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«A Dissertation Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of ...»

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THE ELECTRONIC AND OPTICAL PROPERTIES OF COLLOIDAL LEAD-

SELENIDE SEMICONDUCTOR NANOCRYSTALS

A Dissertation

Presented to the Faculty of the Graduate School

of Cornell University

In Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

by

Jeffrey Matthew Harbold

January 2005

© 2005 Jeffrey Matthew Harbold

THE ELECTRONIC AND OPTICAL PROPERTIES OF COLLOIDAL LEAD-

SELENIDE SEMICONDUCTOR NANOCRYSTALS

Jeffrey Matthew Harbold, Ph. D.

Cornell University 2005 Quantum dots of the IV-VI semiconductors, and specifically lead selenide, strongly confine both electrons and holes, leading to a dramatic modification of the bulk semiconductor properties. This dissertation is devoted to the study of the electronic and optical properties of colloidal lead-selenide nanocrystals or quantum dots.

We begin by discussing the synthesis and characterization of high-quality colloidal lead-selenide nanocrystals with a narrow size distribution and well- passivated surfaces. With diameters between 3 and 8 nanometers, these lead- selenide quantum dots exhibit size-quantized transitions in the infrared region of the electromagnetic spectrum and exhibit bright band-edge photo- luminescence tunable from approximately 1000 to 2000 nanometers. These properties are extremely promising for applications.

The current theoretical understanding of the electronic states of IV-VI semiconductor quantum dots is based on envelope function approaches and tight-binding methods. While successful in explaining many features of the electronic structure, all current calculations fail to explain the presence of additional peaks in the optical absorption spectrum of lead-selenide and lead- sulfide quantum dots. We re-examine the leading explanations for these unexplained transitions and also consider a new possibility, that of enhanced electric quadrupole transitions.

In addition, the degeneracy of the lowest optical transition in IV-VI quantum dots is predicted to split by the intervalley coupling of the 4 equivalent L-valleys in the first Brillouin zone. Low-temperature photo- luminescence and size-selective photoluminescence experiments reveal, for the first time, a splitting in the emission spectra of lead-selenide and lead- sulfide nanocrystals. These observations are consistent with a theoretical treatment of the splitting of the lowest transition in lead-selenide quantum dots due to intervalley coupling.

The dynamics of electrons and holes are crucially influenced by quantum confinement. In the strong confinement limit, a dramatic reduction in the excited state (or intraband) relaxation rate of carriers is predicted to occur.

With its sparse electronic states and simple energy spectra, lead-selenide quantum dots represent an ideal material system in which to study the intraband carrier relaxation. We present the first measurements to directly time-resolve the intraband relaxation of electrons and holes in lead-selenide nanocrystals. Prior theories cannot explain the observed picosecond time-scale intraband relaxation and we discuss several possible explanations.

BIOGRAPHICAL SKETCH

Jeffrey Harbold was born in York, Pennsylvania in 1975 to Larry and Linda Harbold. Jeffrey first revealed his future scientific aspirations when, after a classroom demonstration by his second grade teacher Mrs. King, he announced to his parents, “I love science”. Following high school, Jeffrey moved to Rochester, New York to attend the Rochester Institute of Technology from which he earned his B.S. in Physics in 1998. Still loving the climate in the Northeast, Jeffrey then entered the Ph.D. program in Applied Physics at Cornell University in Ithaca, New York. He joined the Wise research group in the summer of 1999 and spent the subsequent years learning about ultrafast lasers, nonlinear optics, and semiconductor quantum dots.

Having surpassed his tolerance for snow and cold, Jeffrey will trade in his snow boots for flip-flops in January, 2005 as he pursues career and adventure on the beaches of Southern California.

–  –  –

First, I thank my advisor Frank Wise for his encouragement and advice. His hands-on style of management and way of showing me how to “do science” cannot be understated. I could not have asked for a better mentor and I am indebted to him. I also thank Roger Loring and Joel Brock for serving on my special committee.

My work over these past years, has largely been the result of fruitful collaborations. On the nonlinear glass project (not mentioned elsewhere in this dissertation), I thank J. Sanghera and I. Aggarwal of the Naval Research Laboratories, B. Aitken and N. Borrelli of Corning, Inc., and T. Birks and W.

Wadsworth of the University of Bath for providing excellent samples and participating in many helpful scientific discussion. I am very grateful to K. Du and T. Krauss at the University of Rochester for their role, from day-one, in the quantum dot project and also for giving me access to their laboratories on numerous occasions. I thank K.-S. Cho and C. Murray of the IBM T. J. Watson Research Center for providing quantum dot samples and all the folks at Evident Technologies for supporting our quantum dot work both directly and indirectly.





I am fortunate to have spent my time at Cornell in the company of such wonderful companions in the Wise group. I thank: Sylvia S., for eagerly giving me the “keys” to my first laser and for all the wisdom she imparted to me while rushing to complete her degree; Kale B., for endless scientific discussions and hours upon hours spent contemplating our current and future lives over cups of coffee, bottles of Coke, and donuts too numerous to count;

Omer I., for assistance in computer modeling on the tapered fiber project and

–  –  –

single screw; P. Fischer, for his wealth of scientific knowledge, his optimism, and also the reminders that should we both become disillusioned with work at some point in the future, we always have our secret get-rich scheme to fall back on; Yi-Fan C., for his enthusiasm, his sense of humor, and his more recent role as career counselor; Hyungsik L, for our numerous discussions on work, careers, the arts, and life; Dave H., for sharing enjoyment of CHEM 794 problem sets with me and for usually trumping any problem that I had in the lab with something much worse; Stephen C. and Yul H. for their assistance on the quantum dot work; Jeff M., for Monday Night Football; and in no particular order Joel B., Alexis W., Lyuba K., Andy C., and Shian Z. in addition to those listed above, for making the Wise group a stimulating and fun place to be.

My successes are one testament to the vigilance and hard work of numerous teachers (formal and informal) of whom I have been blessed to be a student: M. King, C. Rawlinson, P. Mentzer, R. D. Lehman, T. Frey, T.

DeChristopher, J. Andersen, E. Heilweil, and J. Shortall.

Lastly, I thank my family: Mom, Dad, Dan, grandparents, aunts and uncles. Without their love and support, I could not have achieved this goal.

And after years of their asking again and again, it is with relief that I finally can answer, “now I’m done”.

–  –  –

1 Introduction

1.1 Strong confinement in semiconductor nanocrystals

1.2 Organization of the dissertation

References

2 The synthesis and characterization of colloidal PbSe nanocrystals...............14

2.1 Introduction

2.2 Synthesis

2.3 Experimental results

2.4 Conclusions

2.5 Acknowledgements

References

3 The assignment of the second optical transition

3.1 Introduction

3.2 Prior work

3.3 Parity-forbidden transitions between isotropic states versus parityallowed transitions between anisotropy-induced states

3.4 Quadrupole transitions

3.5 Conclusions

References

4 Photoluminescence studies of lead-salt nanocrystals

4.1 Introduction

4.2 Sample preparation and initial characterization

4.3 Global photoluminescence measurements

4.4 Size-selective photoluminescence measurements

4.5 Discussion

4.6 Conclusions

References

5 Time-resolving the intraband relaxation of strongly-confined electrons and holes in colloidal PbSe nanocrystals

5.1 Introduction

5.2 Experiment

5.3 Discussion

5.4 Conclusions

5.5 Acknowledgements

References

vii6 Future directions

7 Appendix

Interband transition quadrupole matrix element for lead-salt quantum dots

References

–  –  –

Figure 1.1: The general form of the bulk band structures and the electronic states that result from the making a QD from (left:) II-VI and III-V semiconductors and (right:) IV-VI semiconductors…………………….

………..4 Figure 1.2: The general form of the first few electron and hole energy levels of PbSe or PbS QDs. The states are labeled (right side) by the quantum numbers j (total angular momentum) and p (parity). An alternative scheme (left side) labels the states by the orbital angular momentum of the envelope function.

The lowest several dipole-allowed transitions are also depicted……………….6 Figure 1.3: The direct band gap of the IV-VI semiconductors occurs at the 4 equivalent L-points of the Brillouin zone………………………………………….8 Figure 2.1: X-ray diffraction pattern of PbSe QDs indexed to the bulk-rock salt crystal structure. The peaks labeled (*) are due to trace amounts of the unreacted precursor, lead oleate………………………………………………….18 Figure 2.2: Low-resolution STEM micrograph of PbSe QDs (full scale= 320 nm) showing the ordering of an ensemble of NCs…………………………………...20 Figure 2.3: High-resolution STEM micrograph of a PbSe QD (full scale= 6.4 nm) revealing lattice imaging of the NC………………………………………....21 Figure 2.4: Linear absorption spectra of PbSe QDs ranging in diameter from 3 to 8 nm……………………………………………………………………………….23 Figure 2.5: Linear absorption spectrum of ~8 nm PbSe QDs with theoretical calculations15 of the allowed one-photon transitions (gray bars). The location of parity-forbidden transitions among the states found by Kang are indicated (dashed arrows) as are the new states arising from the inclusion of the anisotropy of the bulk band structure (solid arrows).22…………………………24 Figure 2.6: Band edge luminescence from PbSe QDs excited at (bottom to top) 740 nm, 990 nm, and 990 nm. The measured quantum yields ranged from 12 to 81%…………………………………………………………………………………...26 ix Figure 2.7: Luminescence decay for 3-nm PbSe QDs and a single exponential fit to the data (250-ns time constant). The instrument response to the excitation pulse is much shorter than the measured decay (1.1 ns FWHM)……………...27 Figure 3.1: Room temperature linear absorption spectra of PbSe QDs in tetrachloroethylene7 and PbS QDs in glass5. The bars indicate the strength of the dipole-allowed transitions calculated in the 4-band envelope function formalism (normal labels).1 The dashed arrows indicate the predicted energies of the two lowest dipole-forbidden transitions (italicized labels)……………..34 Figure 3.2: The energy of the several lowest optical transitions in PbSe QDs6, 7, 11 as determined from room-temperature linear absorption spectra. The scaling of the axes is described in the text. The horizontal lines indicate the predicted energies of dipole-allowed transitions and dipole forbidden transitions within the isotropic model1 along with parity-allowed transitions within the anisotropic model.2…………………………………………………….37 Figure 4.1: Comparison of the absorption and photoluminescence spectra in solid state film samples to colloidal samples. The solid state film absorption has been corrected for scatter by subtracting a 1/l4 background. The peak in film absorption at ~ 0.9 eV is from oleic acid…………………………………….49 Figure 4.2: The peak of the photoluminescence spectrum shifts to lower energy and narrows with decreasing temperature. The excitation energy is 1.1 eV. The feature observed in all of the traces at 0.9 eV is an artifact of the grating…….52 Figure 4.3: Comparison of the low-temperature photoluminescence spectra from PbS solid state film and colloidal samples at an excitation energy of 1.27 eV. The energy position of the PL structure (~ 0.97 eV and ~ 1.15 eV) does not change with sample preparation, although the strength of these features does………………………………………………………………………………......53 Figure 4.4: The intensity dependence of the photoluminescence spectra was explored using CW and pulsed excitation. The measured spectra are nearly identical despite a difference in peak intensity of approximately 1010. Based on this observation, we rule out contributions from surface states……………….55 x Figure 4.5: Photoluminescence spectra from PbSe QD solid state films at T = 15 K. The dashed curves are the measured PL spectra and the solid curves are the best-fit two-Gaussian decomposition of the measured spectra………………..56 Figure 4.6: Line-narrowed photoluminescence from 4.7-nm PbSe at T = 15 K.



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