Rectification and magnetism in single molecule junctions
Author
Sherif, SiyaEntity
UAM. Departamento de Física de la Materia CondensadaDate
2015-11-18Funded by
This work has been supported by the European Union (FP7) through the program ELFOSSubjects
Átomos - Uniones - Tesis doctorales; Electrónica molecular - Tesis doctorales; FísicaNote
Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 18-11-2015Esta obra está bajo una licencia de Creative Commons Reconocimiento-NoComercial-SinObraDerivada 4.0 Internacional.
Abstract
The electronics industry has been consistently in demand to develop miniaturized
devices by scaling down the size of the electronic components. Gordon E.
Moore observed this trend in size reduction and proposed his famous Moore's
Law [1]. The present top-bottom approaches used in the production line are
reaching the theoretical limit. One way to overcome this limit is to use functionalized
molecules that can perform as electronic components [2]. Aviram
and Ratner rst proposed this concept in 1974 [3]. They suggested that a
molecule could act as a recti er, a primary electronic component that allows
the
ow of current only in one direction. Ever since researchers have been
trying to develop molecule based devices that could replace various electronic
components. Molecules are small and must be considered as quantum mechanical
systems, which opens the possibility of devices based on quantum
properties such as quantum computers, quantum interference devices, etc.
To develop such devices, it is necessary to understand the properties of the
molecules at the individual level. Measuring electronic transport across the
molecule is one of the methods to probe these quantum properties. This is
a challenging task that requires contacting the molecule with external electrodes.
A scanning tunnelling microscopy (STM) is an ideal tool for studying
electronics and magnetic properties of single molecules deposited on surfaces.
The main motivation of this thesis is to investigate recti cation and magnetism
in single molecule STM junctions at both room and cryogenic temperatures.
One of the prerequisites for single molecule experiments using STM is to have
a \good" sample, i.e., well- separated individual molecules spread on a
at
conducting substrate. The rst part of the thesis deals with a technique for depositing
the molecules on a suitable substrate. We have successfully deposited
K12(DyP5W30O110):nH2O molecules on an Au(111) substrate. This was not
an easy task due to the size of the molecule. In the second part we investigate
the recti cation observed in the molecule. The recti cation ratio attained for
this molecule is quite high, more than ten times larger than previously reported
results in molecular junctions. We could also explain unambitiously that the
recti cation in this molecular system is arising from the asymmetric coupling
of the electrodes and molecule. The third part of the thesis deals with the
magnetism in the same molecule. This molecule is single-ion molecular magnet
that exhibit magnetic properties at low temperature in bulk samples. We
addressed the following question: Does the molecule preserve its magnetic behaviour
when deposited on a metallic substrate? Inelastic spin
ip tunnelling
spectroscopy probing the magnetism in the individual molecules showed that
the molecules did preserve their magnetic properties on the surface. The last
part of the thesis, focuses on fabricating a superconducting Graphene substrate
that could eventually be used in single-molecule experiments. We show
that Graphene on lead becomes superconducting by proximity e ect. La industria de la electr onica ha demandado consistentemente desarrollar dispositivos
miniaturizados mediante la reducci on del tama~no de los componentes
electr onicos. Gordon E. Moore observ o esta tendencia en la reducci on del
tama~no y propuso su famosa Ley de Moore [1]. Las aproximaciones `topbottom'
actuales (de arriba-abajo o de lo macro a lo nano) utilizadas en la
l nea de producci on est an alcalzando el l mite te orico. Una forma de superar
este l mite consiste en utilizar mol eculas funcionalizadas que puedan actuar
como componentes electr onicos [2]. Aviram y Ratner propusieron este concepto
por primera vez en 1974 [3]. Sugirieron que una mol ecula puede actuar
como un recti cador, un componente electr onico primario que permite el
ujo de corriente s olo en una direcci on. Se ha intentado desde entonces desarrollar
dispositivos basados en mol eculas que pudieran reemplazar diversos
componentes electr onicos. Las mol eculas son peque~nas y deben considerarse
como sistemas mecanocu anticos, lo que ofrece la posibilidad de desarrollar
dispositivos basados en propiedades cu anticas como ordenadores cu anticos,
dispositivos de interferencia cu antica, etc. Para desarrollar dichos dispositivos,
es necesario entender las propiedades de las mol eculas a nivel individual.
La medida de transporte electr onico a trav es de una mol ecula es uno
de los m etodos para explorar estas propiedades cu enticas. Este es un reto
importante que requiere contactar la mol ecula con electrodos externos. Un
microscopio de efecto t unel (STM) es una herramienta ideal para estudiar las
propiedades electr onicas y magn eticas de mol eculas individuales depositadas
sobre super cies. La principal motivaci on de esta tesis es investigar recti -
caci on y magnetismo en uniones moleculares individuales formadas con un
STM a temperaturas ambiente y criog enicas.
Uno de los requisitos para realizar experimentos en una unica mol ecula utilizando
un STM es tener una \buena" muestra, lo que signi ca tener mol eculas
bien separadas unas de otras distribuidas sobre un sustrato conductor plano.
La primera parte de la tesis trata sobre la t ecnica para la deposici on de
mol eculas en un sustrato adecuado. Hemos depositado satisfactoriamente
mol ecuals de K12(DyP5W30O110):nH2O sobre un sustrato de Au(111), aunque
no fue una tarea sencilla debido al tama~no de la mol ecula. En la segunda parte
investigamos la recti caci on observada en dicha mol ecula. La ratio de recti-
cation que alcanza es bastante elevada, m as de diez veces mayor que los
resultados de uniones moleculares publicados previamente. Tambi en hemos
podido explicar inequ vocamente que la recti caci on en este sistema molecular
surge del acoplamiento asim etrico de los electrodos y la mol ecula. La
tercera parte de la tesis trata sobre el magnetismo de esta misma mol ecula,
la cual es un im an molecular con un unico i on que presenta propiedades
magn eticas a baja temperatura en muestras macrosc opicas. Por este motivo
nos planteamos la siguiente pregunta: > conserva la mol ecula su comportamiento
magn etico cuando se deposita sobre un sustrato met alico? Espectroscop
a inesl astica de esp n (inelastic spin tunneling spectroscopy) para
explorar el magnetismo de mol eculas individuales mostr o que las mol eculas s
conservaban sus propiedades magn eticas en la super cie. La ultima parte de
la tesis se centra en la fabricaci on de un sustrato de grafeno superconductor
que podr a utilizarse en experimentos de mol eculas individuales. Mostramos
que el grafeno sobre plomo se vuelve superconductor por efecto de proximidad
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