Energy Level Alignment and Electron Transport Through Metal/Organic Contacts: From Interfaces to Molecular Electronics

210 Pages·2013·5.88 MB·English

by  Enrique Abad (auth.)

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Enrique Abad
Energy Level Alignment
and Electron Transport
Through Metal/Organic
Contacts
From Interfaces to Molecular Electronics
Doctoral Thesis accepted by
the Autonomous University of Madrid, Spain
123
Author Supervisor
Dr. EnriqueAbad Prof.Dr. FernandoFlores Sintas
Autonomous Universityof Madrid Autonomous Universityof Madrid
Madrid Madrid
Spain Spain
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-642-30906-9 ISBN 978-3-642-30907-6 (eBook)
DOI 10.1007/978-3-642-30907-6
SpringerHeidelbergNewYorkDordrechtLondon
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(cid:2)Springer-VerlagBerlinHeidelberg2013
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To my parents,
it could not have been anybody else
Inspect every piece of pseudoscience and
you will find a security blanket, a thumb
to suck, a skirt to hold. What does the
scientist have to offer in exchange?
Uncertainty! Insecurity!
Isaac Asimov
Nature is a merciless and harsh judge of
the theorist’s work. In judging a theory, it
never rules ‘‘Yes’’ in best case says
‘‘Maybe’’, but mostly ‘‘No’’. In the end,
every theory will see a ‘‘No’’
Albert Einstein
All our science, measured against reality,
is primitive and childlike—and yet it is
the most precious thing we have
Albert Einstein
Supervisor’s Foreword
The growing field of organic electronics relies on the use of organic conjugated
molecules as components of multilayer devices, whose performance depends
critically on the energy barriers controlling the carrier transport between layers.
Those barriers are determined by the relative alignment of the molecular levels
betweenthematerialsformingthecontacts,eitheraMetal/OrganicoranOrganic–
Organic interface. This thesis work is focused on analyzing theoretically the
energy level alignment of different Metal/Organic interfaces.
The organic molecule/metal interaction is, in principle, amenable to density
functional theory (DFT)calculations;however, using local density approximation
(LDA)orgeneralizedgradientapproximation(GGA)fortheexchange-correlation
potentialtendstoyieldfortheorganicmoleculetransportenergygapsthataretoo
small by several eVs. This problem is related to the fact that Kohn–Sham eigen-
values (as calculated in those approximations) are not a proper representation of
thequasiparticleexcitationenergy:inconjugatedorganicmoleculesthedifference
between the HOMO and LUMO Kohn–Sham eigenvalues is significantly smaller
than the transport energy gap measured experimentally due to the self-interaction
correction that arises because the Kohn–Sham LDA–DFT (or GGA–DFT) gap
referstoaN-electroncalculation,whiletheexperimentaltransportgapisrelatedto
thetotalenergydifferencesofthemoleculegroundstateswithN+1,NandN-1
electrons.
AnotherproblemlimitingtheapplicationofDFTtoMetal/Organicinterfacesis
relatedtotheimportanceinthesesystemsofvanderWaalsinteractionswhichare,
in many cases, necessary to get an accurate description of the organic molecule
adsorptiondistanceandenergy.ThereasonisthatvanderWaalsisanon-localand
long-range interaction, while exchange-correlation functionals in standard DFT
methodsarelocalandshortrange,withatypical exponentialdecay.Therefore,in
many Organic/Metal interfaces one has to introduce corrections to standard DFT
methods in order to get an accurate description of the Metal/Organic interaction.
Inthisthesiswork,thesetwoproblems havebeenaddressedintroducinginthe
standard LDA approach corrections in the direction of increasing the organic
molecule energy gap and of including an appropriate van der Waals interaction.
ix
x Supervisor’sForeword
Regardingtheenergygapproblem,ithasbeenshownhowtheKohn–Shamenergy
gap of the molecule has to be corrected by its charging energy: this quantity is
calculated from the charge transfer between the metal and the molecule, and the
energy level shift of the LUMO and HOMO levels as obtained from a consistent
LDA calculation; in this consistent approach, the molecule energy gap is finally
calculatedbymeans ofascissoroperatorthatintroduces theappropriatecharging
energyinthoselevels.RegardingthevanderWaalsinteraction,akindofcorrected-
LDAcalculationhasbeenintroducedwhichcombinesaweakchemicalinteraction
between the metal and the organic molecule with a semiempirical van der Waals
attraction.Thewayofcalculatingtheweakchemicalinteractiontriestoavoidthe
doublecountingthatwouldappearincludingboththeexchangecorrelationenergy
provided by a conventional LDA approach and the correlation energy associated
with the long-range van der Waals potential introduced independently.
All these ideas have been combined in this thesis work in a full calculation of
different Metal/Organic interfaces; in particular the following systems have been
considered:C /Au(111),benzene/Au(111),pentacene/Au(111),TTF/Au(111)and
60
TCNQ/Au(111), at the molecular level and for an organic monolayer.
AlltheseresultshavealsobeeninterpretedwithintheUnified-InducedDensity
of Interface States (IDIS) model, whereby the Metal/Organic level alignment is
controlled by the potentials created by the charge transfer between the two
materialsandbythe‘‘pillow’’effectassociatedwiththePauliprincipleappliedto
the overlap between the charges of both materials. It has been found that the
Unified-IDISmodelisvalidnotonlyfortheMetal/Organicinterfacebutalsointhe
molecular limit, with a lonely molecule adsorbed on a metal.
Madrid, Spain, March 2012 Prof. Dr. Fernando Flores Sintas
Acknowledgments
It is curious that in all theses only one author is mentioned, when actually, they
comefromthecoordinatedworkofalotofpeople.Theauthoristheonethathas
contributed the most, of course, but this thesis would not have been successful
withoutthecollaborationofotherpeople.Thisistheplacetotakeallthesepeople
into account.
First, I would like to acknowledge this work to my Ph.D. advisors: Fernando
Flores and José Ortega. Fernando showed me the way a physicist thinks. It is
amazinghowthis‘‘physicalintuition’’thatprofessorstrysohardtoteachusduring
undergraduate courses, is innate in his case. His skill for taking the most relevant
parts of a physical problem, make reasonable approximations, and give some
physical sense to hours and hours of computational time is incredible; as it is his
dedicationtothiswork.Despitehislongcareer,hestillgetsexcited,likeafreshman
physicsundergraduatestudent,whenfacinganewproblem.Joséhastakencareof
the‘‘ends’’ofmyformation.Bothfromthepointofviewofthetechnicalissues—his
guidance through the FIREBALL code, and the physical meaning of its approxi-
mations—andfromthepointofviewofwhatbeingascientistmeans—tobecritical
withotherpeoples’work,andwiththeownwork;butalsoknowingthatalthougha
resultisnotexactorperfect,thisdoesnotmeanthatwecannotlearnalotaboutit.
Also,forhisaidduringmyvisittotheUSduringfall2008.
ThehelpfromCesarGonzálezhasalsobeencritical.Hehadtoanswerthehuge
quantity of questions that I made to him during my first two years of this thesis,
and he had a critical reading of the manuscript (moreover, he was my host in
Praguein2008).Also,ithasbeenhelpfultoknowPavelJelínek,andallhiswork
onFIREBALLandtoworkwithhimduringMay2008(andviaonlineduringthe
rest of my thesis). The work of Dani González both in FIREBALL and in ‘‘xeo’’,
as well as his guidance during the first year of my thesis is acknowledged too.
I also want to thank James Lewis, to introduce me in his group during fall 2008,
andallowmetocollaborateinthenewversionofthecode.Thecollaborationwith
JoséIgnacioMartíneztheselasttwoyearsisgreatlyacknowledged,aspartofthis
xi
xii Acknowledgments
work would not have been possible without him (and without Barbara). Also,
collaboration withYannickDappeinvanderWaalscalculationshasbeencritical.
Finally, I want to thank Pau Burset, for answering my ‘‘many-body’’ questions,
GiulioBiddauforhisparticularpointofviewofC physics,LauraCanoandher
60
thesis on organic conductors, which has helped me to put mine in perspective,
Milica for her advice on the ‘‘research world’’ outside graduate school and for
answering thousands of English language questions during my thesis writing, and
Laura Ramos for her help with the bureaucracy that research involves. And of
coursetoJoseAntonioIbáñez,myphysicsteacherinHighSchool,whodiscovered
me my vocation, and Nerea Zabala from the Basque Country University, who
introducedmetotheresearchworld.ThefinancialsupportfromtheConsejeríade
EducacióndelaComunidaddeMadrid,andtheFondoSocialEuropeothroughthe
‘‘Ayudas para la realización de contratos de personal investigador de apoyo’’
program, and EU through the FP7 project MINOTOR (Grant FP7-NMP-228424).
This research was also funded by the Comunidad de Madrid through projects
S2009/MAT-1467, 0505/ MAT/0303, and Ministerio de Ciencia e Innovación
through projectsMAT2007-60966 andFIS2010-16046.
But there have been more than research in my life during these years. A lot of
people have made my life more pleasant in the department. In particular, Floren,
César, Michael and Barbara, my office mates; and all the ‘‘fifth floor’’ of ‘‘C–V’’;
the ones that have been ‘‘always’’ there (Pau, Laura, Alfonso, Diego, Juan
Antonio, Linda, Stefan, Rafa…), the ones that left (Yannick, Antonio, Sebas,
Giulio,César,Remi,Dani…)andthe‘‘new’’people(Nacho,Milica,Delia,Lucía,
Blanca…). Concerning my life outside the department (there is life outside!) I
could not forget José Angel (we are the exception, where sharing a flat had made
usbetterfriendsinsteadofturningusintoenemies),Kike,Rocíoandtheircouples:
GemaandPedro.Peoplefrommyoldtheatertroupe(Gemma,Sara,Eva)andfrom
the new one (Nines, Pilar, Jose María, María, Sole, Alicia… and of course, Pilar,
‘‘the boss’’). And my present and past flatmates, and ‘‘Torredebabel’’ users:
Thomas, Josu, Guillaume, Michaela, Ruth, Maggie, Linda, Alessia, Roberto,
María, Natalia, Sheila, Gorka, Diego…. Thank you all for making my life more
satisfying in this city I have fallen in love with: Madrid.
I want to thank also the people who have been at my side, in spite of the
distance. My group of friends: Tere, Idoia, Nuria, Rober, María, Andrea, Tania,
Iratxe, Natxo…. in particular, my best friends: Igone and Edurne. I can’t imagine
my life without them, but I guess that it would have been less pleasant. Also,
people from San Ciprián (Lete, Félix, Dani… and Olalla, that right now is
‘‘another one’’). I want to thank Tanja for making 2010 nicer, and the first month
of thesis writing.
And the best for the end. The ones who my thesis is dedicated to: my parents.
Forraisingme,teachingmetheethicsofhardwork(withoutthat,Iwouldn’thave
been able to do this), and for supporting me in this adventure, even if this meant
thattheironlychildhadtoleavethenest.Iwanttothankalsomygrandmotherand
the rest of my family for their support and help.
At this point I only have to say: Thank you everybody!!!!