Need to develop nanoscale electronic devices has resulted in the concept of molecular electronics[1] where single or a small group of organic molecules may serve as functional building b
Trang 1Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications
Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India
1 Introduction
In recent years, there has been significant progress in data
processing and information technology This has been possible by
scaling down typical size of electronic devices, mainly transistor
that enables to increase the number of devices per chip and
consequently increases computational capacity and speed
How-ever, there are several factors that limit the continuing
miniatur-ization of silicon-based devices to about 100 nm range These
include technological limitations of device patterning, limitations
to size of metal interconnects and insulator thickness and p–n
junction depletion region width, etc Further, reduction in size also
leads to increased statistical errors in device characteristics as well
as reduced switching energy where quantum and thermal effects
may increase errors Need to develop nanoscale electronic devices
has resulted in the concept of molecular electronics[1] where
single or a small group of organic molecules may serve as
functional building blocks of electronic circuits providing
electro-nic functions such as rectification, switching, memory and
transistor action, etc Important problems in nanoscale molecular
electronics that need to be overcome before these devices may be
put to applications include development of methods to place the
molecules at desired positions and connecting them to other
molecules [2] It is clear that the development of molecular
electronics requires, design and synthesis of molecules with
electronic functionality, study of the electrical properties of such
molecules and techniques for self-assembly of circuits
In view of significant challenges to be overcome before fully functional electronic circuits can be fabricated, an interim possibility is to integrate molecular devices with silicon electronics
to yield hybrid devices where part of the circuit is based on molecules[3] In this regard, the study of self-assembly techniques for deposition of organic molecules on silicon and measurement of characteristics of mono or multilayers of these molecules are important in that the studies will demonstrate molecules with electronic functionality and techniques of self-assembly that are useful for hybrid electronics as well as future placing of molecules
at desired locations and interconnects In this paper, results of some studies carried out in our laboratory towards (a) synthesis of molecules, (b) self-assembly and electrochemical deposition techniques for preparation of mono and multilayers of these molecules on silicon and (c) study of their electrical properties have been presented
Depending upon the nature of interaction between organic molecules and substrate (Si in present studies), there are two types of methods for deposition of mono and multilayers Thermal evaporation or Langmuir–Blodgett (LB) deposition is based on physisorption of molecules on substrate [4] Physi-sorbed films prepared in this manner have low mechanical and chemical stability On the other hand, self-assembly and electrochemical deposition techniques result in chemisorption due to covalent bond between substrate and molecules[5] These bonds are strong and are better suited for further processing of the prepared devices Therefore, self-assembly and electroche-mical deposition techniques have been employed for the present studies
Various chemical processes and type of molecules can be used for the preparation of self-assembled monolayers (SAMs) on silicon substrates with native silicon oxide or bare silicon surface[6] In
A R T I C L E I N F O
Article history:
Available online 11 June 2009
Keywords:
Molecular electronics
Electrochemical grafting
Self-assembly
Functional molecules
A B S T R A C T
For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices In this paper, we review work done in our laboratory on study of characteristics of some functional molecules For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques Both commercially available and specially designed and synthesized molecules have been utilized for these investigations We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers
ß2009 Elsevier B.V All rights reserved
* Corresponding author Tel.: +91 22 25593863; fax: +91 22 25505151.
E-mail address: drgupta@barc.gov.in (S.K Gupta).
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Applied Surface Science
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p s u s c
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Trang 2the method used by us, high density of silanol (Si–OH) groups are
prepared by Piranha cleaning process with some variations on
native silicon oxide or thin thermally grown oxide layer Organic
molecules that end in silane group (–SiX3, X55Cl, OCH3or OC2H5)
can be self-assembled on such surfaces by dipping substrates in
103M concentration of molecules in solvents such as
dicyclo-hexyl or n-hexane Direct deposition of SAMs on silicon can be
carried out by preparation of hydrogen terminated substrates
followed by hydrosilylation reaction with organic molecules
having unsaturated carbon bond (C55C) as end group These
organic molecules can also be deposited on hydrogenated silicon
by electrochemical techniques[7] We have used some of these
techniques for preparation of mono/multilayers
Various molecules that have been reported to have electronic
functionality include alkanes as insulating material [8], redox
active molecules for memory devices[9,10,11],s–p and donor
bridge acceptor (D-s-A) structures for molecular diode[6,12], and
s–p–sstructure for negative differential resistance (NDR)[6,13]
Some of these devices have been fabricated and characterized by us
will be described
2 Devices developed
Preparation techniques and characteristics of mono and
multilayers of several molecules have been studied Various
devices such as (a) dielectric layers, (b) memory and switching
devices, (c) rectifier and (d) negative differential resistance devices
will be described
2.1 Dielectric material
Dielectic monolayers of different molecules were prepared by
self-assembly and electrochemical deposition techniques and
results obtained by both methods are discussed
2.1.1 Self-assembly
Three different alkyltrichlorosilane molecules have been used
to prepare and study their dielectric properties [14] SAMs of
octyltrichlorosilane (C8), dodecyltrichlorosilane (C12) and
octa-decyl trichlorosilane (C18) have been deposited on the native oxide
of heavily doped n type (n++) silicon (resistivity: 1 mVcm) Si
substrates were cleaned in piranha solution (2:1 solution of H2SO4
and H2O2) to obtain OH terminated surface and were immersed in
a 1 mM concentration of desired alkyltrichlorsosilane solution in
toluene, for a duration of 3–48 h for different molecules For
electrical measurements, a mercury drop was used as top
electrode
The thicknesses of different films were measured by
ellipso-metry and were found to be 1.5, 1.9, 2.3 and 3.0 nm for C8, C12, C18
and native Si oxide, respectively These are in good agreement with
expected values for alkyl chains[15,16] Water contact angle for
freshly cleaned Si was found to be <108, indicating the hydrophilic
nature of the native oxide layer After the deposition of
alkylsilanes, the contact angle increased to 1098 indicating
hydrophobic nature of CH3terminated surface[17,18] The atomic
force microscopy (AFM) images (typical image for C12 molecules
shown in the inset of Fig 1a), demonstrated densely packed
structure of the monolayers with average surface roughness of
1 A˚ Fourier transform infrared spectroscopy (FTIR) peak
posi-tions of symmetric (ns) and asymmetric (na) stretching modes of
CH2group provide information about the molecular order in the
monolayers For a well-ordered (i.e crystalline) monolayer, the
peak positions ofns and naare at 2851 1 and 2918 1 cm1,
respectively; while for highly disordered monolayers these peak
positions shift to 2855 1 and 2924 1 cm1, respectively[6] In our
case, as shown inFig 1a, the values ofn andn are at 2848 and
2915–2919 cm1, respectively, confirming the formation of the well ordered monolayers
Typical J–V characteristics recorded for the Hg/SAM/SiO2/ Si(n++) structures are shown in Fig 1b The monolayers have current density (at 1 V) in the range of 105 to 108A/cm2
compared to 0.1–1 A/cm2through native oxide indicating that the alkylsilanes behave as good dielectric materials The data presented inFig 1has been analyzed using Simmons theory of tunneling for a thin insulating film sandwiched between two
Fig 1 (a) FTIR spectra of C8, C12 and C18 monolayers and (b) fitting of typical experimental J–V data (symbols) to Simmons tunneling model (curves) Inset of (a) shows AFM image and line scan for a C12 monolayer.
Fig 2 Current density J (measured at 1 V) as a function of thickness of the monolayers.
Trang 3similar metal electrodes, i.e M–I–M junction[19] It may be noted
that in the present case, the work functions of the two electrodes,
Hg (4.5 eV) and Si (4.1 eV), are nearly same The fitting of
experimental J–V data to Simmon model with two variable
parameters namely barrier height (F) and effective mass of
electron (m*) is also shown inFig 1b The values of m*= 0.3 meand
Fin 1.59–1.67 eV range (for different molecules) were found The
measured current density (J) showed an exponential dependence
on molecular length as shown by the data inFig 2 The tunneling decay parameter (b) estimated from the slope of this plot was found to be 0.28 0.02 A˚1
2.1.2 Electrochemical deposition
It is difficult to prepare dense and ordered monolayers with self-assembly process for shorter alkyl chains due to reduced van der Waals interactions among chains In addition, the presence of the native oxide layer precludes the study of a true silicon/organic monolayer interface To overcome these problems, we have
Fig 3 Cyclic voltammograms recorded during growth of C8 molecules on Si in the
potential range of 0 to 1 V at a scan rate of 0.05 V s 1
for different number of scans.
Fig 4 (a) Current–voltage characteristics for Al/H/Si, and Al/OTS/Si structures Inset
shows AFM image and a line scan for OTS monolayer (b) Impedance spectroscopy
data of Al/C8/Si structure The solid line is a fit to data using equivalent circuit
Fig 5 (a) XRD pattern of an APTMS multiplayer (12-layered) Inset shows AFM image and a line scan for typical monolayer (b) N 1s XPS spectra recorded for typical monolayer and multilayer (12-layered) samples The second peak in multilayer XPS data is attributed to NH
Trang 4deposited octyltrichlorosilane (C8 also denoted by OTS) monolayer
on hydrated Si (1 1 1) by electrochemical method The study
demonstrates that electrochemical process allows direct grafting
of silane molecules on Si via formation of Si–Si bonds
For electrochemical depositions, hydrogen-terminated Si (n++
doped) surfaces were prepared by boiling in piranha solution to
obtain OH-terminated surface followed by dipping in 40% NH4F
solution The OTS monolayer was deposited in cyclic voltammetry
(CV) mode under high purity argon gas[20] CV scans were carried
out using a 1:1 solution of 0.1 M tetrabutylammonium perchlorate
(TBAP) electrolyte and 5 mM OTS in dry methanol Control
experiments were also carried out using TBAP solution alone CV
scans were carried out between 0 and 1 V (reference electrode Ag/
AgCl) at a scan rate of 0.05 V s1 Total of 50 scans were made
Representative CV plots are shown inFig 3 It is seen that the current
is very high (2 mA) for scan 1 but reduces by two orders of
magnitude after 30 scans Reducing current indicated reduced area
available for the deposition of molecules A possible electrochemical
reaction, for the deposition of molecules is: formation of
‘‘nucleo-philic Si’’ atoms at the surface by librating hydrogen free radicals and
their reaction with ‘‘electrophilic Si’’ atoms of OTS molecule to form
Si–Si bond with release of chlorine ions[20] Coverage of surface
with molecules was estimated by scan current (Fig 3) to be 97% at
the end of the 30th scan This agreed with AFM scans showing dense
monolayers after 30 scans (Fig 4a) Thickness of monolayers
measured by ellipsometry was found to be 13.2 A˚ in agreement with
theoretical length of the molecule (13.6 A˚)
J–V characteristics of Al/OTS/Si structures are shown inFig 4a
These show very low currents through the dielectric layer Results
of impedance spectroscopy measurements on Al/OTS/Si are shown
inFig 4b The data have been analyzed using the equivalent circuit
(inset ofFig 4b) consisting of parallel resistance and capacitance
network for each of oxide and molecular layers A capacitance of
7.1 107F/cm2 was obtained for the monolayer Very small
values of resistance (110V) for oxide layer showed its absence in
the structure as expected for direct monolayer deposition on
silicon The study demonstrates that dense monolayers of alkyl
chains can be deposited by electrochemical techniques
2.2 Molecular memory
Molecular memory based on hysteresis/switching in current
voltage characteristics has been observed in self-assembled
multilayers of 3-aminopropyltrimethoxysilane (APTMS) and
elec-trochemically deposited monolayers of a specifically designed and
synthesized molecule,
5-(4-undecenyl-oxyphenyl)-10,15,20-tri-phenylporphyrin (TPP-C11)
2.2.1 I–V hysteresis in multilayers of APTMS
Hydroxyl group (OH) terminated Si(p++) substrates (resistivity
<1 mVcm) were prepared by dipping in piranha solution A very
small static deionized water contact angle (<28) of these surfaces revealed their hydrophilic character Immediately after prepara-tion, the substrates were transferred into a glove box and immersed into a 0.2% (v/v) solution of APTMS in anhydrous toluene[21] One to twelve layers of APTMS were deposited and the thickness was controlled by changing the grafting time Immediately after the deposition, the substrates were ultrasoni-cally cleaned in chloroform for 5 min to remove physisorbed molecules
The thickness of the APTMS samples was been measured using ellipsometry and was found to be 0.65 and 7.85 nm for films
Fig 6 Proposed mechanism for the growth of APTMS multilayers.
Fig 7 (a) I–V characteristics of a 12-layered APTMS device recorded seven times with device structure in the upper inset The lower inset shows I–Vs of a monolayer device that does not exhibit hysteresis (b) Hysteresis and NDR effect recorded for different maximum positive voltages.
Trang 5deposited for 0.5 and 120 h, respectively Considering the
theoretical thickness of the APTMS molecule (0.65 nm) these
sample correspond to a monolayer and 12-layered APTMS samples
The XRD pattern of 12-layer sample (Fig 5a) showed a Bragg peak
at 13.78 corresponding to interlayer spacing of 6.4 A˚, indicating
layer-by-layer self-assembly of the multilayer Typical N 1s XPS
spectra recorded for the monolayer and 12-layered samples are
shown in Fig 5b For monolayer samples, a single peak at
400.3 eV corresponding to nitrogen in the NH2group[22,23]was
observed, while for multilayer sample two peaks were observed
The low binding energy peak (400.4 eV) could be due to the
nitrogen in the NH/NH2group and the second peak (402.2 eV) is
attributed to positively charged quaternary nitrogen of the form –
NH3 The AFM showed surface roughness of <1 A˚ for monolayers
(Fig 5a) and 7 A˚ (i.e nearly equal to the monolayer height) for
12-layered sample Based on the results obtained from XPS, XRD and
AFM, a possible formation mechanism of APTMS multilayers via
self-assembly is schematically shown inFig 6 [24] The
layer-by-layer formation of multilayer-by-layers is mediated through NH3 ions
The I–V characteristics of the Hg/APTMS multilayer/Si(p++)
devices were recorded at room temperature by scanning the bias in
the sequence: 5 V ! 0 V ! +2.7 V ! 0 V ! –5 V Several,
succes-sive measurements showed highly reproducible characteristics As
shown inFig 7a, a pronounced hysteresis and negative differential
resistance (NDR) were observed in the negative bias region in I–V
characteristics for a multilayer device On the other hand, an
APTMS monolayer (inset of Fig 7a), did not exhibit hysteresis
indicating that the observed hysteresis could be associated with
trapped NH3 ions (as observed in XPS data) in the multilayers I–Vs
were also measured with different maximum positive voltage
during scan (while keeping the maximum negative voltage at
5 V) and the results are shown inFig 7b It is seen that both the
magnitude of the hysteresis and the peak-to-valley ratio of NDR
increases with the value of applied maximum positive voltage
+Vmax
Now we demonstrate that the I–V hysteresis for multilayer
devices can be used for resistive random access memory (ReRAM)
applications The memory effect may be demonstrated under
‘write–read–erase–read’ operations In the present case, +3 and–5 V
pulses for 10 s were applied, respectively, to ‘write’ the
high-conducting state and ‘erase’ to a low-high-conducting one These states
were monitored (‘read’) by measuring the device current at 4 V
The results shown inFig 8demonstrate that the device may be used
as random access memory The device has on/off ratio of 2, which is
better than that reported (1.3) for resistive memory devices
fabricated using oligo (phenylene–ethynylene) molecules[25–28]
2.2.2 Electrical switching in electrografted TPP-C11 TPP-C11 molecules with structure shown in the inset ofFig 9a were synthesized in our laboratory The synthesized molecules were characterized by FTIR and Nuclear magnetic resonance (NMR) techniques and no impurity peaks were observed The molecule consists of a vinyl (C55C) terminated 11-carbon atom alkyl-chain attached to the conjugated porphyrin ring The vinyl group was attached to enable electrografting of the molecule on H-terminated Si Monolayers of TPP-C11 were deposited[29]on H-terminated Si (prepared as discussed earlier) using cyclic voltammetry (CV), carried out using a 1:1 (v/v) solution of 0.1 M TBAP electrolyte and 1 mM TPP-C11 in dry dichloromethane The
CV were run under inert ambient in the potential range between 0 and 1 V at a scan rate of 0.05 V/s using three-electrode system with Ag/AgCl as a reference electrode
Fig 9 (a) CV characteristics obtained during of TPP-C11 deposition Insets show structure of TPP-C11 molecule as well as AFM image and line scan of a monolayer (b) Deposition mechanism of the molecules on Si.
Fig 10 Typical J–V plot of TPP-C11 grafted on Si Inset shows FTIR spectra of
Trang 6Typical CV scans are shown inFig 9a An irreversible oxidation
peak was observed at 0.3 V, for the first scan and it disappeared
rapidly as the number of scans were increased and eventually
vanished for 50th scan No such peak appeared when the CV
was run using the TBAP solution alone indicating that the peak
is associated with the bonding of TPP-C11 with Si surface
Disappearance of the peak after large number of scans is attributed
to non-availability of bonding sites for the molecules AFM image
of a monolayer shown inFig 9a indicate very small roughness A
possible electrochemical reaction, taking place in two steps, is
shown inFig 9b[7] In the first step, application of a negative
potential to the working electrode makes ‘nucleophilic Si’ atoms at
the surface by releasing hydrogen free radicals These nucleophilic
Si atoms, in the second step, react with vinyl group of the TPP-C11
molecules to form Si–C bond A very low current at 50th scan
indicates that the TPP-C11 monolayer has fully covered the Si
substrate The FTIR spectra recorded for the TPP-C11 monolayers
(see inset ofFig 10) exhibited a clear N–H stretching frequency at
3305 cm1 confirming the presence of porphyrin group As
expected symmetric and asymmetric stretching modes of CH2
group at 2881 and 2935 cm1, respectively, were also observed
Typical J–V plots recorded for TPP-C11 monolayers using Hg
drop as top contact are shown inFig 10 These show hysteresis and
switching behavior It may be noted that the J–Vs recorded for
monolayers of C11 molecules (not shown here) did not show any
hysteresis This indicates a predominant role of porphyrin ring
This electrical bistable behavior can be utilized for the molecular
memory effects similar to that of APTMS multilayers
2.3 Molecular diode Bilayers of fullerene and meso-5,10,15,20-tetraphenyl porphyrin (TPP) deposited on Si were found to show rectification character-istics Bilayers of a derivative of TPP, i.e meso-5,10,15,20-tetra (3-fluorophenyl) porphyrin (TFPhP) with C60 also showed similar behavior Both the molecules TPP and TFPhP were synthesized by us and their characterization by NMR and FTIR did not show any impurity peaks Initially fullerene (C60) molecules were deposited on H-terminated Si (n++) substrates using an electrochemical process (Fig 11a) Irreversible peak at 0.73 V indicated the deposition of
C60 Subsequently, TPP (or its derivative) layer was deposited by self-assembly using 20mM solution of TPP/TFPhP in dichloromethane carried out for a period of 24 h at room temperature Typical AFM image of a bilayer is shown in the inset ofFig 11a The films had an average roughness of 1 nm The bilayers showed rectification behavior with a rectification ratio of >200 at 1.8 V (Fig 11b) The rectification is tentatively attributed to alignment of LUMO of fullerene and HOMO of TPP
2.4 Negative differential resistance Diazonium salt: N-(2-(4-diazoniophenyl) ethyl)-N-hexyl-naphthalene-1,8:4,5-tetracarboxydiimide tetrafluoroborate (DHTT) with structure shown inFig 12a was synthesized and electro grafted
on H terminated Si (n++) substrate by cyclic voltammetry (Fig 12b) Irreversible peak at 0.75 V indicates cleavage of dinitrogen and
Fig 12 (a) Structure of molecule DHTT, (b) CV characteristics obtained during its Fig 11 (a) CV characteristics obtained during C 60 deposition Inset shows AFM
image of a bilayer (b) Typical I–V characteristics of TFPhP/C60 supramolecular
Trang 7formation of Si–C bond XPS spectra (not shown here) showed N 1s
peak at 401 eV indicating the deposition of molecules Bilayer
formation was further confirmed by ellipsometry and atomic force
microscopy The J–V plots recorded using mercury drop as top
contact showed hysteresis and negative differential resistance
(Fig 12c) during first scan Subsequent scans showed changes in
characteristics that are being investigated
3 Conclusions
Application of self-assembled and electrochemically deposited
monolayers for investigation of electrical properties of organic
molecules for molecular electronics has been presented Many
functional molecules such as
5-(4-undecenyl-oxyphenyl)-10,15,20-triphenylporphyrin (TPP-C11), meso-5,10,15,20-tetraphenyl
por-phyrin (TPP) and its derivatives as well as diazonium salt have been
synthesized Conditions for self-assembly of various molecules as
well as their deposition by electrochemical techniques have been
worked out Dielectric behavior, hysteresis, memory, switching,
negative differential resistance and rectification have been
demon-strated in some of the molecules
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