implications for nanodiamonds applications in imaging, sensing, quantum information, or energy conversion. They also significantly enhance our fundamental understanding of photoluminescence emission processes in nanodiamonds.

REFERENCES

Stehlik, M. Varga, M. Ledinsky, V. Jirasek, A. Artemenko, H. Kozak, L. Ondič, V. Skakalova, G. Argentero, T. Pennycook, J. Meyer, A. Fejfar, A. Kromka, B. Rezek: Size and purity control of HPHT nanodiamonds down to 1 nm. J. Phys. Chem. C 119 (2015) 27708–27720.

Stehlik, S., Ondic, L., Berhane, A.M., Aharanovic, I., Girard, H.A., Arnault, J., Rezek, B. Photoluminescence of Nanodiamonds Influenced by Charge Transfer From Silicon and Metal Substrate. Diamonds and Related Materials, 2016, Volume 63, pp 91-96.

S. Stehlik, T. Petit, H. A. Girard, J.C. Arnault, A. Kromka, B. Rezek: Nanoparticles assume electrical potential according to substrate, size and surface termination. Langmuir 29 (2013) 1634-1641.

Photoluminescence of nanodiamonds influenced by charge transfer from silicon and metal substrates. Advances in Engineering

 

Journal Reference

Stepan Stehlik1, Lukas Ondic1, Amanuel M. Berhane2, Igor Aharonovich2, Hugues A. Girard3, Jean-Charles Arnault3, Bohuslav Rezek1,4. Photoluminescence of Nanodiamonds Influenced by Charge Transfer From Silicon and Metal Substrate. Diamonds and Related Materials, 2016, Volume 63, pp 91-96.

Show Affiliations
  1. Institute of Physics ASCR, Cukrovarnicka 10, 16200 Prague 6, Czech Republic
  2. School of Physics and Advanced Materials, University of Technology, Sydney, Australia
  3. Diamond Sensors Laboratory, CEA LIST, F-91191 Gif sur Yvette, France
  4. Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 16627 Prague, Czech Republic

 

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