Optoelectronic Properties of Graphene-Based van der Waals Hybrids

277 Pages·2020·14.74 MB·English

by  Kallol Roy

Springer Theses
Recognizing Outstanding Ph.D. Research
Kallol Roy
Optoelectronic
Properties
of Graphene-Based
van der Waals Hybrids
Springer Theses
Recognizing Outstanding Ph.D. Research
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Kallol Roy
Optoelectronic Properties
of Graphene-Based van der
Waals Hybrids
Doctoral Thesis accepted by
Indian Institute of Science, Bangalore, India
123
Author Supervisor
Dr. Kallol Roy Prof. ArindamGhosh
Department ofPhysics Department ofPhysics
Indian Institute of Science Indian Institute of Science
Bangalore, India Bangalore, India
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-59626-2 ISBN978-3-030-59627-9 (eBook)
https://doi.org/10.1007/978-3-030-59627-9
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To
Maa, Baba, Rituparna, Rimjhim, family
and friends, Arindam da, and all teachers
’
Supervisor s Foreword
Light-matter interaction in a solid is at the heart of optoelectronics where incident
light causes the flow of electricity, or conversely, the charge carriers of opposite
signs combine to produce light. Efficient conversion of light into electricity, and
viceversa,areessentialrequirementsforphotovoltaicdevices,light-emittingdiodes
and optical sensors, and even the emerging domain of quantum technologies
through the realization of single photon emitters and detectors. While silicon and
other bulk semiconductors are traditional backbones of optoelectronic devices and
sensors, a quest for newer material aims to revolutionize our perception of func-
tional optoelectronics.
Thediscoveryofgraphenein2004,andsubsequentlythatoftheatomicallythin
semiconductors from transition metal chalcogenides in 2009, triggered an
unprecedented global activity in material science. Many of the chalcogenides
possessanoutstandingabilitytoabsorblightevenwhenthethicknessisreducedto
a single molecular layer. This led to excellent optoelectronic performance, and the
foundation of flexible optoelectronics was laid. Yet, the true paradigm shift in
devicefunctionalitycamewiththeadventofvanderWaalsheterostructures,where
atomically thin sheets of two or more dissimilar materials were brought within
sub-nanometer distance. Although originally the van der Waals heterostructures
were employed to enhance electronic transport in graphene transistors, it did not
takelongtorealizethatthisuniquebottom-upmaterialsynthesisprotocolcanyield
a new genre of devices with unprecedented performance and control. The Ph.D.
thesisofDr.KallolRoy,titled‘Optoelectronicpropertiesofgraphene-basedvander
Waalshybrids’,istheoneofthefirstdemonstrationsoftheimmensepowerofvan
der Waals heterostructures, where a binary hybrid of graphene and molybdenum
di-sulphide(MoS )isshowntobenearlytenordersofmagnitudemoreefficientin
2
converting light into electricity than typical bulk semiconductors.
The crucial ingredient of the light-matter interaction in graphene/MoS hybrids
2
is the inter-layer charge transfer across the interface when it is illuminated with
visiblelight.Theexceptionalphoto-responsivityarisesasaresultofmassiveoptical
gain because the transferred charge (electron) that contributes to current in gra-
phene,circulatesoverabilliontimesinthecircuitbeforeitcanrecombinewiththe
vii
viii Supervisor’sForeword
holes left behind in the MoS layer. The key novelty of the hybrid-based opto-
2
electronic architecture outlined in this thesis lies in that it combines the graphene
and the chalcogen layer to exploit the ‘best of both worlds’, namely, the high
electronicmobilityofgrapheneandthestronglightabsorptionbythechalcogenide
layer. The work also raises serious questions on the mechanism of charge transfer
acrossvanderWaalsinterfaces,whichisrelevantnotjusttooptoelectronics,butto
a plethora of other device concepts with van der Waals heterostructures.
When Dr. Kallol Roy started research for the Ph.D. degree the technology to
realize van der Waals heterostructure was at its infancy. It has thus been a rather
steep learning process, for both him and me (the beneficiaries of which include
subsequent generations of students and postdocs in my laboratory), motivated to
create heterostructures with clean atomic interfaces. The initial part of the thesis
provides a fascinating in-depth description of a purely homebuilt system, centered
aroundanexistingopticalmicroscope,thatperformsthistaskwithgreatcontroland
versatility. The fruits of this versatility are apparent in the latter part of the thesis
whereDr.Roybuildsfarmorecomplexheterostructureswithsequentialoverlaying
offiveormoreseparatelayers.Thepurposehasbeentoextendthecapabilityofhis
hybrid optoelectronic devices to the ultimate sensitivity limit and realize single
photon detection with these. By using gapped bilayer graphene to capture the
photo-generated electron transferred from MoS , and thereby greatly reducing the
2
dark current and noise, he demonstrates a photon counter with single-shot current
measurements. The thesis explains how such devices function as a
number-resolving single photon detector, which has been a precious goal for
secured quantum information transfer protocols.
Dr.KallolRoy’sthesisisanexcitingexpositionofhownovelmaterialsynthesis,
innovative instrumentation and careful measurements can be combined tonucleate
a new paradigm in on-chip material engineering. Configuring exotic device func-
tionalityandsimulating fundamental concepts with vanderWaalsheterostructures
have progressed in dramatic pace during the past few years. In that context, this
thesis will provide an early perspective to the newcomers as well as experts in the
field alike.
Bangalore, India Arindam Ghosh
Abstract
Light-matter interactions in atomically thin van der Waals materials have attracted
significant attention in the recent days [1–8]. Although the thickness does not
exceedfewnanometers,suchatomicallythinmaterialsaloneorincombinationwith
other nanostructures show exciting and unexpected photodetection properties [9–
18].
Fabrication of atomically sharp junctions can be achieved with 2D van der
Waalsheterostructures,whichsignificantlyenhancesthescopetodesignanewtype
of physical systems, where novel phenomena can be studied [17, 19–23].
Heterostructures also combine properties of dissimilar materials resulting in
improveddeviceperformancesandhence,canbeappliedtomultiplefields[24–26].
This thesis encompasses photoresponse study of various atomically thin
heterostructures made of graphene, bilayer-graphene (BLG) and MoS .
2
Agraphene-on-MoS heterostructure,madeofmonolayergrapheneandfewatomic
2
layers of MoS , combine superior electronic transport properties of graphene with
2
the optical properties of MoS . Such hybrids exhibit enormous photoresponsivity,
2
with values as high as (cid:1)1010 A W-1 at (cid:1)130 K and (cid:1)108 A W-1 at room
temperature, which make these the most photoresponsive material available till
date. The presence of tunable persistent photoresponse allows these to function as
optoelectronic memory devices, where the persistent state shows a near perfect
charge retention within the experimental time scale of operation ((cid:1)12 hrs).
Noise-free large-gain (109(cid:3)1010) mechanism is one of the salient features of
graphene-MoS hybrids.DevicesmadefromBLG-on-MoS hybridsfurtherhelpin
2 2
improvingthephotoresponsivegaininthesedevices,andalargephotoresponsivity
((cid:1)109 A W-1) is maintained even when operating these devices at low channel
bias (V \50 mV), or at a low range of channel current (I \10 nA). In an
DS DS
optimized operating condition, where circuit noise is lower than the signal from a
single photoelectron, BLG-on-MoS devices function as a number resolved single
2
photon detector. High specific detectivity and low noise equivalent power of these
devices allow investigation of photon noise present in an optical source.
ix
x Abstract
Along with the optoelectronic property study, various optical and electrical
characterizationsareadaptedthatexplaintheinterfacepropertiesofgraphene-MoS
2
heterostructures. For example, Raman spectroscopy and photoluminescence study
at the interface suggest strong interlayer coupling and efficient dissociation of
excitons, respectively, which play a key role in attaining large photoresponse.
Interfacial barrier characteristics are also investigated in a vertical graphene-MoS
2
geometry, which shows that the barrier height can be tuned by applying an elec-
trostatic field.
Various experimental techniques and instruments, such as heterostructure fab-
rication technique and setup, optical cryostat, etc., were developed in house to
accomplish experimental investigation, which are discussed in details.
The results of photoresponse study in van der Waals materials have opened up
the possibility of designing a new class of photosensitive devices which can be
utilized in various optoelectronic applications such as in biomedical sensing,
astronomical sensing, optical communications, optical quantum information pro-
cessing and in applications where low-intensity photodetection and number
resolved single photon detection attracts tremendous interest.
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