Olga I. Micic

Highly efficient band‐edge emission from InP quantum dots

High quality InP quantum dots with diameters ranging from 25 to 45 A, have been prepared; these quantum dots (QDs) show high quantum yields for band‐edge photoluminescence (lowest energy HOMO‐LUMO transition). The wavelength of the blue‐shifted band‐edge emission ranges from about 575 to 730 nm depending on QD size. The quantum yield for photoluminescence is 30% at 300 K and 60% at 10 K; the multiexponential decay of this emission exhibits lifetimes ranging from 5 to 50 ns. Deep red‐shifted emission due to trapping of carriers in defect states on the QD surface which exhibits lifetimes above 500 ns, has been eliminated by treating the QDs with a dilute solution of HF or NH4F.

Synthesis of extremely small InP quantum dots and electronic coupling in their disordered solid films

Extremely small colloidal InP quantum dots (QDs) with diameters ranging from 15 to 23 A were synthesized, and the optical properties of close-packed arrays of these dots were studied. The isolated QDs in dilute colloidal solution exhibit pronounced discrete absorption spectra, indicating a narrow size distribution. The absorption spectra of close-packed solids of ∼18 A diameter QDs with interdot spacings of 9 and 18 A show that the absorption onsets and excitonic peaks are, respectively, redshifted and broadened in going from dilute solution to close-packed solids. These results can be explained by electron delocalization in disordered close-packed solids; the spacing of electronic levels in the QDs is reduced and produces a redshift in the absorption spectra.

Synthesis, structure, and optical properties of colloidal GaN quantum dots

Colloidal chemistry was used to synthesize GaN quantum dots. A GaN precursor, polymeric gallium imide, {Ga(NH)3/2}n, which was prepared by the reaction of dimeric amidogallium with ammonia at room temperature, was heated in trioctylamine at 360 °C for one day to produce GaN nanocrystals. The GaN particles were separated, purified, and partially dispersed in a nonpolar solvent to yield transparent colloidal solutions that consisted of individual GaN particles. The GaN nanocrystals have a spherical shape and mean diameter of about 30±12 A. The spectroscopic behavior of colloidal transparent dispersion has been investigated and shows that the band gap of the GaN nanocrystals shifts to slightly higher energy due to quantum confinement. The photoluminescence spectrum at 10 K (excited at 310 nm) shows band edge emission with several emission peaks in the range between 3.2 and 3.8 eV, while the photoluminescence excitation spectrum shows two excited-state transitions at higher energies.

Anti-Stokes photoluminescence in colloidal semiconductor quantum dots

We report anti-Stokes photoluminescence (photon energy up-conversion) from size-quantized CdSe and InP nanocrystalline colloids. The observed up-conversion is highly efficient and occurs at very low excitation intensities. With low temperatures the intensity of the up-converted photoluminescence decreases while that of the usual Stokes photoluminescence increases; the up-converted photoluminescence is also restricted to energies corresponding to the band gaps of the quantum dots that are present in the colloid ensemble. The anti-Stokes photoluminescence is explained by a model that involves surface states.

Size-dependent Raman study of InP quantum dots

Raman spectrum of a quantum dot (QD) is characterized by transverse (TO) and longitudinal (LO) optical modes as well as surface optical modes, occurring between the TO and LO modes. We have studied in detail the size-dependence of the Raman spectrum of InP QD of diameter larger than 35 A. The LO phonon frequency decreases while the TO phonon frequency increases with decreasing QD size. The linewidth of the LO phonon broadens and the broadening becomes increasingly asymmetrical towards the low frequency side as the QD size decreases. By analyzing the Raman intensity ratio of the LO phonon to its overtone, we find that the electron-phonon coupling decreases with decreasing QD size.

Synthesis and characterization of binary and ternary III–V quantum dots

Abstract Quantum dots of InP, GaP, GaInP 2 , and GaAs with diameters ranging from 20–80 A can be synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The synthesis is achieved by heating appropriate organometallic precursors with stabilizers in high boiling solvents for several days to produce QDs, which can then be dissolved in nonpolar organic solvents to form transparent colloidal QD dispersions. The high sample quality of the InP and Gap QDs results in excitonic features in the absorption spectra; excitonic features could not be observed for GaAs or GaInP 2 QDs. The GaP and GaInP 2 QD colloids exhibit very intense (quantum yields of 15–25%) visible photoluminescence at room temperature. The photoluminescence for InP QDs preparations show two emission bands: one band is in the visible at the band edge of the QD, and a second band appears above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band can be removed after controlled addition of etchant; subsequently, very intense band-edge emission (quantum yield 30%), which is tunable with particle size, is obtained. The QDs were characterized by TEM, SAXS, AFM, powder X-ray diffraction, steady-state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy.

Observation of the quantum confined ground state in InP quantum dots at 300 K

Colloidal suspensions of InP quantum dots (diameters 25 and 35 A) show steplike absorption spectra which are blue‐shifted by about 1 eV with respect to bulk material. Time resolved femtosecond spectroscopy at room temperature identifies the first quantum confined state. The nonlinear bleaching signal rises within 300 fs and persists longer than 200 ps.

Temperature effect on the photoinduced reduction of methyl viologen with several sensitizers and the evolution of hydrogen from water

Abstract Irradiation by visible light of an aqueous solution containing a photosensitizer, methyl viologen (MV 2+ ) and ethylenediaminetetraacetic acid leads to the formation of the reduced form of methyl viologen (MV + ). The quantum yield for the formation of MV + depends strongly on the time during which the formation is observed owing to the reaction of MV + with oxidative products and its reduction to MV 0 . Proflavin, acridine yellow and ruthenium(II)tris(2,2-bipyridyl) were used as photosensitizers and showed the same ability to promote hydrogen evolution. When CdS was used as a sensitizer a factor of 10 less hydrogen was obtained than when the dyes were used. The redox catalysts platinum, PtTiO 2 RuO 2 and PtCdS in colloidal systems showed approximately the same activity towards the reduction of water. The reduction of MV 2+ and the evolution of hydrogen were enhanced at higher temperature (70 °C). The optimum conditions for water reduction on redox catalysts in colloidal systems under continuous illumination are analysed.

Anomalies in the linear absorption, transient absorption, photoluminescence and photoluminescence excitation spectroscopies of colloidal InP quantum dots

Abstract We report photoluminescence (PL), linear absorption and femtosecond, transient bleaching spectra for a colloidal solution of indium phosphide (InP) quantum dots (QDs) at ambient temperature. The PL quantum yield is shown to depend significantly upon the excitation wavelength and the PL excitation spectrum deviates markedly from the absorption spectrum. The cooling of electrons and holes to the lowest energy excited state is determined, by transient bleaching spectroscopy, to occur with an efficiency that is independent of the excitation wavelength. These results are discussed in terms of a threshold for a non-radiative decay process that resides above the bandgap and a PL quantum yield that depends upon the size of QD.

GaInP2 overgrowth and passivation of colloidal InP nanocrystals using metalorganic chemical vapor deposition

We have used metalorganic chemical vapor deposition to deposit thin GaInP2 passivating films on both isolated and close-packed arrays of colloidal InP/GaInP2 core-shell nanocrystals. Conformal GaInP2 layers of 10–20 nm were grown on the nanocrystals after organic capping molecule removal by a thermal annealing treatment. We show that the InP nanocrystals retain their crystallinity, shape and luminescence efficiency after being exposed to growth temperatures of 600 °C. The GaInP2 nanocrystal composite showed strong photoluminescences indicating effective passivation of surface states. In close-packed nanocrystal arrays, the emission band is redshifted compared to films of isolated nanocrystals indicating electron coupling between dots embedded in GaInP2.