Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

UCPL process would not show such a temperature dependence. (2) TheUCPL occurs at very low light intensity, and the lifetime of the surface statewould have to be in the millisecond region to permit a stepwise two-photonprocess; the lifetime of the trap emission was measured to be in the ns ranger.(3) Although the energy required to upconvert the hole is in the range of 300meV, which is large compared to the bulk phonon energies of InP and mightimply the need for many phonons, surface phonons associated with hydrogen-bondingto P surface atoms can be as large as 300 meV [56]; thus, only oneor two phonons may be required for the upconversion if hydrogen orequivalent type of bonding is associated with the P-DB (such bonding couldarise from the chemical treatment processes for the QD colloids). (4) Becausethe UCPL line shape does not show a peak but rather a continuous rise, thismeans the In-DP state has an energy distribution and the Boltzmann factorfor the upconverted hole population in the UVB is not determined by the 300-meV gap. (5) Phonon localization into surface defects is a known process [57]and may contribute to the high relative efficiency of the UCPL process.Recent ODMR [12] and EPR experiments [48] support the model forUCPL discussed earlier. The EPR results show that a nonradiative, permanenthole trap near the valence band develops at the surface of InP QDs afterthey have been exposed to light and aged. The EPR signal from this hole trapis removed upon electron injection into the QDs from sodium biphenyl andthe UCPL is also quenched. The EPR signal from the hole trap is also absentin freshly prepared InP QDs, as is UCPL. The EPR results also show that anelectron trap at the surface is present in untreated InP QDs and that this trapis removed by HF treatment; this result is consistent with the ODMR resultsshowing surface electron traps attributed to phosphorus vacancies.D. Photoluminescence BlinkingFluorescence intermittency (PL blinking) in single QDs has been observed inboth colloidal nanocrystals [58–64] and in epitaxially grown quantum dots[65–67]. The effect is manifested as intermittent photoluminescence with thetime between light emission being on and off, varying from 10 ms to 100 s; theFigure 12 (a) Plot of conduction-band minimum (CBM), valence-band maximum(VBM), In dangling bond (In-DB), and P dangling bond (P-DB) energy levels as afunction of QD size (solid lines). Data points are the sum of the experimental anti-Stokes shift plus the absolute calculated VBM energy; the data points agree with thepredicted plot, which is equivalent to P-DB energy minus (CBM In-DB). At 15 A˚ ,the anti-Stokes shift is predicted to be zero, and this agrees with the experimentalresult. (From Ref. 51.)Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.