The broad goal of this proposal is to explore the interplay between localized high-spin states of an individual molecule and conduction electrons in order to develop molecular electronic devices for local magnetic field sensing, ultra-high-density information storage, and quantum information processing. Single-molecule magnets are characterized by a large total spin and a strong intrinsic anisotropy. They present some unique characteristics, such as quantum tunneling of the magnetization and Berry phase interference. Although extensively studied in crystalline form, some of their key properties remain elusive. For instance, it is unclear how quantum tunneling of the magnetization influences electronic conduction through these molecules. Understanding this property is crucial for any electronic device development. The proposed research program addresses these issues by combining chemical synthesis with experimental and theoretical physics to probe quantum properties of isolated single-molecule magnets. The molecules will be attached to nanometer-gapped metal electrodes and gated electrically to form a single-electron transistor. Device fabrication will make use of lithographic and electromigration techniques. The molecule?s electric conduction will be studied both statically and dynamically to reveal excited molecular states, the effect of different ligands, the Kondo effect, spin-polarized transport, the Berry-phase blockade, quantum oscillations of the magnetization, and decoherence,. The proposed study emphasizes exploring these phenomena toward practical devices. In particular: (i) to employ the intense magnetic field tunability of the Berry phase to obtain high-sensitivity local magnetic field nanosensors; (ii) to develop reading and writing procedures for molecular bits in high-density magnetic memories; and (iii) to demonstrate quantum logic gate operations in a molecular qubit. The team has extensive experience with single-molecule magnets and in quantum electronic transport. Preliminary results have demonstrated the team?s ability to fabricate suitable devices and to measure the IV characteristics of isolated molecules in the Coulomb blockade regime. Available facilities permit efficient device fabrication with a short turnover time. The facilities available to the team include low temperatures, high magnetic fields oriented in arbitrary directions, continuous-wave and pulsed high-frequency microwave excitations, and ultra-fast pulsed voltage gating.

Intellectual Merit: Molecular electronics is rapidly becoming a separate research field within Applied Sciences and Engineering. The main effort so far has been on carbon-based systems or isotropic molecules containing a small net spin. This proposal focuses on molecules that are intrinsically magnetic due to their large spin and strong axial anisotropy. The research encompasses chemistry, physics, device fabrication and development, as well as fundamental studies at low temperatures and high magnetic fields. The proposed studies will lead to a better understanding of the quantum properties of isolated single-molecule magnets and how magnetism can be combined with electronic transport in a single-electron transistor setup.

Broader Impact: The proposal will advance our knowledge of single molecule-based electronic devices. These devices have great potential for ultra-high density integration and quantum information processing, which may lead to new and revolutionary technologies. Several graduate and undergraduate students will be trained in the interface between inorganic chemistry and fundamental and applied physics within an environment that constantly crosses the boundaries of these disciplines.

Project Report

Magnetic materials are a multi-billion annual industry worldwide and impact almost every aspect of a modern technological society. They find numerous uses in a wide range of applications, and they are a huge industry in the USA. Current trends in miniaturization of magnetism-based devices have made magnetism a major sub-discipline of nanoscience, and alternative routes to extremely small magnets of nanoscale dimensions (i.e., nanomagnets) are desirable. The availability of molecules that can function as individual nanoscale magnets, so-called single-molecule magnets (SMMs), has opened up their application in new technologies. In the present research, the focus has been on their use as individual components in spintronics-related systems by the collaborating physics group at the University of Central Florida. Emphasis has been on small Mn3 and Mn4 SMMs that are easy to prepare in large amounts and whose peripheral ligands can be readily modified by standard chemical methods. To enhance the ability of these molecules to bind to graphene surfaces and to straddle nano-gaps between gold nanoelectrodes, a family of modified SMMs were prepared and characterized structurally. The main concentration of effort was in introducing aromatic ligands of varying size, from simple benzoate to the highly conjugated pyrenecarboxylic acid, and with differing flexibilities. These combined modifications provide a family of related SMM molecules differing in the total size dimensions and the extent of pi conjugation, allowing deposition on graphene surfaces with different interaction strengths, and in nanogaps of varying spacings. These applications were also assisted by the high solubility of the Mn3 and Mn4 SMMs bearing such aromatic ligands, facilitating their use by the physics collaborators for their studies. The P.I. and his group also carried out a range of broader impact activities throughout the funding period, spanning education, domestic and international conference organization, and outreach. The P.I.’s group supported the planning and execution of the chemistry demonstrations and other activities in the annual Chemistry Day at the Mall in Gainesville, FL, targeted at K-12 students, their teachers and parents, and the local community and media. This is directed at increasing the public’s awareness of the impact of chemistry in our daily lives, and to foster greater awareness and interactions between the chemistry department and the local community. The P.I. had two undergraduates in his group at all times getting research experience, and hosted each year a Florida high-school student for summer research as part of the University of Florida’s Student Science Training Program (SSTP). The P.I. organized two conferences a year for each of the years of NSF support. He organized the annual "Florida Inorganic and Materials Symposium" (FIMS) student meetings of Florida higher education institutions spanning fourteen undergraduate colleges, community colleges, and PhD granting universities, including the historically black Florida A&M university. The P.I. also co-organized two biennial international workshops, the "Current Trends in Molecular and Nanoscale Magnetism (CTMNM)" meeting ( Florida, 2010 and Chalkidiki, Greece, 2012), a specialized magnetism workshop spanning chemistry and physics, and the broad "North America-Greece-Cyprus Workshop on Paramagnetic Materials (NAGC)" (Patras, Greece, 2011). Both the CTMNM and NAGC workshops targeted and included many oral presentations from students and postdoctorals. Both conferences bring together chemists and physicists interested in the use of nanoscale magnets, of both the traditional and molecular variety, in a number of studies and applications, and in many cases fostered new collaborative ventures.

National Science Foundation (NSF)
Division of Electrical, Communications and Cyber Systems (ECCS)
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Usha Varshney
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University of Florida
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