PbSe Nanocrystals (NCs) -from synthesis to applications- by Razvan-Ionut Stoian Oklahoma State University, Department of Physics Motivation General properties of the PbSe nanocrystals (theoretical aspects) Synthesis methods Applications Future Directions

Motivation The achievement of strong spatial confinement of the charge carriers (see the “Theoretical background” slide) ↔ non-linear optical properties Applications (mainstream) Biology Optical Sensors Lasers Nano-electronics Applications (specific) telecommunication applications low power, low threshold lasers (optical pumping) (1550nm domain) Goals achievable by the synthesis of small sized PbSe NCs

Literature Review Important steps in the study and synthesis of PbSe NCs W. D. Lawson– PbSe Thin Films prepared by evaporation[1] 1995 – Sasha Gorer- Chemically deposited, nanocrystalline PbSe Films [2] 1997 – Kang and Wise – Detailed calculations on PbSe NCs energy bands [3] 1997 – Lipovskii et al. - Synthesis of PbSe NCs in phosphate glasses [4] 2001 – Wang et al. - Hydrothermal synthesis of PbSe NCs [5] 2002 – Dui et al. - PbSe NC synthesis by organic precursors [6] 2004 – Sashchiuk et al. - PbSe NC synthesis by polymeric precursors [7]

Literature Review (cont.) NCs synthesis is a multidisciplinary research field Enabling technologies (characterization methods) Related fields...make use of the NCs, namely: Electrical Engineering – infrared detectors Physics – Fundamental research and Lasers Chemistry Biology – biological markers[8] Nano-electronics [9] TEM - HR-TEM (high res. TEM) STM SEM - HR-SEM (high res. SEM) XRD (X-ray diffraction) XRF (X-ray fluorescence) Absorption spectroscopy

Theoretical background Key words for PbSe NCs: -Excitonic Bohr radius - -(Strong) Spatial Confinement -Electronic density of states (DOS) R : the average dimension of the NC discrete excitonic energy levels optical absorption levels are discrete these abs. levels are situated in the midinfrared domain

Theoretical background (cont.) [10] The effect of the NC size on the temperature dependence of E g [10] Optical transition strengths of a 8.5 nm PbSe NC (calculation) [10]

Synthesis Methods 1. PbSe NC synthesis using phosphate glasses glass host of choice: P 2 O 5 -Ga 2 O 3 -ZnO-AlF 3 -Na 2 O preparatives – glass host and solid PbSe melted at 1150°C PbSe NC size controlled by varying the annealing temperature: °C TEM micrograph [4] PbSe NC characteristics - size : 2-15 nm - dispersion: ±7% - - low processabilty

Synthesis Methods 2. The hydrothermal method 2Pb 2+ + N 2 H 4 +4OH - → 2Pb + N 2 +4H 2 O Pb +Se → PbSe Experimental setup TEM of PbSe NC [5] PbSe NC characteristics: - size : min 23 nm -high processabilty -

Synthesis Methods 3. PbSe NC synthesis by organic precursors [6] Experimental setup NCs size controlled by the preparation temperature: °C PbSe NC characteristics: - size : 3-8 nm -high processabilty - STEM on PbSe [6]

Synthesis Methods -comparisons-

Applications Infrared detectors µm Biological markers [8] LEDs and mid-infrared lasers Low power, eye-safe lasers Low power, low threshold (optically pumped) laser

Applications (cont.) Whispering gallery mode emission (PL) [11] Laser 1550 nm silica bead coated with PbSe NCs optical fiber PL from a PbSe coated silica bead [11]

Future directions Advancement of the NC synthesis implies: NCs will have extremely small sizes (<1nm) NCs will exhibit a true monodisperse character New theoretical models will be developed Advancement in Nano-electronics (large scale integration) Challenges to be overcome: a better control of the parameters that “tweak” the NCs characteristics - temperature during the synthesis - the purity of the reagents

References [1] Lawson, W.D. et al. Journal of the Electrochemical Society, 1960, 107, p [2] Gorer, S., Albu-Yaron, A. and Hades, G. Journal of Physical Chemistry, 1995, 99, p [3] Kang I. and Wise, F. J. Opt. Soc. Am. B, 1997, Vol. 14, 7 [4] Lipovskii, A et al., Applied Physics Letters, 1997, Vol. 71 (23), p [5] Wang, C., Zhang, G., Fan, S., Li, Y. Journal of Physics and Chemistry of Solids, 2001, Vol. 62, p [6] Dui, I. et al., Nanoletters, 2002, Vol. 2, 11, p [7] Sashchiuk, A. et al. Nanoletters, 2004, Vol.4, 1, p [8] Smith, A.M., Gao, X. and Nie, S. Photochemistry and Photobiology, 2004, 80 [9] Wehrenberg, B.L.,Yu, D., Ma, J. and Guyot-Sionnest, P. Journal of Physical Chemistry. B, 2005, Vol. 109, p [10] Wise, F. W. Acc. Chem. Res., 2000, Vol. 33, p [11] Finlayson, C.E. et al., 2006, Semiconductor Science and Technology, 2006, Vol. 21