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Title: LDRD-LW Final Report: 07-LW-041 "Magnetism in Semiconductor Nanocrystals: New Physics at the Nanoscale"

Abstract

The work conducted in this project was conducted with the aim of identifying and understanding the origin and mechanisms of magnetic behavior in undoped semiconductor nanocrystals (NCs), specifically those composed of CdSe. It was anticipated that the successful completion of this task would have the effect of addressing and resolving significant controversy over this topic in the literature. Meanwhile, application of the resultant knowledge was expected to permit manipulation of the magnetic properties, particularly the strength of any magnetic effects, which is of potential relevance in a range of advanced technologies. More specifically, the project was designed and research conducted with the goal of addressing the following series of questions: (1) How does the magnitude of the magnetism in CdSe NCs change with the organic molecules used to passivate their surface the NC size? i.e. Is the magnetism an intrinsic effect in the nanocrystalline CdSe (as observed for Au NCs) or a surface termination driven effect? (2) What is the chemical (elemental) nature of the magnetism? i.e. Are the magnetic effects associated with the Cd atoms or the Se atoms or both? (3) What is/are the underlying mechanism(s)? (4) How can the magnetism be controlled for further applications? To achievemore » this goal, several experimental/technical milestones were identified to be fulfilled during the course of the research: (A) The preparation of well characterized CdSe NCs with varying surface termination (B) Establishing the extent of the magnetism of these NCs using magnetometry (particularly using superconducting interference device [SQUID]) (C) Establishing the chemical nature of the magnetism using x-ray magnetic circular dichroism (XMCD) - the element specific nature of the technique allows identification of the element responsible for the magnetism (D) Identification of the effect of surface termination on the empty densities of states (DOS) using x-ray absorption spectroscopy (XAS), with particular emphasis on elucidating small changes in the d-electron count. Characterizing changes in the d-electron density can yield important insight into the mechanisms of magnetism in materials. As the three attached manuscripts illustrate (presented in preprint form to ensure no infringement of copyright), each of these milestones was successfully illustrated and the results published in the scientific literature during the course of the project. The research team members were able to determine, from a series of XAS, XMCD and SQUID magnetometry measurements, that CdSe NCs are paramagnetic and that the magnitude of magnetic susceptibility is dependent upon the type of organic molecule used to passivate the NC surface (i.e. the observed magnetism results, at least in part, from a surface effect that is not intrinsic to the NCs). In addition, they identified that the mechanism by which the magnetic susceptibility is modified - via {pi} back-donation of d-electrons to the organic ligands from the Cd atoms. These findings demonstrate that the magnetic properties are related to the surface Cd atoms and illustrate the means by which the magnetic behavior can be manipulated for specific technological applications. Two of the papers published during the course of the LW project do not contain magnetometry data, but focus on the evolution in electronic structure of the CdSe NCs as a function of particle size. These measurements were crucial in developing an understanding of the electronic behavior of the NCs and, ultimately, in assigning the p back-donation mechanism for inducing controllable paramagnetic behavior. Significantly, the research team has also filed a patent application based upon their research: 'Method for Creating Ligand Induced Paramagnetism in Nanocrystalline Structures' Docket: IL-11858. It is noted that both LDRD-LW and Office of Basic Energy Sciences (OBES) funding is acknowledged in the attached manuscripts. As such, is important to indicate that funds were not comingled during the course of the project. Some of the experimental data presented in the manuscripts was collected prior to the LDRD-LW award by Dr. Meulenberg and Dr. Lee, who were supported by an OBES award at the time. This data was, in part, used by Dr. Meulenberg in his proposal/presentation for the LDRD-LW competition. Moreover, the vast majority of data presented in the three papers was collected at OBES-funded synchrotron facilities which necessitated an acknowledgement to OBES.« less

Authors:
; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
971408
Report Number(s):
LLNL-TR-418378
TRN: US1001312
DOE Contract Number:  
W-7405-ENG-48
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTROSCOPY; ATOMS; AWARDS; ELECTRONIC STRUCTURE; MAGNETIC CIRCULAR DICHROISM; MAGNETIC PROPERTIES; MAGNETIC SUSCEPTIBILITY; MAGNETISM; ORIGIN; PARAMAGNETISM; PARTICLE SIZE; PHYSICS; SYNCHROTRONS

Citation Formats

Meulenberg, R W, Lee, J I, and McCall, S K. LDRD-LW Final Report: 07-LW-041 "Magnetism in Semiconductor Nanocrystals: New Physics at the Nanoscale". United States: N. p., 2009. Web. doi:10.2172/971408.
Meulenberg, R W, Lee, J I, & McCall, S K. LDRD-LW Final Report: 07-LW-041 "Magnetism in Semiconductor Nanocrystals: New Physics at the Nanoscale". United States. https://doi.org/10.2172/971408
Meulenberg, R W, Lee, J I, and McCall, S K. 2009. "LDRD-LW Final Report: 07-LW-041 "Magnetism in Semiconductor Nanocrystals: New Physics at the Nanoscale"". United States. https://doi.org/10.2172/971408. https://www.osti.gov/servlets/purl/971408.
@article{osti_971408,
title = {LDRD-LW Final Report: 07-LW-041 "Magnetism in Semiconductor Nanocrystals: New Physics at the Nanoscale"},
author = {Meulenberg, R W and Lee, J I and McCall, S K},
abstractNote = {The work conducted in this project was conducted with the aim of identifying and understanding the origin and mechanisms of magnetic behavior in undoped semiconductor nanocrystals (NCs), specifically those composed of CdSe. It was anticipated that the successful completion of this task would have the effect of addressing and resolving significant controversy over this topic in the literature. Meanwhile, application of the resultant knowledge was expected to permit manipulation of the magnetic properties, particularly the strength of any magnetic effects, which is of potential relevance in a range of advanced technologies. More specifically, the project was designed and research conducted with the goal of addressing the following series of questions: (1) How does the magnitude of the magnetism in CdSe NCs change with the organic molecules used to passivate their surface the NC size? i.e. Is the magnetism an intrinsic effect in the nanocrystalline CdSe (as observed for Au NCs) or a surface termination driven effect? (2) What is the chemical (elemental) nature of the magnetism? i.e. Are the magnetic effects associated with the Cd atoms or the Se atoms or both? (3) What is/are the underlying mechanism(s)? (4) How can the magnetism be controlled for further applications? To achieve this goal, several experimental/technical milestones were identified to be fulfilled during the course of the research: (A) The preparation of well characterized CdSe NCs with varying surface termination (B) Establishing the extent of the magnetism of these NCs using magnetometry (particularly using superconducting interference device [SQUID]) (C) Establishing the chemical nature of the magnetism using x-ray magnetic circular dichroism (XMCD) - the element specific nature of the technique allows identification of the element responsible for the magnetism (D) Identification of the effect of surface termination on the empty densities of states (DOS) using x-ray absorption spectroscopy (XAS), with particular emphasis on elucidating small changes in the d-electron count. Characterizing changes in the d-electron density can yield important insight into the mechanisms of magnetism in materials. As the three attached manuscripts illustrate (presented in preprint form to ensure no infringement of copyright), each of these milestones was successfully illustrated and the results published in the scientific literature during the course of the project. The research team members were able to determine, from a series of XAS, XMCD and SQUID magnetometry measurements, that CdSe NCs are paramagnetic and that the magnitude of magnetic susceptibility is dependent upon the type of organic molecule used to passivate the NC surface (i.e. the observed magnetism results, at least in part, from a surface effect that is not intrinsic to the NCs). In addition, they identified that the mechanism by which the magnetic susceptibility is modified - via {pi} back-donation of d-electrons to the organic ligands from the Cd atoms. These findings demonstrate that the magnetic properties are related to the surface Cd atoms and illustrate the means by which the magnetic behavior can be manipulated for specific technological applications. Two of the papers published during the course of the LW project do not contain magnetometry data, but focus on the evolution in electronic structure of the CdSe NCs as a function of particle size. These measurements were crucial in developing an understanding of the electronic behavior of the NCs and, ultimately, in assigning the p back-donation mechanism for inducing controllable paramagnetic behavior. Significantly, the research team has also filed a patent application based upon their research: 'Method for Creating Ligand Induced Paramagnetism in Nanocrystalline Structures' Docket: IL-11858. It is noted that both LDRD-LW and Office of Basic Energy Sciences (OBES) funding is acknowledged in the attached manuscripts. As such, is important to indicate that funds were not comingled during the course of the project. Some of the experimental data presented in the manuscripts was collected prior to the LDRD-LW award by Dr. Meulenberg and Dr. Lee, who were supported by an OBES award at the time. This data was, in part, used by Dr. Meulenberg in his proposal/presentation for the LDRD-LW competition. Moreover, the vast majority of data presented in the three papers was collected at OBES-funded synchrotron facilities which necessitated an acknowledgement to OBES.},
doi = {10.2172/971408},
url = {https://www.osti.gov/biblio/971408}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Oct 19 00:00:00 EDT 2009},
month = {Mon Oct 19 00:00:00 EDT 2009}
}